U.S. Geological Survey Open-File Report OF 03-008

Gulf of Mexico Planktic Foraminifer Core-top Calibration Data Set:  Raw Data

By Harry J. Dowsett^[1], Charlotte A. Brunner^[2], Stacey Verardo¹, and Richard Z. Poore¹

INTRODUCTION AND BACKGROUND

Paleoceanographers use the distribution of planktic foraminifers in sediment samples to estimate past oceanographic and paleoclimatic conditions (see Murray, 1995 for review).  Analysis of climate and environmental variability on the decadal to millenial scale requires a taxonomically stable and well-dated core-top calibration data set.  Databases used in global reconstructions of the last glacial maximum (Cline and Hays, 1976; CLIMAP, 1976; 1981), last interglacial (CLIMAP, 1984), and middle Pliocene (Dowsett et al., 1999) do not always meet these requirements.  In this report we present planktic foraminifer faunal census data and AMS ¹⁴C data which can be be used in investigations of climate variability in the Gulf of Mexico region (eg. Poore et al., in review). More comprehensive interpretation and analysis of these data, aimed at developing a temporally and taxonomically stable data set will follow in other publications (see Dowsett et al., 2002).

MATERIALS & METHODS

Core-top samples included in this report were originally retreived during Gulf of Mexico cruises of the RV Vema and RV Robert Conrad (Lamont Doherty Earth Observatory, LDEO), RV Trident (University of Rhode Island, URI), RV Gyre (Texas A&M University, TAMU), RV Knorr (Woods Hole Oceanographic Institute, WHOI), RV Ida Green (University of Texas Marine Science Institute) and the RV Marion-Dufresne (French Polar Institute),  dating as far back as 1954.   Samples are from piston cores, trigger weight cores or gravity cores. Core sites selected for this study represent a range of depth and environment and are distributed througout the Gulf of Mexico (Figure 1 and Appendix 1).

The faunal data assembled here are a combination of planktic foraminifer counts from Gulf of Mexico core-top samples processed by the U.S. Geological Survey (USGS), URI, and Brown University (under the direction of Nilva Kipp).  The processing technique is standard but differences between the labs are noted below.   Additional information regarding methodology can be found in Imbrie & Kipp (1971), Brunner and Cooley (1976), Brunner (1979, 1982), and Dowsett & Poore (2001).  Careful attention was paid to the taxonomic concepts of the various authors so that the resulting data set is internally taxonomically-consistent.

Raw samples aquired by the USGS were processed by first oven drying (<=50°C) and then soaking in dilute Calgon or H₂O₂ solution for several hours to disaggregate the sediment.  Disaggregated sediment was washed through a 63µm mesh and oven dried at <=50°C.  Dry residue was then dry-sieved at 150µm with the >150µm fraction reserved for faunal analysis.  When necessary (>300 individuals in the >150µm fraction) samples were split using a CARPCO or OTTO microsplitter to obtain a representative sample of 300 specimens.  Next (for samples analyzed at the USGS), individuals were fixed on a standard 60-square micropaleontological slide based upon their designation as species.  Samples analyzed at URI were counted directly from a strew on a tray.

FAUNAL DATA

The taxonomies of Parker (1962, 1967), Blow (1969), Kennett and Srinivasan (1983), and informal notes of Nilva Kipp, were employed for identification. Faunal census data are reported here using  the following taxonomic categories:

Orbulina universa d'Orbigny

Globigerinoides conglobatus (Brady)           

Globigerinoides ruber (d'Orbigny).  White and pink varieties of this species are tallied together.

Globigerinoides tenellus Parker       

Globigerinoides sacculifer (Brady).  We include in this category specimens assignable to Globigerinoides quadrilobatus (d'Orbigny) and Globigerinoides trilobus (Reuss). 

