Late Holocene climate variability includes the Little Ice Age (LIA, 450-150 BP) and the Medieval Warm Period (MWP, 1100-700 BP) that are characterized by contrasting hydrologic and thermal regimes. The degree of interaction between the North American continent and the ocean during these two abrupt climate events is not well known. Marine sedimentary records from basins proximal to major rivers integrate climate signals across large spatial scales and can provide a coherent, high-resolution assessment of the oceanic and continental responses to changing climate and hydrologic conditions. The Pigmy Basin in the northern Gulf of Mexico is ideally situated to record inputs from the Mississippi River and to relate these inputs to changing hydrologic conditions over North America during the LIA and MWP. Hydrologic variability recorded over the North America continent is directly dependent on the moisture balance (E/P) over the sub-tropical Gulf of Mexico (a major source of moisture to the North America continent). Warm, moist air masses from the south interact with cold/dry air masses from the north over the North American continent to produce storm fronts. Increased evaporation over the Gulf of Mexico leads to enhanced precipitation over the North American continent, due to the intensification of atmospheric circulation, which influences meridional moisture flux from the Gulf of Mexico to the North American continent. This study focuses on the sedimentary record spanning the last 1400 years and utilizes a multi-proxy approach incorporating organic and inorganic geochemical analyses to define intervals of varying continental inputs and to assess changes in the moisture balance (E-P) within the Gulf of Mexico.

Understanding the cause of abrupt climate change in the geologic past can help assess the potential magnitude and variability of future changes in regional and global climate. The research presented here focuses on some of the first records of hydrologic variability in the central North American continent during an interval of Marine Isotope Stage 3 (24-57 thousand years before present (ka)). Sediment core MD02-2551 from the Orca Basin, northern Gulf of Mexico, is used to document the first detailed melting history of the southern margin of the Laurentide Ice Sheet (LIS) during MIS 3, and to record terrestrial inputs from the Mississippi River related to changes in evaporation-precipitation over the mid-continent, from 28-45 ka.Paired measurements of oxygen isotopes and Mg/Ca-SST on the planktonic foraminifera Globigerinoides ruber (pink) are used to calculate the oxygen isotopic composition of seawater and test one of the key hypotheses for abrupt climate change. Five rvals of freshwater input from 28-45 ka do not match the abrupt Dansgaard-Oeschger temperature oscillations recorded in Greenland ice. Rather, summer melting of the LIS may have occurred during Antarctic warming and likely contributed to sea-level variability during MIS 3. A detailed assessment over one of the meltwater events, using the oxygen and carbon isotopic composition of G. ruber and the deeper dwelling Neogloboquadrina dutertrei, demonstrate that meltwater was confined to the surface layers and likely had an impact on the biological pump in the Gulf of Mexico. A similar oxygen isotopic composition of seawater record determined from the year-round white G. ruber suggests that melting was not limited to the warmest summer months. The timing of LIS meltwater input is decoupled from an interval of enhanced wet conditions over the North American continent and increased Mississippi River discharge, as shown by a suite of organic and sedimentologic proxies. Increasing summer insolation on the orbital scale may have led to a northward migration of the Intertropical Convergence Zone and an intensification and westward shift in the conical position of the Bermuda High, which shuttles moisture to the North American continent and contributes to flooding in the Mississippi River drainage basin.