64. MICROFOSSIL DISTRIBUTION IN COARSE-FRACTION (>150 µm) ANALYSIS OFDEEP SEA DRILLING PROJECT SITE 480, CENTRAL GULF OF CALIFORNIA:

PRELIMINARY RESULTS1

Steve Crawford and Hans Schrader, School of Oceanography, Oregon State University, Corvallis, Oregon

ABSTRACT

We examined the relative abundance of various components in the coarse fraction (> 150 µm) from a selected por-tion of the DSDP Site 480 piston core. The components consist mainly of diatoms, radiolarians, benthic and planktonicforaminifers with minor amounts of sponge spicules, terrigenous material, volcanic glass(?), dehydrated gypsumcrystals, and spines of unknown biological origin. The examination shows that the siliceous organisms abound in thelaminated sediments and that the calcareous organisms are more abundant in the nonlaminated sediments. Seasonalupwelling is responsible for the deposition of laminated sediments. The upwelling creates a strong oxygen-minimumzone, restricting the occurrence of burrowing benthic organisms and benthic foraminifers.

INTRODUCTION

Hydraulic piston coring recovered a nearly continu-ous, 152-meter sediment section from the Guaymas Ba-sin in the central Gulf of California (Fig. 1). Site 480 isin a highly productive area (Zeitzschel, 1969), where astrong oxygen-minimum layer in the water column im-pinges on the slopes of the seafloor (Calvert, 1966). Thelack of oxygen eliminates the burrowing benthic endo-fauna and thus preserves the laminae formed by sea-sonal upwelling during the dry season and preserves ter-rigenous material during the wet season (Byrne and Em-ery, 1960). Down-core, the laminated sections are sepa-rated by nonlaminated zones, probably caused by in-creased oxygen in the water. The oxygen increase allowsthe establishment of a benthic community of organismswhose burrowing destroys the laminations (Schrader etal., 1980).

This chapter examines the relative abundance of vari-ous components in the sediment coarse fraction greaterthan 150 µm. Trends in the abundance of coarse-frac-tion components can then be used to examine ecologicalchanges. We are concerned with an 8.6-meter section ofHole 480 from a depth of 7.8 meters to 16.4 meters. Thesection represents approximately 12,000 years—assum-ing that one pair of laminae indicates one year and thateach pair of laminae averages about 0.7 mm in thickness(Schrader et al., 1980).

The sampling interval of 10 cm allows us to examinelarge-scale ecological changes occurring over hundredsof years. Changes occurring on this scale are primarilydue to world-wide climatic fluctuations rather than localor regional short-term changes. We chose this sectionfor preliminary study because it has sections of lami-nated and nonlaminated sediments.

METHODS

Surface scrapings from Hole 480 were taken at 10-cm intervalsover the length of the core. Each 10-cm interval was represented by a

Curray, J. R., Moore. D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt.Printing Office).

110°

Figure 1. DSDP Site 480. Depth contours are in fathoms (1 fathom =1.83 m).

composite sample weighed before and after drying. We then preparedthe samples for diatom analysis by boiling them in H2O2 and sodiumpyrophosphate to remove the organic matter and disperse the clay-sizematerial. We sieved the samples through a 150 µm mesh to obtain thecoarse fraction. The remainder of the material was processed fordiatom analysis (Schrader and Gersonde, 1978), and the coarse frac-tion was dried and placed in microscopical counting slides. Counts ofradiolarians, benthic foraminifers, and planktonic foraminifers weremade and multiplied by a factor (1 g/sample wt.) standardizing thedata to one-gram dry sample.

MICROFOSSIL DISTRIBUTION

The vertical distribution of microfossils in the inter-val from 780 cm to 1640 cm is shown in Figure 2. Thisfigure should be used only for recognizing general trendsin the distribution of the microfossils. The scale is log-arithmic and too small for accurately plotting and read-ing the actual fossil counts. The original data (Table 1)should be consulted for more specific information.