Sphaeroidinella dehiscens (Parker & Jones)

Globigerinella aequilateralis (Brady)

Globigerinella calida (Parker)           

Globigerina bulloides d'Orbigny     

Globigerina falconensis Blow

Globigerina digitata Brady

Globigerina rubescens Hofker        

Turborotalita quinqueloba (Natland)

Neogloboquadrina pachyderma (Ehrenberg).  Right and left coiling varieties are counted separately in this report.      

Neogloboquadrina dutertrei (d'Orbigny)

Globorotaloides hexagona (Natland)

Pulleniatina obliquiloculata Parker & Jones

Globorotalia inflata (d'Orbigny)

Globorotalia truncatulinoides (d'Orbigny). Right and left coiling varieties are counted separately in this report.      

Globorotalia crassaformis (Galloway & Wissler)

Neogloboquadrina pachyderma - Neogloboquadrina dutertrei (P - D ) intergrade.  Specimens of right-coiling Neogloboquadrina with more than four chambers in the final whorl, transitional between Neogloboquadrina pachyderma (Ehrenberg) and Neogloboquadrina dutertrei (d'Orbigny). 

Globorotalia hirsuta (d'Orbigny)

Globorotalia scitula (Brady)

Globorotalia menardii (Parker, Jones, and Brady) s.l.  Our Gl. menardii complex includes

Gl. menardii, Globorotalia tumida (Brady) s.l. and Globorotalia ungulata Bermudez.

Candeina nitida d'Orbigny  

Globigerinita glutinata (Egger) s.l.

Hastigerina pelagica (d'Orbigny)    

Other.  Unidentified specimens or specimens that are rare within the Gulf of Mexico assemblages.

Raw counts of planktic foraminifers in each of 135 samples  are provided in Appendix 1.

AMS ¹⁴C DATA

Archival core-top material from many of the cores shown on Figure 1 and listed in Appendix 1, is no longer available.  Therefore, direct dating of core-top assemblages is for the most part, impossible.  In many cases, we were able to obtain samples close to the core-top and estimate core-top ages by extrapolation.   Several numerical techniques were devised to determine the probability of assemblages representing "modern" core-top conditions and will be discussed elsewhere (see also Dowsett et al., 2002).

Samples selected for Accelerator Mass Spectrometry (AMS) ¹⁴C dating were processed as indicated above.  Unless indicated otherwise (Table 1), dates were obtained from mixed planktic foraminifers hand picked from the >150µm washed residue.  Graphite targets for AMS dating were made at the USGS in Reston, Virginia.  The carbon from these samples was captured as CO₂ by acidification of the entire sample with 85% phosphoric acid (H₂PO₄) in a vacuum chamber.  The CO₂  was then dried by forcing the gas through a bath cooled (using alcohol and dry ice) to approximately -80°C.  The dried CO₂ was converted to pure carbon in the form of graphite by placing a measured volume (equivalent to 1mg carbon) in a chamber with iron powder, hydrogen, and zinc as a catalyst at 575°C for ten hours.  The sample carbon (precipitated on the iron) was pressed into aluminum targets for AMS analysis. 

Dating was done at the Lawrence Livermore Laboratory Center for Accelerator Mass Spectrometry (CAMS) in Livermore, CA (Roberts et al., 1997).  Ages were reported in radiocarbon years (BP) using the Libby half-life of 5568 years.  AMS ¹⁴C dates were converted to calendar years (BP) by calibration to the INTCAL98 database (Stuiver et al., 1998) and a estimated marine reservoir correction of 400 years.

Table 1 lists results of AMS ¹⁴C dating.  An initial analysis of some of the data presented here can be found in Dowsett et al. (2002).

Table 1.  AMS ¹⁴C  results.  Samples composed of mixed planktics unless otherwise noted.

