Dansgaard-Oeschger Cycles in the Gulf of Mexico: A Clue to Abrupt Climate Change?

Recent evidence suggests that low-latitude climate variability plays a significant role in abrupt climate change during the last glacial cycle, particularly during the deglaciation. However, there have been few low-latitude marine records that cover the abrupt climate transitions known as Dansgaard-Oeschger (D-O) cycles that occurred during Marine Isotope Stage 3 (MIS 3; 24-57 ka). Defining the extent of D-O cyclicity in the low latitudes may provide insight into the mechanisms that are responsible for abrupt climate transitions. A 32-m sediment core (MD02-2551) from the Orca Basin, Gulf of Mexico, collected aboard the R/V Marion Dufresne in July 2002, provides new information to address the role of subtropical Atlantic sea-surface temperature (SST) in relation to high-latitude climate change during MIS 3. The location of Orca Basin at the mouth of the Mississippi River is also ideal to record variations in meltwater input from the Laurentide Ice Sheet during the last glacial period. Radiocarbon dates on a 6 m interval of the core, which covers ~30-40 ka, suggests that the average sedimentation rate is >50 cm/1000 years, allowing for 30-year resolution sampling. Paired δ ¹⁸O and Mg/Ca data on the planktic foraminifer Globigerinoides ruber (pink variety) provide SST and δ ¹⁸Oseawater estimates during a series of D-O cycles. Four distinct cycles exist in the isotopic data, which have a similar pattern and likely correspond to Interstadials 5-8, as defined in records from the Greenland ice core. These cycles have an amplitude of >1 ‰ , with values consistently reaching -2 ‰ during Interstadial 8, one of the warmest and longest Interstadials recorded in Greenland ice. The Mg-derived SST has a reduced variability with respect to the isotopic data, suggesting that the large δ ¹⁸O shifts are a function of changes in salinity, probably due to a combination of evaporation/precipitation processes and meltwater input from the Laurentide Ice Sheet during Interstadial events. The amplitude of the isotopic data and the very negative δ ¹⁸O values (-2 ‰ ) point to meltwater input as being the primary control on salinity. The presence of D-O cycles in the Gulf of Mexico has important implications for understanding abrupt climate change on the millennial time scale and for defining the relationship between high and low latitude climate variability, particularly as it relates to meltwater input from ice sheets. Defining the phasing of D-O cycles in the Gulf of Mexico relative to Greenland will make it easier to assess the relative importance of thermohaline circulation and greenhouse gas concentration changes on global climate change.