The first column shows the distribution of laminatedand nonlaminated sections as well as a gap in the strati-graphic column. We do not know whether this gap rep-resents a missing section or if it was caused by a separa-

1245

S. CRAWFORD, H. SCHRADER

780-

Diatoms

< = >

Radiolarians

OO o

88 8COCN-I—I All! II I I

PlanktonicForaminifers

880-

980-

1080-

1180-

1280-

1380-

1480-

1580-

I

Figure 2. Numbers of microfossils in one-gram dry sample plotted against depth. (Diatom abundance was recorded as less than, equal to, orgreater than other microfossils in the sample. Radiolarians, benthic foraminifers, and planktonic foraminifers are recorded on a logarithmicscale. Column A is radiolarians/radiolarians + planktonic foraminifers. Column B is benthic foraminifers/planktonic + benthic foraminifers.Horizontal lines on the stratigraphic column represent laminated sections, and diagonal lines represent nonlaminated sections. The blank area isa stratigraphic gap.)

tion of the core material during the coring process. Thelack of data in the interval from 1020 to 1030 cm wascaused by an anomalously low sample weight, whichin turn provided a very high standardizing factor. Thisyielded obviously erroneous counts, so we excluded thissample. No data are available for the 1210 to 1220-cmsample.

The relative abundance of diatoms is shown in the sec-ond column of Figure 2. The diatoms were not countedindividually, but we made visual estimations of theirabundance compared to the other fossils. Figure 2 showsthat diatoms are generally in greater abundance than allother microfossils in laminated sections and in lesserabundance in nonlaminated sections. Most of the dia-toms consist of species of Coscinodiscus (not Coscino-discus nodulifer) in laminated and nonlaminated sedi-ments. Occasional blooms of Thalassiothrix longissima

occur in the laminated sections, for example, in the1000- to 1010-cm interval.

Radiolarians are common in every sample and show adistribution pattern similar to that of the diatoms. Ra-diolarians, in general, are much more abundant in lami-nated sections. Their average abundance in the nonlami-nated zones is 400 to 500 per gram of dry sample. In thelaminated sections, they average 1000 to 2000 per gram.It was not obvious that any change in predominant spe-cies occurred or that there was any change in the nassel-larian/spurnellarian ratio when we compared the lami-nated and nonlaminated sections—this based on a cur-sory visual examination. A closer inspection might re-veal differences.

Benthic foraminifers show a distribution pattern op-posite to that of diatoms and radiolarians. In laminatedsections, they average under 200 per gram of dry sam-

1246

COARSE-FRACTION ANALYSIS

Table 1. Counts and ratios of microfossils for each 10-cm depth in-crement from Cores 3 and 4, DSDP Hole 480.

Sample(interval in cm)

3-1, 0-1010-2020-3030-4040-5050-6060-7070-8080-9090-100100-110110-120120-130130-140140-150

3-2, 0-1010-2020-3030-4040-5050-6060-7070-8080-9090-100100-110110-120120-130130-140140-150

3-3, 0-1010-2020-3030-4040-5050-6060-7070-7777-9191-100100-110110-120120-130

4-1, 0-1010-2020-3030-4040-5050-6060-7070-8080-9090-100100-110110-120120-130130-140140-150

4-2, 0-1010-2020-3030-4040-5050-6060-7070-8080-9090-100100-110110-120120-130130-140140-150

4-3, 0-1010-2020-3030-4040-5050-6060-7070-8080-9090-100100-110110-120120-130

Radiolarians

12131379859

1122659736

13431328501

1231454540422200544202336271543605

201713861559941

144814181580

883287527311250545896

18801255529

1180

398287297355

1278363352244375218354658383533435

1162833542340331477475392408403456647374400313424296262220625564292286311160258389320288350370

BenthicForaminifers

43230

1974132hh1222443

179185

25821976225712562544253023251191543197847299772

0

00

800

Ü. ;14

16380

1751670686856017150

76032218

15474

266921152334290187157

1353162550255944320252176134127182136316

590

32265200310329256

3190

224102138247288344

1505760235136221156

PlanktonicForaminifers

613

1900ü0(]

