Meltwater input into the Gulf of Mexico and 8.2 kyr BP cool event in Greenland

Investigators: Jenna M. LoDico, Benjamin P. Flower, and Terrence M. Quinn

Introduction: Determining the past record of sea surface temperatures and salinities is fundamental for understanding past changes in climate. Oceanographic processes in the Gulf of Mexico have an affect on North American climate. It is part of the Western Hemisphere Warm Pool which provides a source of heat and moisture to the North American continent and Northern high latitudes. Establishing the history of Holocene SSTs from Mg/Ca and d18O of sea water in the Gulf of Mexico will provide insight into the role of tropical climate system on global climate.

 

French research vessel Marion Dufresne


In July 2002, core MD02-2550 was taken by the French RV Marion Dufresne (Figure 1); 9.09 m of sediment was recovered from Orca Basin situated in the northern Gulf of Mexico (figure 2) at a depth of 2248 m. Orca Basin is advantageous for high-resolution paleoclimatology because of very high sedimentation rates greater than 50 cm/1000 yr and a brine layer overlying the sediment that preserves sedimentary laminations (Leventer et al., 1983). The presence of pteropods assures that dissolution of carbonates is minimal. The planktonic foraminifera G. ruber was chosen due to their preference for inhabiting surface waters.

 

8.2 kyr BP Cool Event: Records in Greenland revealed that around 8.2 kyr BP ago temperatures cooled abruptly for a period of about 300-400 years. It has been proposed that this climatic event was caused by a shut down of the thermohaline circulation (THC) in the North Atlantic. Around 8.2 kyr BP a collapse took place of the last dome of the Laurentide Icesheet that covered the Hudson Bay (North America). This collapse would have considerably increased the influx of freshwater into the North Atlantic, causing a freshwater pulse that could possibly have lowered the surface water density, and slowing or shutting down the formation of deepwater and weakening the THC (Barber et al., 1999).

 

Photograph of section 2 of MD02-2550, 150 – 300 cm. 0 – 230 cm sediments are black, finely laminated silty clay; 230 – 300 cm sediments are greenish-gray, laminated silty clay.



Map showing the study area, Orca Basin, 26°56.78 N 91°21.75 W


Methods: The section 2 of MD02-2550 core was sliced 0.5 cm yielding decadal resolution. Sets of 70 foraminifera of the variety Globigerinoides ruber (white variety) were picked from 250 – 350 mm size fraction and split into aliquots for oxygen isotope and Mg/Ca analysis. The samples underwent an extensive cleaning process established by Boyle (1981) and modified by Boyle and Keigwin (1985/1986) and verified for foraminiferal Mg by Hastings et al. (1998). Oxygen isotopes (80 mg) were measured by mass spectrometer (a Finnigan MAT DeltaPlus XL). Foraminiferal Mg, Sr, Ca, and Mn (300 mg) were analyzed on a Perkin Elmer 4300 DV Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) at the College of Marine Science, University of South Florida.

Age model based on 9 AMS 14C dates. Average sedimentation rate
is 34 cm/kyr.

 
 





SST calculations:
Mg/Ca(mmol/mol)=0.38exp[0.09*SST]
(Dekens et al., 2002)


d18O sea water calculations:
T °C=14.9-4.8*(dcarbonate-dseawater)
(Bemis et al., 1998)














d18O (‰ PDB) and Mg/Ca
(mmol/mol) values vs. Depth (cm). 99 %
Replicate analysis was completed for Mg/Ca
values.





Comparison of proxies (Mg/Ca SST, d18O from G. ruber (white variety), Calculated d18O seawater, GISP2 d18O vs. Age (cal kyr BP)). AMS age control points for MD02-2550 are indicated by yellow triangles.


Findings
• SST’s average ~ 28 °C from 6.75 to 10.25 cal kyr BP which is within the current SST range for GOM (23 – 29 °C)
• d18O average ~ -1.6 ‰
• a large excursion of ~ -1.5 ‰ from 8.47 to 8.35 cal kyr BP may represent meltwater input from the collapse of the Laurentide Ice Sheet’s final dome


Future work
• Generate remaining Mg/Ca SST and d18O records for Holocene with additional AMS 14 C dates.
• Faunal assemblages for Holocene for additional support for SST, SSS and nutrient changes.


References

Barber, D. C., A. Dyke, C. Hillaire_marcel, A.E. Jennings, J.T. Andrew, M.W. Kerwin, G. Bilodeau., 1999, Forcing of the Cold event of 8,200 years ago by catastrophic drainage of Laurnetide lakes, Nature, 400

Bemis, B.E., Spero, H.J., Bijma, J. (1998). Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revisited paleotemperature equations: Paleoceanography. 13(2): 150-160.

Boyle, E. A., 1981, Cadmium, zinc, copper and barium in foraminifera tests, Earth Planet, Sci. Lett. 53, 11-35

Boyle, E.A., Keigwin, L.D., 1985/1986, Comparison of Atlantic and Pacific paleochemical records for the last 215,000 yrs: changes in deep ocean circulation and chemical inventories, Earth Planet, Sci. Lett. 76, 135-150

Leventer, Amy, William, Douglas F., Kennett, James P., 1983, Relationships between anoxia, glacial meltwater and microfossil preservation in the Orca Basin, Gulf of Mexico. Marine Geology 53, 23- 40


Acknowledgements
Ethan Goodard, Heather Hill, Sarah Judson, Tom Guilderson