Hydrogen isotopic ratios of algal and terrestrial organic matter in Lake Tulane, FL: from a modern calibration to the reconstruction of paleoclimatic and paleohydrologic conditions

Investigators: Eric Cross, David Hollander, Terry Quinn, Ted van Vleet

ABSTRACT: The deuterium/hydrogen (D/H) composition of source waters (precipitation, lakes, etc.) is controlled by multiple components of the hydrologic cycle, including temperature, atmospheric circulation patterns, and seasonal variability. In the subtropics, migration of the relative position of the Intertropical Convergence Zone (ITCZ) on both seasonal and geologic time scales has been shown to be the dominant control on these hydrologic conditions (Haug et al., 2001). The D/H isotopic composition of algal and terrestrial organic material is directly linked to the hydrologic cycle, and its variability at the bulk and molecular levels has the potential to be used as a proxy for paleoclimatic and paleohydrologic reconstructions. Application of this proxy requires a modern calibration study which evaluates the environmental factors (biological, chemical, climatic) that control D/H isotopic signatures and relevant fractionation processes in aquatic (algal) and terrestrial organic matter. The work presented herein provides such a calibration using a sub-tropical lacustrine environment in North America, and applies this information to interpret climatic changes on both long term (glacial/interglacial), as well as rapid (last 2 Ka) time scales in the geologic past.

• The groundwater-fed nature of this system allows for seasonally invariant dD of waters and algal material, regardless of changes in temperature, rates of biological production, or other localized factors that may vary over the annual cycle. The dD of algal material is thus a direct reflection of the source waters.

• For the purposes of this calibration, the dD of oak and pine leaf biomass can be said to be isotopically identical, and do not vary significantly over the annual cycle, allowing for analysis of terrestrial materials throughout the sedimentary record.

• The dD enrichment of terrestrial biomass relative to algae reflects the intensity of evaporative processes that are unique to the isotopic fractionation associated with terrestrial material. This relationship can be used to observe the degree of relative humidity in a given environment.