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  • 8/14/2019 Sediment Out Washing From the Bering Sea Recorded by Grain Size Using the Laser Diffraction Particle Analyser

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    Sediment transport from the Bering Sea(Meiji Drift) to the North Pacific Ocean

    as recorded by grain size

    O.M JINADU

    Department of Geology & Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK

    ABSTRACT

    A thick sediment body deposit known as the Meiji drift is located in the northwestern

    Pacific Ocean. It was proposed to have been formed from deep water exiting the Bering Sea

    under the influence of thermohaline circulation. The grain size of clay, silt and sandy layers

    from the Ocean Drilling Programme (ODP) site 884 was determined using laser diffraction

    grain analysis to investigate changes in palaeocurrent velocity and sediment source to the

    Meiji drift over the last 10 myr, with emphasis on Quaternary glacial and interglacial

    periods. There was a subtle change in mean grain size during the onset of Northern

    Hemisphere Glaciation (~2.6 Ma). During the last 150 ka grain size show trends that

    indicate the palaeocurrent velocity of deep water in the Meiji drift may have changed

    through time.

    Introduction

    The Meiji sediment drift is up to 1800 m

    thick, over 1000 km long and approximately 350

    Km wide. The Meiji drift deposition was caused

    by the connection between the bottom water and

    the bearing sea circulation pattern. It is

    suggested that the deposition is formed under

    the influence of the thermohaline circulation

    (THC) (Mammerickx 1985; Rea et al., 1995;

    Scholl et al., 2003). It is thought that the

    deposition first began in the Oligocene and

    sediments are still being deposited to this day

    (David w. Scholl et al; 1973 Rea et al 1995).

    Diatoms have been observed at site 884 (Barron

    and Gledenkov, 1995). The sources to the Meiji

    Drift are Bering-derived material during

    glacials and volcanic-arc material from the

    nearby Aleutian and Kamchatkan volcanic

    regions (VanLaningham et al., 2009). The

    isotope recordings from the last Glacial

    Maximum show that there were little or no

    salinity changes (Keigwin, 1987 Keigwin et al;2003). It was also found that the current salinity

    of the surface water at this time is not high

    enough to cause THC in the Bering Sea

    (Warren, 1983; Emile-Geay et al., 2003). There

    are a number of possible explanations for this.

    For example it is possible that the Asian

    monsoon adds a large amount of fresh water to

    the North Pacific via the western boundary

    current.

    The processes that cause a sediment drift

    can be characterized by the documentation and

    analysis of sources of the sediment, and also bythe examination of the changes in grain size.

    Grain size analysis is a common sedimentologic

    tool widely used for the study of marine

    deposits, where particle sizes imitate the

    processes that generated the clasts, including

    weathering, erosion, transport, andsedimentation. Grain size analyses of marine

    sediments have been effectively used in

    combination with other paleoceanographic

    proxies to document past changes in strength ofbottom currents and upwelling i.e. Warner and

    Domak (2002) analysed glacial marine

    sediments from the Antarctic as a

    paleoenvironmental proxy and correlated it with

    the downcore variations of magnetic

    susceptibility. Modern tools such as X-ray

    attenuation (sedigraph), coulter counter

    (electroresistance) and photohydrometer are

    automated instruments that estimate grain size

    distribution. The laser diffraction particle size

    analyser is the latest instrument that offers the

    most effective way to perform rapid analyses of

    very fine grained sediments on very small

    samples.

    Laser diffraction grain analysis was used

    here to analyse the clay, silt and sandy layers of

    the samples taken from the Ocean Drilling

    Project Programme (ODP) site 884b (Rea et al.,

    1995; Fig 1).Measurements were taken from

    samples spanning the last 10 Ma with paying

    particular attention to the last 150,000yrs so as

    examine the Pliocene-Quaternary transition and

    the potential influence of the Milankovitch

    cycles. Fine grained samples were mostly

    focused on because the platy shape of clayparticles gives them more area to be measured

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    (Konert et al., 1997), making it easier to collect

    accurate and relevant data. Furthermore, upto

    40 samples can be run at the same time and data

    can be collectively and rapidly.

