4 research outputs found
Evaporation induced 18O and 13C enrichment in lake systems: A global perspective on hydrologic balance effects
Growing pressure on sustainable water resource allocation in the context of global
development and rapid environmental change demands rigorous knowledge of how regional water
cycles change through time. One of the most attractive and widely utilized approaches for gaining this
knowledge is the analysis of lake carbonate stable isotopic compositions. However, endogenic
carbonate archives are sensitive to a variety of natural processes and conditions leaving isotopic
datasets largely underdetermined. As a consequence, isotopic researchers are often required to
assume values for multiple parameters, including temperature of carbonate formation or lake water
δ18O, in order to interpret changes in hydrologic conditions. Here, we review and
analyze a global compilation of 57 lacustrine dual carbon and oxygen stable isotope records with a
topical focus on the effects of shifting hydrologic balance on endogenic carbonate isotopic
compositions.
Through integration of multiple large datasets we show that lake carbonate δ18O values
and the lake waters from which they are derived are often shifted by >+10‰ relative to source waters
discharging into the lake. The global pattern of δ18O and δ13C covariation
observed in >70% of the records studied and in several evaporation experiments demonstrates that
isotopic fractionations associated with lake water evaporation cause the heavy carbon and oxygen
isotope enrichments observed in most lakes and lake carbonate records. Modeled endogenic calcite
compositions in isotopic equilibrium with lake source waters further demonstrate that evaporation
effects can be extreme even in lake records where δ18O and δ13C
covariation is absent. Aridisol pedogenic carbonates show similar isotopic responses to evaporation,
and the relevance of evaporative modification to paleoclimatic and paleotopographic research using
endogenic carbonate proxies are discussed.
Recent advances in stable isotope research techniques present unprecedented opportunities to
overcome the underdetermined nature of stable isotopic data through integration of multiple isotopic
proxies, including dual element 13C-excess values and clumped isotope temperature
estimates. We demonstrate the utility of applying these multi-proxy approaches to the interpretation
of paleohydroclimatic conditions in ancient lake systems. Understanding past, present, and future
hydroclimatic systems is a global imperative. Significant progress should be expected as these modern
research techniques become more widely applied and integrated with traditional stable isotopic
proxies
Meltwater pulse recorded in Last Interglacial mollusk shells from Bermuda
The warm climate of Bermuda today is modulated by the nearby presence of the Gulf Stream current. However, iceberg scours in the Florida Strait and the presence of ice-rafted debris in Bermuda Rise sediments indicate that, during the last deglaciation, icebergs discharged from the Laurentide Ice Sheet traveled as far south as subtropical latitudes. We present evidence that an event of similar magnitude affected the subtropics during the Last Interglacial, potentially due to melting of the Greenland Ice Sheet. Using the clumped isotope paleothermometer, we found temperatures ~10°C colder and seawater δ18O values ~2‰ lower than modern in Last Interglacial Cittarium pica shells from Grape Bay, Bermuda. In contrast, Last Interglacial shells from Rocky Bay, Bermuda, record temperatures only slightly colder and seawater δ18O values similar to modern, likely representing more typical Last Interglacial conditions in Bermuda outside of a meltwater event. The significantly colder ocean temperatures observed in Grape Bay samples illustrate the extreme sensitivity of Bermudian climate to broad-scale ocean circulation changes. They indicate routine meltwater transport in the North Atlantic to near-equatorial latitudes, which would likely have resulted in disruption of the Atlantic Meridional Overturning Circulation. These data demonstrate that future melting of the Greenland Ice Sheet, a potential source of the Last Interglacial meltwater event, could have dramatic climate effects outside of the high latitudes