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FieldworkCover Story

Calibrating Proxies for the Study of Holocene Climate Change in the Northern Gulf of Mexico


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Don Hickey and Eric Tappa deploy a sediment trap
Above: Don Hickey (USGS, left) and Eric Tappa (University of South Carolina) deploy a McLane Mark 78 sediment trap from the R/V Pelican in the Gulf of Mexico. Photograph by Wendy Kelly (USGS). [larger version]

To better anticipate future changes in the Earth's climate, it is necessary to differentiate between natural and anthropogenic influences on the climate system. Paleoclimate reconstructions provide information regarding the timing and magnitude of natural climate variability. Researchers use proxy data from such natural sources as tree rings, corals, and marine sediment to extend the climate record beyond that of the approximately100-yr-long instrumental record. The accuracy of reconstructions of past environmental conditions, including the temperature and productivity of the ocean, depends largely on the reliability of these proxy records. To make optimal use of the information derived from the sedimentologic record, sedimentary properties need to be calibrated to current oceanographic and biologic conditions by measuring modern mass fluxes (rates at which particles sink through the water column) of carbonate, biogenic opal, organic carbon, and terrigenous material and by analyzing microfossil assemblages and shell chemistry.

Researchers from the U.S. Geological Survey (USGS) office in St. Petersburg, Florida, and the University of South Carolina recently began a study to calibrate commonly used proxy records—such as oxygen-isotope and Mg/Ca ratios in planktonic foraminifera—from Gulf of Mexico sediment. They deployed a sediment-trap mooring in the northern Gulf of Mexico to measure the flux and chemistry of planktonic foraminiferal shells and organic material collected in the trap. Data will be compared with concurrent hydrographic and climatic observations. The results will provide better calibration of standard climate proxies, leading to improved interpretation and correlation between marine and continental paleoclimate records.

deployment of SeaBird Carousel Water sampler recovery of box cores
Above left: Deployment of SeaBird Carousel Water sampler equipped with twelve 5-liter Niskin bottles and a conductivity-temperature-depth (CTD) sensor. Photograph by Wendy Kelly (USGS). [larger version]

Above right: Recovery of box cores. Photograph by Wendy Kelly (USGS). [larger version]

In January 2008, Kathy Tedesco, Don Hickey, and Wendy Kelly (USGS) participated with Eric Tappa (University of South Carolina) in a cruise aboard the research vessel (R/V) Pelican, operated by the Louisiana Universities Marine Consortium (LUMCON). They deployed a mooring equipped with a McLane Mark 78 sediment trap in the northern Gulf of Mexico near the Pigmy Basin in approximately 1,300 m of water. The trap is positioned at a depth of approximately 800 m on the mooring to guarantee collection of deeper-dwelling species of planktonic foraminifera (for example, Globorotalia spp.). Samples will be analyzed for carbonate, biogenic silica, and organic carbon contents; foraminiferal assemblages; and stable-isotope and Mg/Ca ratios.

Seawater samples were collected with a SeaBird Carousel Water sampler equipped with twelve 5-liter Niskin bottles for chemical analyses, such as the oxygen-isotopic composition of seawater, plus a conductivity-temperature-depth (CTD) sensor and a dissolved-oxygen sensor to provide seasonal profiles of temperature, salinity, and dissolved oxygen. Two box cores were taken at the mooring site to help develop a climate record for the region during the Holocene.

The Gulf of Mexico is a semi-enclosed basin surrounded by the gulf coast of the United States, Mexico, and Cuba. The climatology of this region is driven by seasonal migration of the Intertropical Convergence Zone (ITCZ), an area of low pressure that forms where the northeast trade winds meet the southeast trade winds near the Earth's equator. Surface-ocean circulation is dominated by the Loop Current, which enters the gulf between Cuba and the Yucatán Peninsula, then loops east and south before exiting through the Straits of Florida. The position of the Loop Current is driven by seasonal fluctuations in the ITCZ and the prevailing wind direction, such that during Northern Hemisphere winter the current is farther south and during Northern Hemisphere summer the current penetrates deep into the gulf. The sensitivity of the region to the seasonal migration of the ITCZ makes it an ideal location for studying Holocene climate variability.

Gulf of Mexico, showing location of sediment-trap mooring, generalized paths of prevailing moisture-bearing winds, and position of the Intertropical Convergence Zone
Above: Gulf of Mexico, showing location of sediment-trap mooring (yellow triangle), generalized paths of prevailing moisture-bearing winds (bold arrows), and position of the Intertropical Convergence Zone (ITCZ; dashed lines) for Northern Hemisphere winter (blue) and summer (black). [larger version]

Previous studies in the Caribbean Sea and Gulf of Mexico show evidence of linkages between the position of the ITCZ and changes in planktonic foraminiferal flux, shell chemistry, and sediment geochemistry during the Holocene. The main objective of this project, to calibrate sediment proxy records, will lead to a better understanding of past changes in the position of the Intertropical Convergence Zone and its relation to global climate fluctuations during the Holocene.

Future cruises to the mooring site are currently scheduled aboard the R/V Pelican for April, August, and October 2008 and January 2009, at which times the trap will be recovered, samples retrieved, and the trap redeployed. A USGS Mendenhall Postdoctoral Research Fellowship awarded to Kathy Tedesco supports this research.

To learn more about paleoclimatology, visit the National Oceanic and Atmospheric Administration's Paleoclimatology Web site. To learn more about proxies, read Wefer, G., Berger, W.H., Bijma, J., and Fischer, G., 1999, Clues to ocean history; a brief overview of proxies, in Fischer, G., and Wefer, G., eds., Use of proxies in paleoceanography; examples from the South Atlantic: Springer-Verlag, Berlin, p. 1-68 (can be previewed at URL http://books.google.com/books?id=e8lIokyIG7gC&printsec=frontcover#PPA1,M1). For information about additional USGS climate-change studies, visit "Climate Change Science."

Related Web Sites
Climate Change Science
USGS (U.S. Geological Survey)
NCDC Paleoclimatology Branch
National Oceanic and Atmospheric Administration
USGS CMG Field Activity 08CEV01 Metadata
USGS (U.S. Geological Survey)

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Calibrating Proxies for Holocene Climate Study

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