    Study area

    The samples were taken from the Meiji Drift,

    which is located on the eastern flanks of the

    Emperor Seamount chain in the northwest Pacific

    Ocean. Fig 1 and 2 show the proximal contributing

    terrigenous sediment sources to the Bering/NW

    Pacific region. The major rivers basins in the region

    are the Anadyr, Yukon, Kuskokwim and

    Kamchatka Rivers.

    Using HYDROTEND model by Syvisky et al

    (2003) it was noted that approximately 2-8million

    metric (Mt) is delivered to the Meiji drift each year

    from three rivers (Kuskokwim, Anadyr and

    Kamchatka) while Eberl (2004) predicted the

    Yukon has delivered 55 Mt of sediments per year to

    the ocean during historic times. Most of the

    sediments input to the Meiji Drift are from fluvial

    sources but some other materials may be adverted

    to the Meiji region by the Kuroshio Current and the

    Alaskan Stream (Figure 1). Deposits such as ice-

    rafted debris (IRD) are found in this region

    Krissek, 1993; Bigg et al., 2008) and were seen in

    the core therefore, they were avoided

    (VanLaighman et al 2009).

    Sampling and Analytical method

    Site 884 sediments over the last ten millions

    years are characterized by mostly clay, silt andclayey diatom-rich layers (Rea et al., 1993).

    Looking at some of the samples, they contain

    materials that resemble ash. These were commonly

    found throughout the core sediments.

    The samples analysed for this study were taken

    from the of the ODP hole 884b and 884c with

    depth ranging from 0-472.5 meters below the

    seafloor (mbsf). Most of the samples were takenfrom the finer-grained intervals although there are

    few occasional samples that were taken from coarse

    (IRD?) units.

    In total 53 samples were analysed. Dry samples

    were gently disaggregated with a mortar and pestle

    but with modest pressure to avoid breaking the

    individual grains. About 1-2 grams was weighed

    and placed into a 250 ml bottle. To further

    disaggregate the samples ~50ml of Sodium

    Dodecyl Sulphate was added to every sample to

    remove organic matter and isolate the terrigenous

    2

    Figure 1.Bering Sea and North Pacific Ocean with ocean currents and contributing sediment source areas. White

    arrows show surface currents, while light gray arrows show present deep water currents are known to exist in the

    present day (Owens and Warren, 2001). River Basins: YB = Yukon River Basin; KuB = Kuskokwim River Basin;

    AB = Anadyr River Basin; KaB = Kamchatka River Basin. Undifferentiated river basins contributing to the Bering

    Sea/Meiji Drift: SWA = southwest Alaska; EA = Eastern Aleutians; WA = Western Aleutians; FN = Far northeast

    Russia; KC = Koryak coastal basins; NK; Northern Kamchatka; SK = Southern Kamchatka. Sample and Core

    Locations: 884 = ODP Site 884. Figure from VanLaningham et al (2009).

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    Fractions. Samples were shaken and left for ~3-

    4 days to allow all grains > 0.1 microns to settle.

    Distilled water (~100ml) was added to the samples

    to remove the chemical and to require optical

    concentrations (also known as obscuration) of 5-

    10% as described by Pape (1996) and Buurman et

    al. (1997). Obscuration is the percentage oflight

    that is attenuated because of extinction (scattered or

    absorbed) by the particles.We calculated thissettling time using Stokes law in terms of particles

    settling down- If particles are falling in the

    viscous fluid together by their own weight due togravitythen a terminal velocity also known as thesettling velocity is reached when this friction force

    combined with buoyant force exactly balance the

    gravitational force. This allowed us to calculate

    how long it takes for the particles to settle down in

    the distilled water. Extraneous water (supernatant)

    was drained off carefully. Samples were washed

    three times to achieve the best result and

    obscuration of 10%. Samples were stirred and

    transferred to the rotary auto preparation station of

    the laser grain size analyzer.

