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A sediment trap time series in the northern Gulf of Mexico is used to better assess the control of environmental variables (e.g., temperature and salinity) on the flux of both microfossils and molecular fossils to the sediments. The information gained from sediment trap studies is used to develop better proxy-based estimates of past oceanographic conditions from analyses of microfossils and molecular fossils in sediment cores.
Sediment trap studies help scientists better understand the environmental factors (e.g., temperature, salinity, ocean circulation, nutrient supply, etc.) that control the chemical composition, ecology, and life history of planktonic organisms living in the water column. These planktonic organisms, preserved in ocean sediments as microfossils or molecular fossils, can be used to reconstruct oceanographic conditions in the Gulf of Mexico over the past few centuries to the past few millions of years. Scientists at the USGS are using a long-running sediment trap (2008-2018+) in the northern Gulf of Mexico to calibrate foraminifera, biomarker, and other micropaleontological proxies for use in climate reconstructions.
Gulf of Mexico sediment trap
Planktic foraminifera from the Gulf of Mexico
Planktic foraminifera are single-celled organisms that live in the ocean and build shells made of calcium Carbonate (CaCO3). The oxygen isotopic ratio (ð18O) and the Magnesium to Calcium ratio (Mg/Ca) in their shells can be related back to the temperature and salinity at which they grew, allowing for paleoceanographic reconstruction from sediment cores collected from the seafloor.
The sediment trap allows scientists to investigate the depth habitat and seasonality of each planktic species of foraminifera in the Gulf of Mexico, along with variations in flux from year-to-year. Since there are good observations of temperature and salinity available from satellites and buoys, foraminifera from the sediment trap are also being used to improve the calibration of foraminiferal chemistry and oceanographic parameters.
Biomarkers are molecular organic compounds that are specific to certain groups of organisms. Alkenones are biomarkers produced by coccolithophores, a type of calcareous phytoplankton or algae. The relative proportion of C37 alkenones with 2 double bonds and 3 double bonds can be calibrated back to the temperature of the surface ocean. This makes alkenones a valuable and widely used proxy for past ocean temperature. However, the seasonal production of coccolithophores can vary in different regions, making detailed studies of alkenone flux important to applying this proxy in the subtropical Atlantic Ocean.
GDGTs (glycerol dibiphytanyl glycerol tetraethers) are compounds produced primarily by marine archaea. An index called TEX86, based on the distribution of cyclopentane rings in GDGTs, is related to sea surface temperature globally. The TEX86 proxy, and flux of GDGTs varies across the global ocean, making regional studies particularly important to understanding the strengths and limitations of this temperature proxy.
Alkenone and GDGT Flux in the Gulf of Mexico Sediment Trap
This research is part of the Climate and environmental change in the Gulf of Mexico and Caribbean project.
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