Paleoceanographic Proxy Calibration

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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

sediment trap mooring deployed in the Gulf of Mexico

The sediment trap mooring has been deployed in the Gulf of Mexico (27.5 ºN, 90.3 ºW) at 700 meters water depth from 2008-2018. It is programmed to rotate every 7-14 days throughout the year, resulting in a weekly to bi-weekly resolution time series of sinking particle flux. This mooring is funded by the USGS Climate Research and Development Program. (Public domain.)

photos of sediment sampling, the R/V Pelican, and sediment analysis

Samples are recovered from the sediment trap once every 9-12 months. Cruises to recover and redeploy the mooring take place on the UNOLS vessel, R/V Pelican, operated by LUMCON, in Cocodrie, Louisianna. On these cruises water is collected for isotopic and trace metal analysis, as well as to measure parameters of the carbonate system (dissolved inorganic carbon, total alkalinity, pH). Sometimes live foraminifera are collected with a plankton net for additional geochemical and genetic analyses. (Public domain.)

Planktic foraminifera from the Gulf of Mexico

Photo of planktic foraminifers from the Gulf of Mexico sediment trap seen through the microscope

Photograph of planktic foraminifers collected from the sediment trap. (Credit: Jessica Spear. Public domain.)

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.

 

 

 

 

graph showing total foraminerfa flux

Total foraminerfa flux. The complete foraminiferal flux data set can be found in Seasonal flux and assemblage composition of planktic foraminifera from the northern Gulf of Mexico, 2008–14, USGS Open-File Report 2016-1115 and Weekly Resolution Particulate Flux from a Sediment Trap in the Northern Gulf of Mexico, 2008–2012, USGS Open-File Report 2014-1035. (Public domain.) 

 

non-encrusted and encrusted specimens of G. truncatulinoides from sediment-trap

Globorotalia truncatulinoids—A winter recorder of surface ocean conditionsScanning electron microscopic (SEM) images of non-encrusted and encrusted specimens of G. truncatulinoides from sediment-trap. Figs. 1–4 are from sediment-trap. Figs. 1–2 are non-encrusted G. truncatulinoides; Figs. 3–4 are encrusted G. truncatulinoides. Scale bar for each figure is 200 μm. Figure is adapted from and more information is available in Spear and others, 2011.

Scanning electron micrographs of Globigerinoides ruber

Globigerinoides ruber morphotypes​​​​​​​Scanning electron micrographs of Globigerinoides ruber (White) morphotypes. (1) a and b. G. ruber (W) sensu lato; (2) c and d. G. ruber (W) sensu stricto. Scientists at the USGS investigated the difference in the oxygen isotopic composition of the 2 morphotypes of G. ruber in the sediment trap. This figure and additional details can be found in Thirumalai and others, 2014.

Biomarkers

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.

Structure of the 5 GDGTs that comprise the TEX86 index

Structure of the 5 GDGTs that comprise the TEX86 index. Each compound has either 1, 2, 3 or 4 cyclopentane rings (highlighted in red). Figure adapted from Richey and others, 2011. (Public domain.)

gas chromatograph paired to a flame ionization detector and a mass selective detector

This gas chromatograph (GC) paired to a flame ionization detector (FID) and a mass selective detector (MSD) at the USGS St. Petersburg Coastal and Marine Science biomarker lab is used to analyze biomarkers such as alkenones in marine sediments from the Gulf of Mexico. (Public domain.)

​​​​​​​Alkenone and GDGT Flux in the Gulf of Mexico Sediment Trap

Time series of (a) TEX86 and (b) UK'37 in sinking particles, and flux of (c) GDGTs and (d) alkenones at 700 m water depth

Time series of (a) TEX86 and (b) UK'37 in sinking particles, and flux of (c) GDGTs and (d) alkenones at 700 m water depth in the northern Gulf of Mexico. SST from HadISST (grey dashed line), local buoy data (black dashed line), and CTD casts during sediment trap recovery/deployment cruises is shown in panels a and b. Total flux of isoprenoid GDGTs (Σ GDGT 0, I, II, III, V and V′) (c) and C37 alkenone (ΣC37:3, C37:2) (d) are plotted with the total mass flux from January 2010 to December 2013. This figure and additional details can be found in Richey and others, 2016.

This research is part of the Climate and environmental change in the Gulf of Mexico and Caribbean project.