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Quaternary International 467 (2018) 277e291

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Quaternary International

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Palynomorph evidence for tropical climate stability in the Gulf ofPapua, Papua New Guinea, over the latest marine transgression andhighstand (14,500 years BP to today)

Marie L. Thomas a, b, *, Sophie Warny a, c, David M. Jarzen d, Samuel J. Bentley Sr. a, e,Andr�e W. Droxler f, Brandon B. Harper f, g, Charles A. Nittrouer h, Kehui Xu i

a Department of Geology and Geophysics, E235 Howe-Russell Geoscience Complex, Louisiana State University, Baton Rouge, LA 70803, USAb Hess Corporation, 1501 McKinney St., Houston, TX 77010, USAc LSU Museum of Natural Science, Louisiana State University, Baton Rouge, LA 70803, USAd Paleobotany & Paleoecology, Cleveland Museum of Natural History, 1 Wade Oval Drive, Cleveland, OH 44106, USAe Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USAf Department of Earth Science, 6100 Main St., Rice University, Houston, TX 77005, USAg ConocoPhillips, 600 N. Dairy Ashford Rd., Houston, TX 77079, USAh School of Oceanography, Box 357940, 1503 NE Boat St., University of Washington, Seattle WA 98195-7940, USAi Department of Oceanography and Coastal Science, 2165 Energy, Coast & Environment Building, Louisiana State University, Baton Rouge, LA 70803, USA

a r t i c l e i n f o

Article history:Received 19 December 2016Received in revised form6 November 2017Accepted 19 December 2017Available online 28 December 2017

Keywords:PalynologyGulf of PapuaPapua New GuineaClimateDinoflagellate cystPollen

* Corresponding author. Hess Corporation, 150177010, USA.

E-mail address: mlthomas716@yahoo.com (M.L. T

https://doi.org/10.1016/j.quaint.2017.12.0351040-6182/© 2017 Elsevier Ltd and INQUA. All rights

a b s t r a c t

Three sediment cores (MV-41, MV-46, and MD-50) from the Gulf of Papua (GoP), Papua New Guinea,were analyzed to assess changes in climatic, oceanographic, and sedimentological conditions over thelast 14.5 kyr. Palynomorphs, which were isolated from sediment core samples, were collected atapproximately 0.5-m intervals using a strong acid and oxidant (MD-50)/non-oxidant (MV-41, MV-46)procedure. Radiocarbon (14C) stable isotope geochronology, magnetic susceptibility, stable isotopeanalysis (MD-50 only; Oxygen-18 [18O] and Carbon-13 [13C]), and clay mineral maturity analysis werealso completed for each core. Palynological data indicate that climatic conditions at sea level haveremained warm, wet, and stable for the past 14.5 kyr with sea surface temperatures in the GoP above14 �C. Potential decreases in vegetative cover marked the Younger Dryas interval (12.5e11.5 kyr BP), asindicated by reduced pollen and spore recovery. The end of the latest marine transgression (and thesubsequent return to eustatic sea level highstand) is clearly delineated by increases in marine paly-nomorph recovery and decreases in mangrove pollen at approximately 5 kyr BP. An increase in sea-sonality and potential El Ni~no Southern Oscillation variability is observed in MD-50's oxygen isotoperesults at ~5 kyr BP. This is not supported by the palynomorph record, likely because of the samplinginterval and dilution by tropical pollen flora, which indicates stable climatic conditions throughout thelast 14.5 kyr. Sediment transport pathways in the GoP remained fairly constant throughout the timeinterval, which is supported by the lack of major changes in palynomorph assemblage composition.

© 2017 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

The National Science Foundation (NSF)-funded Source-to-Sink(S2S) initiative and the MD148-PECTEN (Past Equatorial Climate:Tracking El Ni~no) expedition conducted cruises in the Gulf of Papua(GoP), Papua New Guinea (PNG) to better understand how climate

McKinney St., Houston, TX

homas).

reserved.

and oceanographic conditions have changed in this region over thelatest sea level cycle and how these factors have influenced thesedimentary record (NSF MARGINS Program, 2004). This studyaims to accomplish three major objectives, which are also part ofthe greater goals of the S2S and PECTEN initiatives: 1) reconstructclimate and oceanographic changes over the last 14.5 kyr; 2)determine the advent of anthropogenic influence; and 3) assesschanges in sediment sources over the study interval. Previousstudies focused on sediment flux and source (e.g., Francis et al.,2008; Jorry et al., 2008; Muhammad et al., 2008; Slingerland

M.L. Thomas et al. / Quaternary International 467 (2018) 277e291278

et al., 2008; Howell et al., 2014; Septama and Bentley, 2016a;Septama et al., 2016), but little attention has been paid to cli-matic, oceanographic, and anthropogenic changes (Carson et al.,2008; Febo et al., 2008), which can be well documented by thepalynologic record. Quaternary palynology of deep sea sedimentsin the GoP has also not been performed by other researchers.

Palynology, which is the study of organic-walled microfossils,can provide much data on climate and oceanographic changes.Pollen, which is produced by vegetation, is a sensitive indicator ofclimate because vegetation distribution shifts directly in responseto climatic factors such as insolation, precipitation, and tempera-ture (Traverse, 2007). Marine palynomorphs, including dinoflagel-late cysts, foraminifer linings, copepod eggs, and tintinnids,respond to changes in oceanographic conditions, including oceantemperature, salinity, productivity, and sediment input (Traverse,2007; Zonneveld et al., 2013). Because palynological preparationscontain a wide variety of both marine and terrestrial taxa, paly-nological analysis can be useful in reconstructing climate andoceanography over the time interval of study (e.g., Warny andWrenn, 2002; Warny et al., 2003, 2012; Feakins et al., 2012;Ferguson et al., 2016). This study focuses on palynomorphs recov-ered from cores MV-41, MV-46, and MD-50 (Fig. 1). We predictpollen and spore assemblages will be dominated by warm and wettaxa during the Holocene with increasing representation of cooland dry climate taxa during colder climatic intervals pre 10.5 kyr,similar to global records, which suggest that prior to the Holocene,temperatures were cooler and conditions were more arid (e.g.,Thompson et al., 1995; Broecker,1996; Lea et al., 2000). Using stableisotope analysis to delineate the contribution of C3 (warm and wet)versus C4 (warm and arid) vascular plant matter, Febo et al. (2008)found a dominant C3 plant signature in offshore GoP cores, whichcould mean that conditions have remained wet over the last 10.5kyr. Therefore, a detailed analysis of the GoP pollen and spore florais necessary to determine how cool and dry the climate actuallybecame prior to the Holocene period.

