NEAR INTERFACE 14C GRADIENTS IN BULK ORGANIC MATTER FROM SEDIMENTS BENEATH THE COLLAPSED LARSEN ICE SHELVES: CONTROL BY LABILE ORGANIC CARBON DISTRIBUTIONS (NOT AGE)

David John DeMaster, North Carolina State University Raleigh, Marine, Earth and Atmospheric Sciences, Raleigh, NC, United States, Enrique Isla, Institut de Ciencies del Mar, Barcelona, Spain, Richard S Taylor, North Carolina State University, Marine, Earth and Atmospheric Sciences, Raleigh, NC, United States and Craig R Smith, University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States
Abstract:
The 14C content of bulk sedimentary organic matter has been used as an important geochronological tool in a variety of marine sedimentary environments, including numerous high-latitude settings. Our research has documented a sharp and consistent gradient in the bulk sedimentary organic 14C age in the upper 5-10 cm of Antarctic continental shelf sediments, which is not a direct result of age following sediment deposition. Rather, the gradient in 14C age (which can be as much as 15,000 years) is the result of the variation in the relative amounts of young planktonic organic carbon (14C reservoir age of ~1300 years) and of old refractory organic carbon (reaching ages as much as 20,000 years in sediments beneath collapsed ice shelves). Following ice shelf collapse, new production of planktonic organic matter occurs in the water column for the first time in tens of thousands of years, and this organic material settles to the seabed, where it is mixed into the sediment column by bioturbation. The resulting gradient in 14C age of the bulk organic matter (not a result of age since deposition), however, can be used to determine the relative amounts of recently deposited marine plankton (one end member) from the old refractory sedimentary organic carbon that occurred prior to ice shelf collapse (the other end member). 14C measurements are a very useful tool for resolving the amount of labile organic carbon in Antarctic sediments (especially beneath collapsed ice shelves), because the inventory of labile organic carbon in the seabed increases over time following ice shelf collapse. The resulting data can provide important quantitative information regarding how the Antarctic seabed is responding to climate changes such as those causing ice shelf collapse. Comparative measurements are currently being made contrasting the distribution of sedimentary labile organic matter determined by these high-resolution 14C measurements with the 14C age distribution of organic matter combusted during ramped pyrolysis analyses from the same depth and core.
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