^a radiocarbon years

^b calendar years (Stuiver et al., 1998)

^c samples with no entry in this column are too young/old to be calibrated

ACKNOWLEDGEMENTS

We thank Lynn Wingard and Thomas Cronin for thoughtful reviews of this work.  We thank Bethany Boisvert, Kate Pavich, and Jessica Darling for help with sample procurement and processing. John McGeehin provided assistance with AMS ¹⁴C analyses.  We are grateful to Nial Slowey and William Bryant (TAMU), Rusty Lotti (LDEO), Arman Silva and Steven Carey (URI) for providing material.  The LDEO core lab is funded under NSF Grant OCE97-11316 and Office of Naval Research Grant N00014-96-10186.  The URI core lab is funded under NSF Grant OCE-0002226.  This work was supported by the USGS Earth Surface Dynamics Program. 

REFERENCES

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Brunner, C.A., 1979.  Distribution of planktonic foraminifera in surface sediments of the Gulf of Mexico.  Micropaleontology, 25(3): 325-335.

Brunner, C.A., 1982.  Paleoceanography of surface waters in the Gulf of Mexico during the Late Quaternary.  Quaternary Research, 17: 105-119.

Brunner, C.A. and Cooley, J.F., 1976.  Circulation in the Gulf of Mexico during the last glacial maximum.  Geological Society of America, Bulletin, 87: 681-686.

CLIMAP, 1976.  The surface of the ice-age earth.  Science, 191: 1131-1137.

CLIMAP, 1981.  Seasonal reconstructions of the Earths surface at the last glacial maximum.  In: McIntyre, A., Map and Chart Series 36, Geological Society of America.

CLIMAP, 1984.  The last interglacial ocean.  Quaternary Research, 21: 123-224.

Cline, R. and Hays, J. (eds.), 1976.  Investigation of Late Quaternary paleoceanography and paleoclimatology.  Geological Society of America Memoir 145.

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Dowsett, H.J., Brunner, C.A., Poore, R.Z. and Boisvert, B.A., 2002.  Gulf of Mexico planktic foraminifer core-top data.  EOS Transactions AGU, 83(19), Spring Meeting Supplement, Abstract GS41A-09.

Dowsett, H.J. and Poore, R.Z., 2001.  Planktic foraminifer census data from the northwestern Gulf of Mexico.  U.S. Geological Survey Open File Report 01-108: 1-6.

Imbrie, J. and Kipp, N.G., 1971.  A new micropaleontological method for quantitative paleoclimatology:  Application to a late Pleistocene Caribbean core.  In: Turekian, K.K. (ed.), The Late Cenozoic Glacial Ages.  New Haven, Yale University Press: 72-181.

Kennet, J.P. and Srinivasan, S., 1983.  Neogene planktonic foraminifera:  a phylogenetic atlas.  Hutchinson Ross, New York, 265p.

Murray, J., 1995.  Microfossil indicators of ocean water masses, circulation and climate, In, Bosence, D. and Allison, P. (eds.), Marine palaeoenvironmental analysis from fossils, Geological Society Special Publication 83: 245-264.

Parker, F.L., 1962.  Planktonic foraminiferal species in Pacific sediments.  Micropaleontology, 8:  219-254.

Parker, F.L., 1967.  Late Tertiary biostratigraphy (Planktonic Foraminifera) of tropical Indo-Pacific deep-sea cores:  Bulletins of American Paleontology, 8:  115-208.

Poore, R.Z., Dowsett, H.J.,Verardo, S., and Quinn, T.M., in review.  Millenial to century scale variability in Gulf of Mexico Holocene climate records.  Paleoceanography.

Roberts, M., Bench, G., Brown, T., Caffee, M., Finkel, R., Freeman, S., Hainsworth, L., Kashgarian, M., McAninch, J., Proctor, I., Southon, J., and Vogel, J., 1997.  The LLNL AMS Facility, In:  Jull, J., Beck, J., and Burr, G., Eds., Proceedings of the Seventh International Conference on Accelerator Mass Spectrometry, Tucson, AZ, USA, North Holland Press, p. 57-61.