Anthropogenic impacts on palynological assemblages are welldocumented in PNG and surrounding areas. Human occupation ofPNG has been dated from at least 40e45 kyr BP (e.g., Torrence,2012), with a transition to agriculture between 7 and 9 kyr BP(e.g., Haberle et al., 2012). Agricultural activities are represented inthe pollen record by the appearance and increase in cultivatedplants and grassland pollen (resulting from deforestation), as wellas charcoal fragments (caused by burning of fields) (e.g., Denhamand Haberle, 2008; Sniderman et al., 2009; Haberle et al., 2012;Warny et al., 2012). We predict an increase in anthropogenic-related palynomorphs in the palynological record from the begin-ning of agriculture on PNG. Additionally, we expect to observeregional succession patterns with an increase in charcoal followingland clearance by burning, followed by replacement with grasslandtaxa, and then a return to rainforest vegetation.

Palynological analysis can also be used to assess changes insedimentation dynamics (e.g., Pocknall and Beggs, 1990; Courtinatet al., 2002; Hooghiemstra et al., 2006; Skupien and Mohamed,2008). The distribution of palynomorphs is strongly affected bythe same factors that control sediment distribution, includingerosion rates, current speed and direction, and particle density andmorphology. Palynomorphs are transported as part of the silt tovery fine sand grain size of the sediment load (e.g., van der Kaars,2001; Moss et al., 2005; Dai et al., 2014). Reworked paly-nomorphs, which have been sourced from geologic formationsolder than Quaternary, are useful for tracing sediment source andtransport distance (e.g., Riding, 2005).

Howell et al. (2014) used fine-grained sediment mineralogy totrace sediment delivery in two of this study's cores (MV-41 andMV-46). They determined different sediment sources for MV-41

and MV-46, despite their close proximity, likely because of com-plex shelf morphology and sourcing from different river catch-ments. Howell et al. (2014) results were consistent with the morewide-ranging provenance study of Septama and Bentley (2016a),which was based on petrography of coarser sediments. If the hy-pothesis of Howell et al. (2014) is correct, palynological assem-blages should differ significantly between the two cores. The thirdcore, MD-50, is located 280 km away in Ashmore Trough, and it isdominated by carbonate sedimentation (Harper, 2014). This loca-tion has been selected to determine changes in sea surface condi-tions by analyzing marine palynomorph assemblages, particularlydinoflagellate cysts, over the last 14.5 kyr.

2. Regional setting

The GoP (8e11�S, 143e147�E) is located southeast of PNG andnorth of Australia (Fig. 1). Sediment discharge from PNG is one ofthe world's largest by volume, at 1.7� 109 tons per year (Milliman,1995). Much of this sediment discharge comes from the rivers,including the Fly, Bamu, Turama, Kikori, Purari, Vailala, and Lake-kamu (from west to east), which enter the GoP (Milliman, 1995;Wolanski et al., 1995; Howell et al., 2014). Siliciclastic sedimentdelivered to the GoP is retained on a >100 km wide continentalshelf in the west; while in the north and east, sediments aredelivered to the continental slope and a series of troughs becausethe shelf is less than 10 km wide (Milliman, 1995; Francis et al.,2008; Septama and Bentley, 2016b). Carbonate sedimentation inthe GoP is predominately located in the south near the Great BarrierReef. Because of the large sediment discharge volumes and mixedsiliciclastic-carbonate sedimentation regime, the GoP can provideimportant insights for studying how sediments are transported tothe ocean from their source to their sink (Milliman, 1995; Septamaand Bentley, 2016a).

Two major oceanographic factors are important on the timescale of this study: currents and the El Ni~no Southern Oscillation(ENSO). The Coral Sea Current (CSC) is the major current affectingsediment transport in the western GoP, moving sediments mostlynortheastward (Brunskill et al., 1995; Wolanski and Alongi, 1995;Wolanski et al., 1995; Keen et al., 2006; Slingerland et al., 2008).In the northern and eastern GoP, transport is more complex wherethe CSC breaks down, and sediments are transported southward offthe shelf onto the continental slope (Brunskill et al., 1995; Walshet al., 2004; Martin et al., 2008; Howell et al., 2014). ENSO condi-tions occur every few years and reduce precipitation, which in turnreduces sediment discharge to the GoP (Rodbell et al., 1999;Kawahata and Gupta, 2004; Ogston et al., 2008). La Ni~na yearsresult in increased precipitation and sediment discharge (Howellet al., 2014). We predict that palynomorph assemblages will pro-vide insight into changes in current and ENSO conditions over thelatest marine transgression. They could also be used to assessmonsoon fluctuations (e.g., Sridhar et al., 2015).

Sedimentological conditions in the GoP have been studiedextensively in recent years (e.g., Muhammad et al., 2008;Slingerland et al., 2008; Howell et al., 2014; Septama and Bentley2016a, b; Septama et al., 2016), and they provide importantcontext for the present study on palynomorph distribution. TheGoP is classified as a mixed siliciclastic-carbonate depositionalsystem (e.g., Dickens et al., 2006; Francis et al., 2008; Septama andBentley, 2016b). Because the three cores evaluated in this study arefrom the northern GoP shelf/northern Pandora Trough and Ash-more Trough, we will focus our discussion on sedimentologicalconditions in these areas (Fig. 1). At the locations of cores MV-41and MV-46 (northern shelf edge and northern Pandora Trough,respectively), river clinoforms reach the shelf break (Wolanski andAlongi, 1995; Howell et al., 2014; Septama and Bentley, 2016b), and

Fig. 1. Core locations in the Gulf of Papua. Digital elevation model from Daniell (2008). MV-41 and MV-46 are located on the continental shelf and slope, and MD-50 is located inAshmore Trough.

M.L. Thomas et al. / Quaternary International 467 (2018) 277e291 279

sediments are deposited on the slope (Brunskill et al., 1995; Walshand Nittrouer, 2004; Muhammad et al., 2008). Howell et al. (2014)provide detailed data on the mineralogy and sedimentology of MV-41 and MV-46. MV-41 is from a shelf-edge deposit composed ofimmature sediments of localized igneous composition that aresourced from the nearby Lakekamu and Vailala river catchmentareas. MV-46, which is located directly down-slope from MV-41, isfrom slope deposits, and sediments have clay mineralogies withhigher maturity, indicating longer distance transport from the east.Howell et al. (2014) have concluded that sediment transport duringthe last marine transgressionwas not directly downslope fromMV-41 to MV-46 and that dispersal pathways have been more hetero-geneous and diagonal to the seabed gradient over the latest Qua-ternary. Septama and Bentley (2016a) documented similar regionalpatterns. The heterogeneity of sediment sourcing for cores MV-41and MV-46 should be apparent in palynological assemblages.

Core MD-50 is located in Ashmore Trough, which receives pre-dominantly carbonate sediments (greater than 80%) that aresourced from several coral reef atolls (Ashmore Reef, Boot Reef,Portlock Reef, and the Great Barrier Reef) (Fig. 1) (Carson et al.,

2008; Francis et al., 2008; Harper, 2014). At present, the troughreceives little siliciclastic sediment, but during times of lower sealevel, siliciclastic sediment input was higher (Carson et al., 2008;Francis et al., 2008; Tcherepanov et al., 2008, 2010; Harper, 2014).Harper (2014) provides sedimentological data on MD-50 and notesthat the core has been dominated by carbonate sediments since atleast 11.5 kyr BP.