    Laser diffraction particle size analyzer technique:

    Measurements were carried out with Coulter LS

    13 320. The laser diffraction particle size analyzer

    (LDPSA) is based on the principle of light beam

    scattering by small particles. The scatter beam is

    divided into two, with each focused into 126

    detectors by a reverse Fourier lens. The diffraction

    pattern formed by laser beam scattering from the

    samples suspended in a liquid medium (aqueous

    mode) is caught by the detector rings of the

    analyser. Diffraction patterns are interpreted using

    the Fraunhofer or Mie theory to calculate grain size

    from the intensity of the diffracted light. The

    instrument measures 118 grain size classes in theranges of 0.04m-2000m. The lower class

    boundary is 0.04m and each following boundary

    is 1.098 times the preceding one (LS 320 Coulter

    manual). Particles smaller than the minimum are

    ignored by the instrument. We note, however, that

    even though the instrument specifications suggests

    it can analyse fine clay particles, McCave et al.,

    (1995) have shown that LDPSA are not effective

    below 0.1 microns.

    For this study the LDPA was used as follows:

    The samples were placed in the rotary autosampler attached to the Coulter LS 320. Calgon

    dispersant was used to enhance dispersal of the

    3

    Figure 2. The proximal contributing terrigenous sediment sources to the Bering/NW Pacific region. The major rivers

    basins in the region are the Anadyr, Yukon, Kuskokwim and Kamchatka Rivers. The numbers in black circle represent

    the amount of sediment load deposited to the Meiji drift each year. From VanLaningham et al 2009

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    grains. The samples were sonicated for ~3 seconds

    immediately before analysis using the built in

    ultrasonicator. This is a final step to disperse any

    clusters formed while the samples are in line. The

    sample tubes were emptied automatically for ~ 5

    sec with an additional flushing by a 3-second water

    stream. The speed of the pump was set to 100 forall analyses. Some of the samples were analysed

    twice to check the reproducibility of the analyses.

    The optical model chosen for grain sizedetermination is the Fraunhofer model,based on the Fraunhofer theory of lightscattering.

    Calculation of statistical parameters

    Statistical parameters are calculated using both

    arithmetic and geometric algorithms. The

    approaches used most commonly are methods of

    moments (Krumbien and PettiJohn, 1938) and thegraphical methods (Folk and Ward, 1957) to

    calculate mean, median, skewness, sorting

    (Standard deviation), coefficient of variation and

    Kurtosis. These statistical parameters were derived

    using the software programme (GRADISTAT)

    (Blott and Pye, 2000) provide with the instrument.

    Result

    The data over the entire record (i.e. from 0 ka to

    10 Ma- see appendix table 1 for raw data) was

    plotted (Figures 3A to 3J). The last 150 ka was

    zoomed in so as to examine the potential influence

    from the Milankovitch cycle variability (Figures3A, 3C, 3E, 3G and 3I). Outliers were observed on

    the plot, especially the higher frequency plot (150

    ka) therefore a three point moving average

    (appendix table 2) i.e. for each point the smoothing

    technique takes one point on each side plus its own

    value and calculates the average of the three. This

    was used to observe the trends.

    The Last Ten Million Years at the Meiji Drift

    The mean and median over the last 10 Ma

    shows tremendous scatter with outliers but there is

    a subtle reduction in grain size around 2.5 Ma. Themean (fig 3a and 3b) and median (fig 3c and 3d)

    grain size showed an increase at 3 Ma to 5 Ma i.e.

    mean sizes increased from 10.5m (7 ) to 32m

    (5 ) while the median showed an increase of

    13.4m to 51.6m. Using the Udden, (1914);Wentworth, (1922) grain size scale, itcan noted that samples increased frommedium silt to coarse silt.

