Paleoclimate Reconstruction from Marine and Lake Sediments

Science Center Objects

Instrumental measurements of climate variables (e.g., precipitation, temperature, ocean circulation, etc.) are only available over the past century or less. In order to quantify the rate and magnitude of natural climate variability going back in time beyond the 20th century, scientists rely on paleoclimate reconstructions. 

Sediment cores collected from high-accumulation rate basins in the deep Gulf of Mexico, along with lake sediment cores from lakes in the tropics and sub-tropics, are used to develop highly-resolved records of past climate variability over the Holocene (the last 10,000 years). Analysis of microfossils, trace metal geochemistry, stable isotope geochemistry and biomarkers is performed at sub-millennial to decadal resolution to generate records of climate variability. 

Paleoclimate from Lake Sediments

vials containing total lipid extracts

Scientists are working on a calibration study to determine how the hydrogen isotopes of lipids from the lake respond to precipitation in Florida. Lake Tulane, located in Highlands County, is and is an ideal location for paleoclimate reconstruction due to high accumulation rates and anoxic conditions at the sediment-water interface.  (Public domain.)

The hydrogen isotopic composition (δ2H) of lipids from leaf waxes and algal lipids can be used as a proxy for hydroclimatic changes in the past. One of the projects that scientists at the USGS St. Petersburg Coastal and Marine Science Center are working on is the reconstruction on precipitation variability in central Florida over the past 2000 years. 

Paleoceanography from Gulf of Mexico Sediments

Scientists at the USGS SPCMC have been focused on generating high-resolution (decadal to multi-decadal) records of sea surface temperature and salinity variability from sediment cores collected throughout the northern and western Gulf of Mexico. The primary proxy used is the magnesium to calcium ratio (Mg/Ca) paried with the oxygen isotopic composition (δ18O) of the planktic foraminifer, Globigerinoides ruber.

Holocene paleoceanographic reconstructions from the Gulf of Mexico:

Pigmy Basin

Fisk Basin

Garrison Basin     

Globigerinoides ruber

SEM image of a Globigerinoides ruber specimen from the Gulf of Mexico. (Credit: Jennifer Flannery. Public domain.)

multicores

Multicores collected from the northern Gulf of Mexico. These cores contain roughly 1 meter of sediment from the seafloor, and represent the past ~2000 years of deposition. USGS scientists slice these cores in 5 mm increments, and analyze the chemical composition of microfossils (e.g., foraminifera) and molecular fossils (e.g., alkenones, GDGTs, leaf waxes, etc.), to generate paleoclimate records over the past 2 millennia. (Public domain.)

Understanding Climate Forcing

The larger goal of this project is to use paleoceanographic reconstructions from the Gulf of Mexico and surrounding region to better understand what mechanisms are forcing climate variability on decadal to centennial time-scales. These reconstructions are also used to assess how phenomena like El Nino Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), Atlantic Meridional Overturning Circulation (AMOC), etc., behave in the pre-anthropogenic era. 

map showing observed sea-surface salinity and rainfall

Long-term correlations between observed sea-surface salinity and rainfall. Correlation map between northern Gulf of Mexico sea-surface salinity (SSS; dashed red box) and global oceanic SSS (ORA-S4 data set; red-blue scale), as well as continental precipitation (GPCC data set; brown-green scale) with locations of proxy records used in the study. Proxy locations are marked with circles (sedimentary records), triangles (speleothems), dashed boxes (tree-ring compilations), stars (circulation proxies), and squares (additional proxies) with color fill indicating sign (fresh – blue; dry/wet–brown/green; purple–weakened poleward transport) during the Little Ice Age (1450–1850 C. E.). Figure from: Thirumalai and others, 2018. (Limited Use by USGS only)