2.1. Time interval of this study

Climate and associated sea level changes play a major role inhow sediments are delivered to the ocean (e.g., Droxler andSchlager, 1985; Glaser and Droxler, 1993; Andresen et al., 2003;Jorry et al., 2008). Four major climatic intervals are evaluated inthis study (Fig. 2), including the Bølling-Allerød Interstadial(~14.5e12.5 kyr BP), the Younger Dryas (~12.5e11.5 kyr BP), Melt-water Pulse-1B (MWP-1B, ~11.5e10.5 kyr BP), and the Holocene(~10.5 kyr BP to present day). Regional relative sea level calcula-tions from dated coral terraces provided constraints for this study(Chappell and Polach, 1991; Ota et al., 1993; Yokoyama et al., 2000;

Fig. 2. Time scale of the last 20,000 years. Figure prepared with TimeScale Creator (TSCreator, 2015; http://www.tscreator.org/). Sea level curve, in meters relative to present, is fromMiller et al. (2005); oxygen-18 (d18O, per mil PDF) composite curve is from Lisiecki and Raymo (2005); carbon-13 (d13C, per mil PDB) curve is from Cramer et al. (2009); Milankovitchcurves are from Laskar et al. (2004); and important climate periods and events are synthesized Thom and Wright (1983), Hope et al. (1983), Denham et al. (2003), and Howell et al.(2014).

Table 1Core data.

Core Length (m) Latitude Longitude Water Depth (m)

MV-41 12.84 �8.37� 145.87� 92MV-46 14.48 �8.44� 145.83� 399MD-50 27.61 �10.17� 145.09� 476

Core information, including length (meters), location (decimal degrees), and waterdepth (meters) (Beaufort, 2007; Harper, 2014; Howell et al., 2014).

M.L. Thomas et al. / Quaternary International 467 (2018) 277e291280

Hanebuth et al., 2000; Muhammad, 2009; Howell et al., 2014). Sealevels in the GoP at the Last Glacial Maximum (LGM) around 22 kyrBP were approximately 120m lower than modern conditions(Fairbanks, 1989; Hanebuth et al., 2000; Yokoyama et al., 2000),resulting in exposure of the continental shelf and delivery of sili-ciclastic sediments directly to the slope (Febo et al., 2008; Septamaet al., 2016; Septama and Bentley, 2016a). The warming climate andmelting ice at high latitudes contributed to the rise in sea level andtransgression on the shelf, which began as early as the Oldest Dryas,~15 kyr BP. During the sea level rise, less siliciclastic material wasdelivered to the slope andwas instead retained on the shelf (Howellet al., 2014; Septama et al., 2016). Periods of rapid rise occurredduring the Bølling-Allerød Interstadial (�80 to �60m) and MWP-1B (�60 to �40m) (Jorry et al., 2008; Howell et al., 2014). TheYounger Dryas, which was a cool period between the Bølling-Allerød Interstadial and MWP-1B, is characterized by lower tem-peratures, drier conditions, slowed rates of sea level rise, and highervolumes of sediment delivery to the slope (Jorry et al., 2008).Relative sea level was between �80 and �60m below current sealevel during this interval. AfterMWP-1B, sea level continued to rise,until modern sea level conditions were reached approximatelybetween 5 and 6.5 kyr BP (Ota et al., 1993; Hantoro, 1997; Lambeckand Chappell, 2001; Liu et al., 2004; Kubo and Syvitski, 2006;Howell et al., 2014). Currently, sediment delivery to the GoP slopeis restricted, and sediments that accumulate there reach throughhemipelagic sedimentation or transport through intermediate andbottom nepheloid layers (Brunskill et al., 1995; Harris et al., 1996;

Walsh and Nittrouer, 2004; Septama and Bentley, 2016a, b;Septama et al., 2016).

3. Material and methods

3.1. Core sampling

Cores MV-41 and MV-46 (Fig. 1, Table 1) were collected using ajumbo piston corer (JPC) onboard the R/V Melville in 2004. MV-41,which is located in 92m water depth on the continental shelfapproximately 2 km from the modern shelf edge, recovered12.84m of sediment. MV-46, which is located in 399mwater depthon the upper slope approximately 8 km from MV-41, recovered14.48m of sediment (Howell et al., 2014). Core MD05-2950 (MD-50) (Fig. 1, Table 1) was collected with an advanced piston corer(CALYPSO II) onboard the R/V Marion Dufresne during the 2005PECTEN cruise. The core site is located in Ashmore Trough near the

M.L. Thomas et al. / Quaternary International 467 (2018) 277e291 281

shelf-slope break in a water depth of 476m. A total of 27.61m ofsediment was recovered (Beaufort, 2007; Harper, 2014).

All cores were logged onboard after collection using a GeotekMulti-Sensor Core Logger (MSCL) to obtain whole-core gammadensity (GD, measured in grams per cubic centimeter) and mag-netic susceptibility (MS, x 10�6 SI) values at 1 cm intervals for MV-41 andMV-46 (Dickens et al., 2006) and at 2 cm intervals forMD-50(Beaufort, 2007).

3.2. Dating and isotope analysis

Eighteen samples from the three cores were collected forradiocarbon (14C) analysis for previous studies (Table 2). Nine ofthese samples were sand-sized or larger particles of woody debriscollected from MV-41 and MV-46, and age results for these twocores were presented in Muhammad (2009) and Howell et al.(2014). 14C analyses for these nine samples were conducted at theAccelerator Mass Spectrometry (AMS) facility at the Woods HoleOceanographic Institute. Four additional radiocarbon dates wereobtained for MV-41 and MV-46 using planktonic foraminifer testsof Globigerinoides ruber and G. sacculifer. Approximately 100 testsper sample were analyzed at the University of Georgia in the Centerfor Applied Isotope Studies, and age results were presented inHowell et al. (2014). Conversion to calendar years (peak probabilityof ± 1s) was performed on these ages using the RadiocarbonCalibration Program of Fairbanks et al. (2005). Six 14C ages for MD-50 were obtained by analyzing approximately 300 tests of G. ruberand G. sacculifer, and the results were presented in Harper (2014).MD-50 ages were converted to calendar years using the CALIB 7.0Radiocarbon Calibration program (Stuiver et al., 2015) and Marine09, a modeled ocean average curve from Reimer et al. (2009). Forour analyses, we placed higher weight on ages measured fromplanktonic foraminifers when the wood dates caused discrepanciesin age determinations. It is possible that the selected wood frag-ments for age determination were reworked from older materialand are not representative of the time of deposition.

Harper (2014) conducted analyses on oxygen (18O) and carbon(13C) stable isotopes in MD-50 using the planktonic foraminiferGlobigerinoides ruber selected from the 250e300 mm fraction.Samples were analyzed for stable isotopes of oxygen and carbonusing a Delta Advantage Isotope Ratio Mass Spectrometer (IRMS)coupled to a Kiel Carbonate Device III. Analytical precision for the

Table 2Radiocarbon dates, including ages based on planktonic foraminifer and(Muhammad, 2009; Harper, 2014; Howell et al., 2014).