303

87438755237351285473510245

58

25140

0000D0

5700

50639

21921760

42831125636563

32888988676909629562

2155138927991935335127632192

693111412351003224970218

00

65310321069429

1212162245631333245645686588

1212972711899782

1026

Radiolarians:Radiolarians +

BenthicForaminifers

9510098

1001001CXJ10010094

LOO34584335521931739299

1009899

100

10010010010010010096

10010097661940

9203239

982926272926392322161826232033232832643065

10010036282350206247506354313121182931243127

BenthicForaminifers:Planktonic +

BenthicForaminifers

889191

103103103[00100

86

75

77837576929298999298

100

ICOICO74

ICOICO97917280

(A661912

8951133324

2239I0161222101020119

15382521

10029162243171627262336283037554425132213

pie; in the upper, nonlaminated section (875-960 cm)and in the upper portion of the lower nonlaminated sec-tion (1140-1190 cm), the benthic foraminifers averageseveral thousand per gram of dry sample. Below 1190cm, their numbers fluctuate considerably and averageseveral hundred per gram of dry sample, even though

the section is completely nonlaminated. The benthicforaminifers have periodic fluctuations of numbers inlaminated and nonlaminated sections. The pulses of in-crease in the laminated sections (810-820, 980-990, 1000-1010, and 1070-1080) contain a low diversity fauna con-sisting of Virgulina and Bolivina species. These pulsescorrespond to increases in radiolarian numbers, but theplanktonic foraminiferal abundance does not change.The pulses of increase in the nonlaminated sections corre-spond to increased numbers of radiolarians and plank-tonic foraminifers. The benthic foraminiferal diversity inthese zones is much greater than in the laminated sec-tions. The proportion of the fauna made up of Virgulinais reduced. Bolivina is the most abundant representative,but species of Uvigerina, Planulina, Bulimina, and othersoccur.

Planktonic foraminifers have a distributional patternsimilar to the benthic foraminifers. They have large num-bers averaging over 1000 per gram of dry sample in thenonlaminated sections and near zero in the laminatedsections. The laminated zones contain a cold-water faunadominated by Globigerina pachyderma mixed with asmaller number of warm water species including Globo-rotalia menardii. This mixture is caused by the seasonalupwelling bringing cold waters to the surface duringpart of the year. The nonlaminated sections are repre-sented by what Bandy (1961) terms a eurythermal faunapredominated by Globigerina bulloides.

The ratio of radiolarians to radiolarians + plank-tonic foraminifers is shown in Figure 2. Except for the1220-1230- and the 1430-1450-cm intervals, the highestratios occur in the laminated zones. Diester-Haass (1977)suggests that this ratio can be correlated to productivityin surface waters: The higher ratios reflect high produc-tivity. Our study agrees: We found the highest ratios insediments reflecting seasonal upwelling.

The last column of Figure 2 shows the ratio of ben-thic foraminifers to total foraminifers. In the upper halfof the stratigraphic section, benthic foraminifers aremore abundant than planktonic foraminifers; in the low-er half, the planktonic foraminifers are more numerous.

Minor components of the coarse fraction consist ofsmall numbers of sponge spicules, terrigenous material(quartz and mica), volcanic glass(?), dehydrated gyp-sum, and spines of unknown biological origin. Thesespines are about 1 mm in length and taper from a bluntto a pointed end. The width of the blunt end averagesabout 0.1 mm. They are flat and have no internal canalor structure. The spines are clear, and a test with HC1confirmed their calcium carbonate composition. Theiroccurrence is restricted to the interval from 890 to 910cm, where they compose about 20% of the sample. Thevolcanic glass(?) consists of a sphere and a tear-drop-shaped fragment of translucent, amber-colored materialin the 1030 to 1040-cm sample. The dehydrated gypsum(bassanite) is variously abundant from 1171 cm to thebase of the section. It does not occur in the upper,nonlaminated zone, or in any of the laminated sections.This mineral occurs in single and twinned crystals whosecomposition we determined by X-ray analysis. Thesecrystals formed within the sediment column as indicated

1247

S. CRAWFORD, H. SCHRADER

by the incorporation of diatom frustules and otherfossils into the crystal form.