    Kurtosis measures the peakness of distribution

    of grain size. Using Krumbien and PettiJohn (1938)

    descriptive terminology as the depth of the core

    increase i.e. from 0-10 Ma (fig 3H); the values for

    kurtosis are small compared to the high frequency

    plot (0-150 ka-fig 3G). The highest value (2.8) is

    seen at 10 Ma and lowest value (0.1) seen at ~5Ma.

    Based on this observation, it can be considered that

    the kurtosis grain size at the end of the core are

    platykurtic to very leptokurtic.

    D10 (fig 3e and 3f) show the cumulative

    percentile at which a specified percentage of grain

    size diameter samples are finer. Over the 10 Ma adecreasing d10 particle size trend between 1.49-2.7

    Mas can be seen. The lowest point is at 2.5 Ma, this

    shows that 10% of the particles are finer than

    0.5m. From 3-5.5 Ma there was an increase in d10

    grain size, the samples shows that 10% of the

    samples are finer than 1m i.e. an increase from

    1m (3 Ma) to 3.5m (5.5 Ma). Towards the end of

    the core i.e. 6 Ma to 10 Ma a decrease was in grain

    d10 grain size was seen. Most sample exhibit grain

    size is finer than 2m (averagely) except the outlier

    observed at 10 Ma.

    All samples are positively skewed (Fig 3I and3J) over the last 10ma. Sizes ranges from

    0.5(lowest) to 2.5(highest). Smaller size are

    observed from 1 Ma-10 Ma compared to 0-150 ka

    which showed better trends of grain size. The 0-10

    Ma plot (fig 3J) only showed a reasonable trend at

    1 to 2.5 Ma where it can be seen that the skewness

    grain size increased slightly i.e. sizes increased

    from +1.0-1.99. Better trends at observed on the 0-

    150 Ka plot (Fig 3I).

    Based on Krumbien and PettiJohn (1938)

    Standard deviation (sorting) show that all sample

    grain sizes are greater than are 4m. This make

    them relatively poorly sorted.

    The Last 150,000 Years at the Meiji Drift

    The mean and median grain size over the last

    150 ka show a few outliers but generally a

    decreasing trend in size can seen i.e. grain size

    went from coarse grain to fine grain silt. From 0-52

    ka the mean have lower grain size i.e. size are in the

    range of 6-7m. Higher grain size can be seen at

    around 56-65 ka (higher than 20m). A subtle

    reduction can be seen at 73 ka (7m). From 75-83

    ka mean grain size increased from 11m to 20 m.

    The last 150 ka (Fig 3g) showed that most of thesamples vary from extremely leptokurtic to

    leptokurtic. The highest point observed at ~30 ka

    with kurtosis value of 7.57m, this can be

    considered as been extremely leptokurtic. A sharp

    decrease can be seen at 38 ka and 61 ka, both

    showing a size of 2.5m (very leptokurtic) and

    1.4um (leptokurtic). From ~88 ka to150 ka most of

    the samples are very leptokurtic (i.e. grain size are

    between 1.50m to 3.00m).

    Using the three point moving average, d10 for

    the last 150ka (fig 3e) showed a decrease in size.

    From ~85 ka to 140 ka most of the samples areappear to be finer than 1.5 m. A minor increase in

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    size can be seen at 80-82 ka i.e. samples increased

    from 1.5m to 2.2m. A subtle reduction can be

    seen at 72 ka, the d10 grain size shows that 10% of

    the samples are finer than 0.5m. The overall trend

    from 0-55 ka showed roughly the same d10 value

    i.e. ~ 0.6m to 0.8m. From ~150 ka to 82 ka the

    skewness grain size appears to be constant i.e. onan average of +1.60. A slight increase can be

    observed around 70 ka. There was a subtle

    reduction at ~60.30 ka (+1.25) and 38 ka (+1.6).

    Towards the top of the core i.e. 0-30 ka most of the

    samples show a higher skewness value i.e. most

    sizes are greater than +2.0.