Core and depth (cm) Planktonic foraminifer 14C age (yr BP)

MV-41: 7 - 15 690 ± 30MV-41: 61 - 63 e

MV-41: 197 - 205 6690± 35MV-41: 552 - 553 e

MV-41: 881 - 883 e

MV-41: 1143 - 1145 e

MV-46: 156 - 157 e

MV-46: 309 - 311 e

MV-46: 413 - 415 e

MV-46: 592 - 599 8470± 30MV-46: 1193 - 1195 e

MV-46: 1331 - 1338 9590± 60

MD-50: 140 - 142 2210± 45MD-50: 360 - 362 4435± 35MD-50: 870 - 872 9360± 70MD-50: 920 - 922 13,590 ± 45MD-50: 1051 - 1053 46,200 ± 1900MD-50: 1090 -1092 43,700 ± 4200

Depths are in centimeters (cm), and ages are in years before present (yr B

equipment is ±0.6‰, and isotope values are reported relative to thecarbonate standard, Vienna Pee Dee Belemnite (V-PDB), in deltanotation, d13C and d18O (Harper, 2014).

3.3. Palynological processing

Cores were sub-sampled at ~50 cm intervals for palynologicalprocessing, which isolates palynomorphs and other organic mate-rial from inorganic sedimentary material. All samples were spikedwith Lycopodium markers (batch number 124,961 for MD-50 and3862 for MV-41 and MV-46) to allow for abundance calculations(Stockmarr, 1971; Traverse, 2007; Lignum et al., 2008). All sampleswere dried and weighed in grams before processing.

Approximately 852 cm of MD-50 were sampled, which rep-resents at least the last 10.5 kyr as revealed by radiocarbondating (Table 2). Samples were processed following a hydro-chloric acid (HCl), hydrofluoric acid (HF), and controlled oxida-tion procedure (Traverse, 2007). This process involved the use of10% HCl and 70% HF to dissolve carbonates and silicates,respectively. Controlled oxidation with Schulze's reagent, amixture of potassium chlorate (KClO3) and nitric acid (HNO3),was performed for 30 s to remove some organic debris and tofacilitate identification of palynomorphs. Samples were sieved at10 mm to remove the fine fraction and improve palynomorphidentification. Heavy liquid separation with zinc bromide (ZnBr2)was performed to separate organic material from the remaininginorganic material that was not dissolved by the HCl and HFtreatment. Organic residues were mounted on glass microscopeslides with clear resin.

Procedures were slightly different for MD-50 compared to MV-41 and MV-46. Various researchers have noted the effects ofoxidation on certain fragile palynomorphs in modern sediments,specifically dinoflagellate cysts (Harland, 1983; Schrank, 1988).Although dinoflagellate cyst recovery is acceptable in MD-50, MV-41 and MV-46 were processed without controlled oxidation toensure maximum recovery of dinoflagellate cysts. Sieves with a 5-mm mesh size were used instead of 10-mm mesh to retain thesmallest palynomorphs, especially Rhizophoraceae pollen (e.g.,Scourse et al., 2005). To introduce as little bias as possible whencomparing palynomorph assemblages among cores, all other stepsin the processing procedure remained the same.

wood fragments with corresponding calibrated ages, for each core

Wood 14C age (yr BP) Calibrated 14C age (yr BP)

e 409± 5810,150± 35 11,818± 87e 7273± 3310,225± 581 11,960± 10810,700± 70 12,639± 6311,800± 140 13,647± 129

4380± 40 4935± 716100± 50 6964± 716950± 100 7778± 102e 9042± 339680± 50 11,120± 92e 10,421± 98

e 2228± 73e 5040± 96e 10,575± 92e 15,819± 118e N/Ae N/A

P).

M.L. Thomas et al. / Quaternary International 467 (2018) 277e291282

3.4. Microscopy and palynomorph identification

At least 300 palynomorphs per slide were counted on anOlympus BX43 microscope at Louisiana State University's CENEX(Center for Excellence in Palynology) laboratory. Palynomorphswere counted using transmitted light at 600x magnification. Allresults and recovery data are presented as absolute abundancevalues and are calculated with the following formula (e.g.,Stockmarr, 1971; Traverse, 2007; Lignum et al., 2008):

specimens per g sediment ¼ counted specimens � tablets added� Lycopodium in 1 tabletcounted Lycopodium� sample weight ðgÞ

Tropical rainforest settings such as those in PNG have highpollen and spore diversity, and many researchers have studiedthese diverse palynofloras, including Paijmans (1975), Flenley(1979), Walker and Flenley (1979), Playford (1982), Davey (1987),Rigby (1997, 2001), APSA Members (2007), and Playford andRigby (2008). Additional pollen identification references includePocknall (1981a, b); Pocknall and Crosbie, 1982, 1988. As species-level identification of diverse tropical vegetation using pollen andspores is difficult, fossil taxa were often grouped into genera orfamily-level designations attributable to modern vegetation. Ifattribution to a specific genus or family was impossible, pollen andspores were grouped according to morphological features (e.g.,tricolporate, tricolpate, monocolpate). Dinoflagellate cysts wereidentified using Zonneveld et al. (2013). Additional marine in-dicators were also identified, including foraminifer linings,copepod eggs, tintinnids, and acritarchs (Traverse, 2007).

3.5. Data analysis

StrataBugs 2.0 was used to create detailed plots of down-corepalynomorph distribution in comparison with previously pub-lished sedimentological data (Figs. 3e5). Each plot contains thefollowing: depth (cm); magnetic susceptibility (x106 SI); sampleintervals (cm); marine indicator abundance (including dinoflagel-late cysts, acritarchs, tintinnids, foraminifer linings, and copepodeggs); the ratio of cool to warm pollen; mangrove abundance(including Rhizophoraceae, Sonneratia spp., Avicennia spp, Nypa,and Acrostichum aureum); total pollen and spore abundance; grassabundance; fungal spore abundance; reworked palynomorphabundance; black debris and charcoal abundance; calibratedradiocarbon ages; and climate events. d13C and d18O values areincluded in the plot for MD-50 (Harper, 2014). The pollen assem-blages observed in the cores are synthesized into cool and dryversus warm and wet climate curves (e.g., Morley and Morley,2011).

Depth versus age plots were also created in Stratabugs and arepresented for each core (Fig. 6). Sediment accumulation rates are incentimeters per kiloyear (cm/kyr). Radiocarbon dates that areexcluded from the curve (one point in MV-41 and one in MV-46)are from reworked wood particles, and the ages determined fromthem are likely not representative of the age of deposition.