CONCLUSIONSThe trends in the relative abundance of the micro-

fossils (Fig. 2) lie in the ecological differences that ac-cumulate and preserve laminated sediments. The lami-nated sediments accumulate under conditions such asthose prevailing in the Gulf. The seasonal wind patternscause upwelling that, in turn, provides nutrients for largediatom blooms (van Andel, 1964). These high produc-tivity conditions (Zeitzschel, 1978) provide nutrients forother planktonic organisms, such as radiolarians andplanktonic foraminifers, to flourish. Under these condi-tions, the diatoms and radiolarians accumulate in largenumbers. Decaying organic matter descends, increasingthe carbon dioxide in the water column and creating astrong oxygen-minimum zone. The increased CO2 cre-ates water that aggressively dissolves the calcium car-bonate (Berger, 1976). Thus, the effects of dissolutionfluctuate with the overlying productivity. As productiv-ity increases, the lysocline becomes shallower and plank-tonic foraminifers dissolve more readily. The same oc-curs in the benthic boundary layer and in the interstitialwaters. But as dissolution occurs, the water becomes in-creasingly saturated with calcium carbonate, thus de-creasing the dissolution effects. Because the oxygen-minimum zone impinges on the sea floor, the number ofbenthic foraminifers in the laminated sections is small.A few species apparently adapted and survived in lowoxygen environments, but their numbers and diversityare low. They are also masked by the large accumula-tion of siliceous organisms. The benthic foraminifersare probably not dissolved because they are rapidlyburied and because the interstitial waters are less aggres-sive than the water column in dissolving calcium car-bonate—particularly in the low-oxygen conditions wherethe interstitial waters are not exchanged with the bottomwaters by burrowing organisms (Berger and Soutar,1970).

The nonlaminated sediments accumulate under con-siderably different conditions. Diatom production is low-er, and with this low primary productivity comes lowerproduction of other planktonic organisms. There arefewer radiolarians in these sections. Benthic and plank-tonic foraminifers are very abundant because of the in-creased oxygen content of the water and decreased dis-solution effects. Their increase is also facilitated by thelack of masking or of dilution by siliceous organisms.

The peaks in abundance of radiolarians, benthic for-aminifers, and planktonic foraminifers in general seemto correspond to one another (particularly in the lowerhalf of the section), suggesting that the environmentalconditions causing the fluctuations affect the bottomwaters as well as the surface waters. The most probablecause of the fluctuations is changing climatic conditionsthat alter the upwelling pattern (= lower primary pro-ductivity and lower nutrient levels), temperature, andsea level. All probably contribute to the fluctuatingabundance of the organisms. The peaks of abundance

are probably a response to periodic increases in primaryproductivity. If these conditions persist long enough,the anoxic environment is established; as a result, fora-miniferal preservation declines and laminated sedimentsaccumulate.

The transition from laminated to nonlaminated andfrom nonlaminated to laminated sediments indicatessome difference in timing between the changes in num-bers of organisms and the sedimentological changes. Inmoving up the section, the first transition is from non-laminated to laminated. At about 1200 cm the benthicforaminiferal population increases dramatically. Plank-tonic foraminifers and radiolarians also increase nearthis point. These increases might be a response to re-newed upwelling conditions that allowed the foramini-fers to flourish until the oxygen-minimum zone was re-established and dissolution effects increased.