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    0

    0.5

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    Skewness

    Age (ka) vs skewness

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    d10

    A e ka

    Age (ka) vs d10E

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    Mediangriansize(m)

    Age (ka) vs Median grain size

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    0

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    Meangrainsize(m)

    Age (ka) vs Mean grain size

    6

    5

    1 23 4

    A

    Onset of

    N.Hemisphere

    glaciation

    Onset of

    N.Hemisphere

    glaciation

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    Skewness

    Age(Ma) vs Skewness

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    Meangrainsize(m)

    Age (Ma) vs Mean grain size

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    Age(Ma) vs Kurtosis

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    Age (Ma) vs d10

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    0 1 2 3 4 5 6 7 8 9 10 11 12Mediangrainsize(m)

    Age (Ma) vs Median grain size

    Figure 3: Graphs showing data plot over the entire record (i.e. from 0 ka to 10 Ma). A: age (ka) against the mean grain size

    (m); B shows age (Ma) against the mean grain size (m);C show plot of age (Ka) against median ;D show age (Ma) against

    median grain size; E: age(ka) against d10; F: age(Ma) against d10;G:age(ka) against Kurtosis; H:age(Ma) against Kurtosis;

    I:age(ka) against skewness; J:age(Ma) against skewness. On the higher frequency plot (C, E, G, and I) the dotted lines are the

    raw value data and the smooth line represent three point moving average. Number 1-6 on the higher frequency plot represent

    the Marine Isotope Stages (MIS; MIS 1, 3 and 5 represents interglacial periods while 2, 4, and 6 represent the glacial period.

    The arrows on plot B, D and F represent the onset of Northern Hemisphere Glaciation that occurred around ~2.6Ma.

    Onset of

    N.Hemisphere

    glaciation

    B

    F

    H

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    Discussion

    The results of the grain size analysis of the

    Meiji drift show a lot of scatter with a few trends

    illuminated. I first focus attention to the median,

    mean, and d10 data sets over the last 10 Ma. Each

    shows a change at around 2.5 Ma to 5.5 Ma. It was

    noted by Rea and Snoeckx (1995) that physicalweathering and erosion suddenly increased the

    amount of IRD and terrigeneous sediments to the

    north pacific to 2.6 Ma. The change from preglacial

    to glacial sediments occurred very rapidly i.e. as

    little as 1000-2000 years (Pruher and Rea 1998). A

    decrease in d10 and mean grain (~2m) at ~2.6 Ma

    are especially striking because this period

    corresponds to the onset of northern hemisphere

    glaciation (~2.6 Ma). Glacial erosion and

    weathering can be use to explain why the sediment

    at 2.6 Ma is fine grained. During glacial erosion

    forms powered rock flour which tends to be veryfine grained (less than 2m).

    The overall decreasing trend over the last 150 ka

    is odd as I expected the variation to track MIS

    stages 6-1(glacial-interglacial variation).

    VanLaningham et al. (2009) showed that the source

    of sediment to the drift changed with the MIS

    stages (Fig 4). But the grain size analyses did not

    show this. There are several possible explanations

    for this is. Either the LDPSA is not a good

    technique to use on silt sized particles (McCave et

    al; 2006) or the sample preparation i.e. chemical

    disaggregation of samples was not done properly.

    Another explanation is that the change in sediment

    source to the Meiji drift (VanLaningham et al.,

    2009) might have caused the decrease in grain size.

    It is difficult to determine which of these is the

    most plausible and future work will have to use a

    Sedigraph and more thorough disaggregating

    treatment to eliminate these potential problems.

    McCave et al; (2006) noted that LDPSA is not

    the ideal instrument for analysing clay and silt

    because coarse clay/ fine silt may be recorded as

    medium to coarse silt i.e. the platy shape of smaller

    grains can be dominated by larger area. At presentthe sedigraph is a good choice of instrument for

    studying deep sea sediment as an analogue for

    current intensity. The technique is based on the

    settling principle of Stokes law which measures

    grain size distribution in terms current velocity.