4. Results

Core MV-41 covers approximately 14 kyr (Fig. 3). Marine in-dicators are dominated almost completely by dinoflagellate cysts ofthe genus Brigantedinium (nearly 13,000 cysts/g), a genus that isoften found in coastal areas and has wide environmental tolerances

(Zonneveld et al., 2013). Foraminifer linings, copepod eggs, andtintinnids are also abundant. The vegetation signal is dominated bywarm climate; tropical taxa, such as Casuarina spp., Tilia spp., palms(dominated by Arecaceae), Rubiaceae (c.f. Psychotria and Timoniusspp.); Malvaceae; and spores. Mangrove vegetation is the majorvegetation representative inMV-41, with Rhizophoraceae being themost abundant. Mangrove recovery is lowest in the Holocene(0e61 cm, as low as 356 grains/g) and Bølling-Allerød Interstadial(673e1252 cm, as low as 118 grains/g) when sea levels are both at a

relative maximum and minimum. Recovery increases during sealevel transgression at the end of the Bølling-Allerød and throughoutthe Younger Dryas and MWP-1B (109e621 cm, up to 1122 grains/gof sediment). The highest total pollen and spore recovery coincideswith a peak in marine taxa during MWP-1B (352 cm, 5244 speci-mens/g). Grassland taxa are recovered in the most modern samples(7e109 cm, 206 grains/g) and across the Bølling-Allerød eYoungerDryas transition (559e785 cm, 124 grains/g). Fungal spores andreworked palynomorphs (dominated by fragments) of undiag-nostic age are abundant in MV-41 samples with abundances ofgreater than 18,000 and 28,000 palynomorphs per gram, respec-tively. Black debris and charcoal fragment abundance peaks at theend of transgression (109 cm, 3733 particles/g), in the YoungerDryas (559 cm; 2542 particles/g), and in the early Bølling-Allerød(1143 cm, 2863 particles/g). Two of these abundance peaks arefollowed by increased grass taxa abundance (7e61 cm and725e85 cm), which could represent recolonization after a distur-bance, such as fire or land clearance.

The sampled interval for core MV-46 covers approximately thelast 10.5 kyr, with assemblages representing the end of MWP-1Band the Holocene (Fig. 4). As for MV-41, marine indicators aredominated by Brigantedinium spp. dinoflagellate cysts (up to 2267cysts/g) and foraminifer linings (up to 5870 specimens/g) withlower occurrences of tintinnids (up to 1101 specimens/g) andcopepod eggs (up to 2245 specimens/g). The highest recoveries formarine taxa were immediately before the end of transgression(257e361 cm, 9293 specimens/g) and at the end of MWP-1B(1141 cm; 8194 specimens/g). Similar to MV-41, pollen assem-blages are dominated by warm, tropical rainforest taxa of the samespecies and families with minor contributions from cool and dryclimate taxa (including bisaccate families of Podocarpaceae andPinaceae, Nothofagus spp., Chenopodiaceae, and Gramineae). Totalspore and pollen abundance is lowest in the youngest samples (aslow as 337 specimens/g) and increases with depth in the core(maximum 4185 specimens/g). Grass taxa occur in low abundances(maximum147 grains/g at 957 cm) and in different intervals than inMV-41. In MV-46, grass taxa are most abundant at the end oftransgression (207 cm) and across the MWP-1B e Holocene tran-sition (957e1245 cm). The Younger Dryas and MWP-1B were notsampled in this core. Fungal spores (almost 30,000 spores/g) aremore common in MV-46 than in MV-41, and reworked paly-nomorphs are less common (samples average between 10,000 and15,000 specimens/g). A clear succession is observed with peaks inblack debris and charcoal (361; 1193; and 1379 cm) followed bypeaks in grassland taxa (309; 1089; and 1246 cm), and then bypeaks in total spores and pollen (257; 777; and 1141 cm), whichcould represent a full secondary ecological succession pattern afterdisturbances such as fires or land clearance.

Fig. 3. Palynomorph assemblages of MV-41. All abundances are in units of palynomorphs per gram of dried sediment. ENSO¼ El Ni~no Southern Oscillation, MWP-1B¼MeltwaterPulse-1B, YD ¼ Younger Dryas, BA ¼ Bølling-Allerød.

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Core MD-50 is characterized by very different palynomorphassemblages from the other two cores (Fig. 5). Palynomorphabundance in MD-50 samples is much lower than MV-41 and MV-46 for all groups, which is likely a reflection of MD-50's distancefrom shore, but it could also be affected by the processing pro-cedure. Marine indicators in MD-50 are dominated by foraminiferlinings (3e1169 specimens/g), copepod eggs (1e172 specimens/g),tintinnids (23e547 specimens/g), and dinoflagellate cysts (9e125specimens/g). The dinoflagellate cysts present include Spiniferitesspp. (2e78 cysts/g), Operculodinum centrocarpum (2e24 cysts/g),O. israelianum (1e4 cysts/g), Lingulodinium machaerophorum (2e13cysts/g), Impagidinium spp. (2e4 cysts/g), and Tuberculodiniumvancampoae (2e22 cysts/g), and Brigantedinium spp. (1e50 cysts/g)Marine indicator abundance increases sharply at the end of trans-gression (350 cm) with a concomitant decrease in mangrove pollen(Rhizophoraceae). The abundance of cool climate taxa increasestoward the top of the core (Fig. 5); however, this is likely a sideeffect of the core's distance from shore and decrease in totalabundance of terrestrial palynomorphs. Bisaccate pollen such asPodocarpaceae, which can be transported much farther because ofthe presence of air bladders (e.g., Muller, 1959), dominate thevegetation signal in the upper section of the core. High abundancesof these types are either indicative of cooler climate or of a trans-gressive phase (greater ‘distance to shore’), as discussed in Warnyet al. (2003). In this case, increased bisaccate abundance is most

likely linked to the Holocene highstand. Total pollen and abun-dances are low throughout the core, with the only peak observedduring MWP-1B (800e850 cm). Spores and bisaccate pollen are themajor component of terrestrial vegetation, particularly in theyounger samples. Three notable peaks in black debris and charcoalrecovery occur at 200, 400, and 800 cm; followed by increasedgrass taxa at 1300 and 550 cm, following the same successionpattern as in MV-41 and MV-46.

Depth vs. age plots, which were prepared for the three cores,highlight the differences in the depositional environments amongMV-41, MV-46, and MD-50 (Fig. 6). MV-41 and MV-46 show highaccumulation rates (approximately 100e500 cm/kyr) during sealevel transgressionwith major decreases (28e32 cm/kyr) when sealevel reached highstand conditions (within 10m of modern day sealevel) at approximately 5e6 kyr BP. MD-50, which was dominatedby carbonate input over the Holocene, has nearly linear sedimentaccumulation rates (from 62 cm/kyr to 92 cm/kyr), which are lowerthan accumulation rates at MV-41 and MV-46 with only a slightdecrease in accumulation after sea level reached highstand condi-tions. Prior to 10.5 kyr BP, sediment accumulation rates for MD-50were much lower, with only approximately 50 cm of sedimentaccumulated during 5.2 kyr (9.5 cm/kyr). No samples were avail-able below 850 cm in MD-50, so we cannot investigate the changesin palynomorph assemblages during this interval of low sedimentaccumulation rates.