In changing from a laminated to a nonlaminated se-quence (about 945 cm), the organisms respond well be-fore the laminae cease to accumulate. The sedimento-logical changes lag behind the biological changes, prob-ably because it takes a significant period of time beforea benthic community is sufficiently established to de-stroy the seasonal laminations. Based on the contacts ofthe upper, nonlaminated section, it appears that it takesapproximately twice as long to establish the benthicforaminiferal community as it does to remove it. Each10-cm interval represents about 150 years in the lami-nated sections and somewhat less in the nonlaminated.It took approximately 400 to 500 years for the benthicforaminifers to reach their peak and only 100 to 200 yearsfor their numbers to decline the same amount. Anotherexplanation is that some sediments were originally de-posited as laminae but were later bioturbated. This oc-curs near the transition from a laminated to a nonlami-nated sequence. X-radiographs of the sediment show thatthin sections of nonlaminated sediments are occasion-ally found within the laminated zones.

A significant change in the benthic/planktonic fo-raminiferal ratio occurs near the middle of the section.In the lower half, the planktonic foraminifers are moreabundant, and in the upper half, the benthonic foramin-ifers are more numerous. This change corresponds tothe previously mentioned changes occurring at about1200 cm. The change in the ratio is caused by the availa-bility of increased nutrient for the benthic community.Prior to the establishment of upwelling conditions, theprimary productivity was low, and less organic matterand nutrients were available to the bottom-dwellingfauna. After primary productivity is increased, nutrientavailability is no longer limiting, and perhaps the con-trolling factor becomes a greater fecundity in the ben-thic foraminifers.

The major nonlaminated zones represent glacial peri-ods that caused major fluctuations in climate and sealevel. Although a detailed, chronological frameworkhas not been established for the core, it appears that thenonlaminated section of the lower half of Figure 2 rep-resents conditions during the last glaciation. Thus, thePleistocene/Holocene boundary occurs somewhere near

1248

COARSE-FRACTION ANALYSIS

the midsection. The idea that nonlaminated sectionsmight represent cooler climatic periods or glacial inter-vals is reinforced by the presence of colder-water diatomfloras in these sediments rather than in the laminatedsections.

ACKNOWLEDGMENTS

We thank Gretchen Schuette and J. C. Ingle for their review of themanuscript and Erwin Suess for a valuable discussion on the origin ofthe gypsum. This study was supported by NSF Grants OCE 77-20624and ATM 79-19458.

REFERENCES

Bandy, O. L., 1961. Distribution of Foraminifera, Radiolaria, anddiatoms in sediments of the Gulf of California. Micropaleontol-ogy, 1:1-26.

Berger, W. H., 1976. Biogenous deep sea sediments: production, pres-ervation, and interpretation. In Riley, J. P., and Chester, R.(Eds.), Chemical Oceanography (Vol. 5): New York (Academic),265-388.

Berger, W. H., and Soutar, A., 1970. Preservation of plankton shellsin an anaerobic basin off California. Geol. Soc. Am. Bull., 81:275-282.

Byrne, J. V., and Emery, K. O., 1960. Sediments of the Gulf ofCalifornia. Geol. Soc. Am. Bull., 71:983-1010.

Calvert, S. E., 1966. Origin of diatom-rich varved sediments from theGulf of California. J. Geol., 76:546-565.

Diester-Haass, L., 1977. Radiolarian/planktonic foraminiferal ratiosin a coastal upwelling region. J. Foraminifera Res., 7:26-33.

Schrader, H., and Gersonde, R., 1978. Diatoms and silicoflagellates.Utrecht Micropaleontol. Bull., 17:129-176.

Schrader, H., Kelts, K., Curray, J., et al., 1980. Laminated diatoma-ceous sediments from the Guaymas Basin slope (central Gulf ofCalifornia): 250,000 year climate record. Science, 207:1207-1209.

van Andel, Tj. H., 1964. Recent marine sediments of Gulf of Califor-nia. In Van Andel, Tj., and Shor, G. G. (Eds.), Marine Geology ofthe Gulf of California: A symposium: Mem. Am. Assoc. Pet.Geol., 3:216-310.

Zeitzschel, B., 1969. Primary productivity in the Gulf of California.Mar. Bioi, 3:201-207.

, 1978. Oceanographic factors influencing the distributionof plankton in space and time. Micropaleontology, 24:139-159.

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