    This can best be related to transport and

    depositional process.Despite all of this grain size parameters in silt

    (2-63m) can be used to interpret palaeocurrent

    velocity and paleoceanography (McCave and

    Manighetti; 1995). These authors noted that, based

    on the dynamics of sediment erosion, depositionand aggregate breakup of the coarser silt (greater

    than 10um) is non cohesive whereas the less than

    10um fraction is cohesive. This means that size

    sorting in response to hydrodynamic processes can

    be used to determine the current velocity i.e. I

    would except smaller fine grained (

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    likely to be found in medium to coarse (10-63m)

    silt fraction.

    Conclusion:

    We use laser diffraction particle analysis to

    examine changes in grain size distribution over the

    last 10 Myr to understand sediment transport to the

    Meiji drift in the North Pacific Ocean. Overall, the

    results showed that LDPSA is probably not the best

    method for taking grain size measurements.

    McCave and Syvisky (1991); McCave et al (1996)

    discussed that the sedigraph is a better instrument,

    especially for silt-sized particles. Acknowledging

    that, I do recognize a few trends of interest. The

    mean, median and d10 grain size show a reduction

    at 2.6 Ma, which can be correlated with the onset

    of Northern hemisphere glaciation. Previous studies

    showed prior to 2.6 Ma, physical weathering and

    erosion increased the rate of sedimentation

    suddenly. Thus, the results obtained here are in

    good agreement with this. A suggested

    interpretation for this is that during glacial

    weathering and erosion process, abrasion might

    have ground the sediment to glacial flour, therefore

    making a very fine particle size that was deposited

    in the Meiji Drift. The result for mean grain size on

    the higher frequency plot (0-150 ka) showed some

    trends as well, but with a fair amount of scatter.

    The grain size results do not agree with previous

    studies because VanLaningham et al. (2009)

    showed that the source of sediment to the drift

    changed with the MIS stages i.e. stages 1-6. But if

    these data are correct, then the lack of size can beused to infer that the velocity of deep water current

    in the Meiji drift did not change very much over the

    last 150 Ka.

    Acknowledgement

    Foremost I would like to thank Dr S.VanLaningham for his

    discussion support and mostly for providing the core samples

    used for this project. Furthermore I would like to show my

    appreciation to Mr Collin Taylor at the Department of geology

    and petroleum geology, University of Aberdeen for technical

    support. Lastly, I would also like to thank Dr D. David K. Rea,

    Professor Emeritus at the Department of Geological Sciences

    University of Michigan for discussing his ideas on the project.

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    Appendix

    9

    Appendix table 1: Show rawa data for the last 10 Ma. Leg= 145, Site=884, Hole= B and C, Core=1, Sec= 1, 0-2cm,

    20cc

    http://www.scopus.com/scopus/search/submit/author.url?author=Pisias,+N.G.&origin=resultslist&authorId=7003985012&src=shttp://www.scopus.com/scopus/search/submit/author.url?author=Duncan,+R.A.&origin=resultslist&authorId=7401604553&src=shttp://www.scopus.com/scopus/search/submit/author.url?author=Clift,+P.D.&origin=resultslist&authorId=7004449923&src=shttp://www.scopus.com/scopus/search/submit/author.url?author=Pisias,+N.G.&origin=resultslist&authorId=7003985012&src=shttp://www.scopus.com/scopus/search/submit/author.url?author=Duncan,+R.A.&origin=resultslist&authorId=7401604553&src=shttp://www.scopus.com/scopus/search/submit/author.url?author=Clift,+P.D.&origin=resultslist&authorId=7004449923&src=s
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    Appendix Table 2. Show 3 point average value data for the last 10 Ma. Leg= 145, Site=884, Hole= B and C,

    Core=1, Sec= 1, 0-2cm, 20cc.