Fig. 4. Palynomorph assemblages of MV-46. All abundances are in units of palynomorphs per gram of dried sediment. ENSO¼ El Ni~no Southern Oscillation, MWP-1B¼MeltwaterPulse-1B.

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5. Discussion

5.1. Bølling-Allerød Interstadial (14.5e12.5 kyr BP)

Changes in palynomorph assemblages in the studied cores allowthe identification of four global climatic intervals and provide abetter understanding of how climate and sea level in PNG and theGoP responded to these intervals (Fig. 2). Only MV-41 records theBølling-Allerød Interstadial (673e1252 cm) and the Younger Dryas(463e673 cm) (Fig. 3). The Bølling-Allerød Interstadial is a warminterval between 14.5 and 12.5 kyr BP that is characterized by rapidsea level rise (Howell et al., 2014), which we interpret in MV-41 tobe supported by high sediment accumulation rates (average of372.5 cm/kyr) during this interval (Fig. 6). Decreased abundances ofall palynomorph groups, especially in the oldest portion of the core(deeper than 1037 cm), were observed and could represent a coolerclimate from 13 to 14 kyr BP (Fig. 3). However, detailed analysis ofthe pollen groups led us to conclude that at sea level in Papua NewGuinea, temperatures were comparatively warm and stable duringthe Bølling-Allerød Interstadial. Pollen assemblages representingwarm climatic conditions similar to present day include Casuarina

spp. (up to 105 specimens/g of sediment at 725 cm), Ilex spp. (up to80 specimens/g at 1037 cm), Tilia spp. (occurring in five samplesthroughout the interval at an average of 80 specimens/g), palms(dominated by Arecaceae, found in every sample throughout theinterval with a maximum abundance of 1467 specimens/g),Rubiaceae (c.f., Psychotria and Timonius spp. [50 specimens/g at1143 cm], Malvaceae [up to 66 specimens/g at 1252 cm], as well as ahigh abundance of spores that indicates wet conditions (up to 3562spores/g]) (e.g., Paijmans, 1975; Flenley, 1979; Walker and Flenley,1979; Rowe, 2007a, 2007b; Rowe et al., 2013). Prominent peaksin fungal spores, which can signify potentially warmer periods (e.g.,Staplin, 1976; Elsik, 1980, 1996), also occur in conjunction withincreases in total spores and pollen, which supports the interpre-tation of a relatively warm and stable climate in PNG during thisinterval.

Low recovery of dinoflagellate cysts and other marine indicatorsin MV-41 (Fig. 3) fits well with the Howell et al. (2014) conclusionthat MV-41 sediments were deposited in a muddy inner-shelfenvironment (0e60m water depth) during the Bølling-AllerødInterstadial. Cysts of Polykrikos schwartzii are recorded throughoutthe interval (up to 341 cysts/g at 848 cm), and these cysts are

Fig. 5. Palynomorph assemblages of MD-50. All abundances are in units of palynomorphs per gram of dried sediment ENSO¼ El Ni~no Southern Oscillation, MWP-1B¼MeltwaterPulse-1B.

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commonly found in eutrophic coastal waters (Zonneveld et al.,2013), which is in agreement with the muddy inner shelf inter-pretation. However, Impagidinium strialatum (31 cysts/g at 903 cm)and Impagidinium spp. c.f. I. aculeatum (50 cysts/g at 1143 cm) areobserved in low abundances in two separate samples in this in-terval, which indicates an open ocean, oligotrophic environmentwith well-ventilated bottom waters (sea surface salinities [SSS]greater than 31.9 practical salinity units [psu] when I. strialatum ispresent) (Zonneveld et al., 2013). The recovery of Impagidinium spp.during the Bølling-Allerød Interstadial in MV-41 can be explainedthrough the reworking of sediments from older intervals or trans-port from offshore environments (potentially obliquely to theseabed gradient as at MV-46) instead of invoking changes todepositional environment and thus sea level, especially because theI. spp. are recovered in lower abundance than P. schwartzii cysts.

5.2. Younger Dryas (12.5e11.5 kyr BP)

The Younger Dryas period is a cool climate interval between 12.5and 11.5 kyr BP with relative stasis in sea level (Howell et al., 2014).We have interpreted the Younger Dryas interval in MV-41 samples(463e673 cm), primarily based on radiocarbon ages and decreasesin palynomorph abundance during this time. Interestingly, coolclimate taxa actually decrease during this interval (5e13% of thetotal pollen assemblage), but the Younger Dryas could be repre-sented by an increase in dry climate grassland taxa in PNG (Gra-mineae are recorded at 74 grains/g) that are only sparsely preservedin MV-41 (e.g., Behling, 1998). A decrease in fungal spore abun-dance (~6500 spores/g on average for the interval) could also reflectcooler conditions (e.g., Staplin, 1976; Elsik, 1980, 1996). Dinofla-gellate cyst recovery does not further inform us on climatic con-ditions in the Younger Dryas, but it does indicate a coastal/shelfdepositional environment, in agreement with the Howell et al.(2014) interpretation, as cysts of Polykrikos schwartzii are found intwo samples (147e199 cysts/g). The decrease in cool climate taxa

and the overall decrease in terrestrial palynomorph abundancecould be caused by the relative quiescence of sea level and lesssediment delivery to the shelf. We can make little further inter-pretation of GoP palynomorph assemblage response to the YoungerDryas climatic interval because only three samples cover this in-terval in one core. Future studies would provide better character-ization of the Younger Dryas by increasing sampling resolution. Theassemblages recovered do hint at the possibility for the global na-ture of the Younger Dryas event (Peteet, 1995).

5.3. Meltwater Pulse-1B (11.5e10.5 kyr BP)

MWP-1B, which is a warm climate interval between 11.5 and10.5 kyr BP that is characterized by high rates of sea level rise(Howell et al., 2014), is represented by samples in all three cores(Figs. 3e5). Samples covering this time period are characterized byhigh abundances of dinoflagellate cyst, including a 300% increase inBrigantedinium spp. in MV-41 (11,163 cysts/g at 352 cm) and MV-46(2267 cysts/g at 1141 cm), supported by a peak in Polykrikosschwartzii cysts (523 cysts/g in MV-46) and Impagidinium spp. (197cysts/g at 1245 cm in MV-46). The abundance of all terrestrialpalynomorph groups increases during this interval, which supportsthe interpretation of warmer climatic conditions. Increased abun-dances of all groups could be caused by increased rainfall from awarmer, wetter climate and subsequently erosion and increasedsediment discharge from land. Additionally, cool/warm climatecurves in MV-41 and MV-46 indicate equable climate, as theabundance of cool climate taxa remains relatively stablethroughout the interval. MD-50 is an exception, but its cool/warmcurve appears to reflect changing sea level conditions rather thanclimatic changes, as discussed previously.

