Increasing Mississippi Discharge During Both the Little Ice Age and Medieval Warm Period: A Gulf of Mexico Perspective on Climate Variability Over North America

Late Holocene climate variability includes the Little Ice Age (LIA, 1550-1850 AD) and the Medieval Warm Period (MWP, 800-1200 AD) that are characterized by contrasting hydrologic and thermal regimes. This climatic paradigm of a dry/cold LIA and a wet/warm MWP is documented at the local and regional scale through the interpretation of tree ring, speleothem and lake records. In contrast, 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. Using molecular organic and inorganic geochemical analyses from a dated sediment core, we document increasing terrestrial plant and lithogenic inputs during both the LIA and the MWP suggesting wet conditions over North America characterized both cold and warm climate regimes. Wet conditions for the LIA and MWP are inferred from 4-fold and 5-fold increases, respectively, in the contribution of terrestrially derived long-chain n-alkanes, and concurrent increases in the percent insoluble residues of the bulk sediments. Associated solely with the wet intervals, algal derived n-alkanes increases by 5-fold and 3-fold during the LIA and MWP, respectively, suggesting that continent-derived nutrients supported marine production. Surprisingly, the ratio of algal:terrestrial organic matter inputs is greater in the LIA than the MWP suggesting that nutrient delivery mechanisms are facilitated preferentially during the wet/cold interval. Geochemical trends in the Pigmy Basin show coherence with the Cariaco Basin Ti record indicating a climatic tele-connection between the low latitude tropics and sub-tropical to temperate region of North America. The climatological significance of LIA and MWP wet events will be discussed in terms of modern meteorological conditions and the long-term changes in solar forcing and seasonal dominance over North America.