MD-50 has the best expression of the palynomorph response tothe end of MWP-1B and rapid relative sea level rise. MWP-1Bsamples in MD-50 have much higher magnetic susceptibilityvalues; warm climate pollen (including Nypa spp., Casuarina spp.,

Fig. 6. Depth versus age plots for a) MV-41, b) MV-46, and c) MD-50. Sediment accumulation rates are in centimeters per thousand year (cm/kyr), and radiocarbon ages withstandard deviations are plotted in years BP.

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Tilia spp., and Sonneratia alba); and an order of magnitude increasein pollen, spores, and black debris and charcoal sourced fromterrestrial vegetation. After relative sea level reachedapproximately �60m below present at the end of MWP-1B(800 cm), the site of MD-50 in Ashmore trough was isolated frommost terrigenous sedimentation, which can be observed in therapid decrease in terrestrial palynomorph groups. MV-41 also re-cords a response to rising sea levels with a sharp decrease in MSvalues, which could be associated with decreased terrigenous de-livery to the GoP basin (Fig. 3).

5.4. Holocene (10.5 kyr BP to present)

The Holocene period, which is dated from 10.5 kyr BP to present,is well recorded by palynomorph assemblages in all three cores(Figs. 3e5). A sharp decrease in mangrove pollen and concomitant

increase inmarine indicator taxa in all three cores (MV-41 [109 cm],MV-46 [207 cm], MD-50 [350 cm]) at approximately 5 kyr BP(Fig. 5) allows us to delineate the end of marine transgression.These results agree with previously published literature on usingmangrove pollen to delineate sea level changes in the followingmanner: mangrove abundance is highest during transgression anddecreases during highstand and lowstand (e.g., Morley and Morley,2011). Peaks in mangrove pollen also indicate stable coastal con-ditions that allow the forests to develop, as well as warm and hu-mid climate (e.g., Limaye and Kumaran, 2012; Pandey et al., 2014).Dinoflagellate cyst increases can be seen during both transgressionand highstand when SSS are stable, near the global average of 35psm (e.g., Zonneveld et al., 2013). While dated coral terraces indicatemarine transgression ends at approximately 6 kyr BP, which is 1 kyrolder than the results here (e.g., Muhammad et al., 2008; Howellet al., 2014), the discrepancy in age dates can be resolved by the

Fig. 7. Sediment transport pathways in the Gulf of Papua over the last 14.5 kyr (after Francis, 2007; Howell et al., 2014). Sediment maturities (illite-smectite ratios from GoP cores,higher ratios correspond to higher maturity) are from Slingerland et al. (2008), Howell et al. (2014), and Xu (2015, personal communication). Bathymetry and topography(DEM¼ digital elevation model) is from Daniell (2008).

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response time taken for palynomorph assemblages to change orpotential intensification of ENSO conditions at approximately 5 kyrBP. The timing of this event also coincides with the Holocenehypsithermal period dated at ~5.55 kyr BP in southeast Australiaand 6.1 kyr BP in northeast Asian (Dodson and Ono, 1997). Duringthis period, temperatures were warmer and precipitation washigher, which is supported by the palynomorph assemblage in thisinterval. Assemblages could also have changed in response to thegreater climatic variability during intensified ENSO (Tudhope et al.,2001), but, this is not supported in the results of this study. Highersampling intervals over the last 5 kyr in each core would allow us tobetter delineate whether climate varied significantly on ENSO timescales, or if the palynomorph response is simply a result of sea levelchanges alone. An intensification of ENSO conditions and climaticvariation is, however, supported by greater variability in the d18Ocurve of MD-50 above 350 cm (Fig. 5).

Investigation of individual dinoflagellate cyst species can pro-vide additional understanding of oceanographic changes in theHolocene, which is the best sampled interval in this study. Dino-flagellate cyst diversity is low in MV-41 and MV-46 (assemblagesare dominated by cosmopolitan Brigantedinium spp.), which leadsto the conclusion that conditions at these core locations were notfavorable for open ocean dinoflagellates, most of which prefer fullymarine conditions and clear water (e.g., Zonneveld et al., 2013). Lowrecovery of cysts in these cores fit well with current understandingof the GoP sedimentary regime, particularly that salinity was toolow at the sites of MV-41 and MV-46 and that sediment input wastoo high (Wolanski and Alongi, 1995; Howell et al., 2014). Disap-pointingly, the low cyst diversity does not help to delineate specificsalinity and temperature changes at these core locations.

Dinoflagellate cyst diversity at MD-50 is higher than at MV-41and MV-46, but it is still relatively low compared to global

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assemblages, which could be a function of processing procedures orunfavorable bottom water conditions (i.e., Zonneveld et al., 2013).Cosmopolitan species, Spiniferites spp. and Operculodinium cen-trocarpum, make up a significant portion of the assemblage in eachHolocene sample. Several species, including Tuberculodinium van-campoae, Lingulodinium machaerophorum, and O. israelianum indi-cate open-ocean, warm equatorial conditions with well-ventilatedbottomwaters (Zonneveld et al., 2013). Operculodinium israelianum,which inhabits regions with greater than 10 �C summer SST (seasurface temperature) and 30.3e39.4 psm year-round SSS; andTuberculodinium vancampoae, which inhabits regions with greaterthan 14.2 �C summer SST and 27.8e39.2 psm year-round SSS(Zonneveld et al., 2013), co-occur in MD-50 samples from 0 to550 cm. The presence of both species together indicates a consis-tently warm environment, as predicted, for an equatorial locationwith little seasonal variability in temperature with SST exceeding14.2 �C and SSS greater than 28 psm at the site of MD-50 (Warnyet al., 2003). Impagidinium spp., which also indicates fully marine,open ocean conditions (Zonneveld et al., 2013), are presentthroughout the core and are absent only in the bottom-most sample(850 cm). In older samples of the core (from 550 to 850 cm), thepersistence of L. machaerophorum (summer SST greater than 10.1 �Cand year-round SSS of 8.5e39.4 psm) and Polysphaeridium zoharyi(summer SST greater than 14 �C and year-round SSS of 28.4e39.4psm), suggests SST remained above 14 �C (Zonneveld et al., 2013).The assemblages indicate that over the last 10.5 kyr, tropical SSTwere warm, and salinities at the site of MD-50 remained marine(SSS greater than 28.4 psm based on the presence of P. zoharyi).

5.5. Evidence for vegetation succession

Although researchers have dated the advent of agriculture onPNG between 7 and 9 kyr BP (e.g., Haberle et al., 2012), evidence foragricultural intensification from an increase in human cultivarpollen as other researchers have shown (e.g., Denham and Haberle,2008; Sniderman et al., 2009; Haberle et al., 2012), is not readilyapparent in the cores selected for this study. Analysis of black debrisand charcoal fragments in conjunction with grassland taxa andtotal spores and pollen appears to illustrate a clear successionpattern of either fire and/or land clearance (whether natural orhuman-influenced), followed by recolonization with grasslandtaxa, and then a return to forest conditions (MV-46 provides twoexcellent examples, Fig. 4). The first succession begins duringMWP-1B at approximately 10 kyr BP and ends by 9.5 kyr BP in theHolocene. The second begins at ~7 kyr BP and ends at 6.5 kyr BP.The duration of these cycles is on the order of 500 years or greaterand could represent regional-scale events. Similar patterns appearin MV-41 and MD-50, but the timing of these events is not exactlythe same, and their duration is from 1 to 5 kyr, which makes itdifficult to ascertain whether these events are related on a regionalscale or represent different sediment sources for each core (e.g.Howell et al., 2014). These patterns could represent land clearancefor agriculture, as noted by Denham and Haberle (2008) andHaberle et al. (2012). However, without the presence of humancultivar pollen in the palynologic record, we cannot definitivelydraw this conclusion.

5.6. Sediment source and transport pathways

Despite a comparison of works on pre-Quaternary paly-nomorphs on PNG (Playford, 1982; Davey, 1987; Rigby, 1997, 2001;Playford and Rigby, 2008), we have not been able to determine agesof reworked palynomorphs in the three cores because they aregenerally fragmented and highly damaged. Although we could nottrack sedimentary provenance using palynomorphs over the study

interval, Howell et al. (2014), Francis (2007), and Septama andBentley (2016a) have provided well-defined sediment transportpathways in the GoP (Fig. 7). These transport pathways have per-sisted over the last 14.5 kyr despite sea level changes, as supportedby little change in the total terrestrial palynomorph assembalge.MV-41 is sourced by low maturity sediments delivered from theVailala and Lakekamu river catchments. MV-46 is sourced by sed-iments of moderate maturity from the Fly, Bamu, Turama, Kikori,and Purari River catchments (Howell et al., 2014), which couldexplainwhy terrestrial palynomorph recoveries are lower at MV-46than at MV-41. Longer time in transport generally results in paly-nomorphs falling out of suspension before reaching the site ofdeposition (e.g., Muller, 1959; Traverse and Ginsburg, 1966). MD-50has little siliciclastic sediment input, and the majority of its sedi-ments are sourced from carbonate sediments derived fromsloughing of the Great Barrier Reef, which was partially exposedduring the LGM and early sea level transgression (Francis, 2007;Francis et al., 2008). Francis et al. (2008) note a relict shelf edgedelta that delivered sediments to Ashmore Trough. Mangrove treescould have inhabited the upper parts of this delta, which explainswhy mangrove pollen is observed in deeper samples of MD-50.Using radiocarbon and palynomorph analysis, we can date thetiming of MD-50 isolation from terrigenous sedimentation and thedrowning of the shelf-edge delta at approximately 10 kyr BP.

6. Conclusions

This study helps us better understand the response of tropicalpalynomorph assemblages in the GoP for four global climatic in-tervals: the Bølling-Allerød Interstadial (14.5e12.5 kyr BP), theYounger Dryas (12.5e11.5 kyr BP), MWP-1B (11.5e10.5 kyr BP), andthe Holocene (10.5 kyr to present) (Howell et al., 2014). The cool/warm climate curve indicates little temperature change at sea levelin the GoP, and it is likely that climates remained warm and stablein the lowlands over the latest sea level transgression and high-stand. The co-occurrence of key dinoflagellate cyst indicators,Operculodinium centrocarpum, O. israelianum, Lingulodiniummachaerophorum, and Polysphaeridium zoharyi, indicates that SSTremained higher than 14 �C and that SSS were higher than 28.4 psmover the last 10.5 kyr. This is supported by the occurrence of warm,tropical rainforest taxa throughout the same interval.

Palynomorphs record the end of marine transgression on theGoP shelf at approximately 5 kyr BP by sharp decreases inmangrove pollen abundance in all three cores and concomitantincreases in the recovery of dinoflagellate cysts and other marineindicators. Increased d18O variability at approximately 5 kyr BP, butwithout associated significant changes in palynological assem-blages at the same sample interval in MD-50, could support anincrease in ENSO activity in the GoP (Fig. 5).

Human impacts on vegetation distribution were not clearlydiscernible in the offshore palynological record, but successionpatterns could represent vegetation recovery after disturbance,such as land clearance for agriculture or the aftermath of a fire.Because these events do not occur at the same time or with thesame duration in each core and we also did not observe humancultivar pollen, we cannot definitively make a connection betweenhuman occupation, grasslands, and forest fires from these data asother authors have done (e.g., Denham and Haberle, 2008; Haberleet al., 2012; Warny et al., 2012). Additionally, the presence of blackdebris and charcoal could be governed more by sedimentationdynamics and sea level changes, similar to the study by Thomaset al. (2015).

The identification of sedimentary provenance from paly-nomorph assemblages was not directly possible in this study,although previous researchers have discussed sedimentary

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transport pathways in the GoP extensively (e.g., Keen et al., 2006;Francis, 2007; Howell et al., 2014; Septama and Bentley, 2016a).The abundance of reworked palynomorphsmay indicate changes insediment flux with increases in reworked forms when sedimen-tation accumulation rates are higher (Figs. 3, 4 and 6). Howell et al.(2014) have concluded that MV-41 and MV-46 have differentsources, although palynomorph assemblages for these two coresare quite similar (Figs. 3 and 4). Francis (2007) has noted thatAshmore Trough, where MD-50 is located, is sourced predomi-nantly by carbonate sediments that were sloughed from the GreatBarrier Reef and smaller reefs to the north and east (Fig. 7). In MD-50, we observe the final drowning of the shelf-edge delta at the endof marine transgression with the disappearance of Rhizophoraceaeand Sonneratia spp. mangroves and the establishment of fullymarine conditions marked by the appearance of Operculodiniumisraelianum and Tuberculodinium vancampoae (Fig. 5; Morley andMorley, 2011; Zonneveld et al., 2013).

This study provides a key control for understanding tropicalclimate change and the influence of that change on palynomorphassemblages deposited in themarine realm of the GoP. Based on thepalynological assemblages, it is apparent that Quaternary climatechange in the PNG/GoP was not nearly as drastic as observed byother researchers, including Thompson et al. (1995), Lea et al.(2000), Ledru et al. (2002). Mainland vegetation assemblages, asrecorded in the GoP, provide a clear picture of a warm, stableclimate for at least the last 11.5 kyr in the equatorial tropics.Additional sampling of the Younger Dryas and Bølling-AllerødInterstadial would further our understanding of the tropical cli-matic changes during these intervals, but an analysis of MV-41samples reveals that the climate likely remained warm and wetup to 14.5 kyr ago.

Acknowledgements

Samples from MV-41 and MV-46 were collected on a NationalScience Foundation (NSF)-funded PANASH (Paleoclimates of theNorthern and Southern Hemisphere) cruise on the R/V Melville in2004. Samples from MD-50 were collected on a 2005 MD148-PECTEN (Past Equatorial Climate: Tracking El Ni~no) cruise on theR/V Marion Dufresne. Sample processing, collection from curationsites, and analysis were primarily funded by the Marathon GeoDEFellowship. Hess Corporation provided the Stratabugs program andlicense for plotting data. AASP e The Palynological Society, Encana,and the Mary Jo Klosterman Professorship also provided financialsupport for this project.

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