New method to reconstruct winter ocean temperatures of the past

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This article is part of the Spring 2019 issue of the Earth Science Matters Newsletter.

As average global temperatures change, the effects differ among locations and from season to season. While some locations see increased winter and summer temperature extremes, others may experience very little change in temperature, but large shifts in precipitation. To accurately project future changes in seasonality, we need to improve our understanding of how seasonality changed during past climate shifts in Earth’s history. Scientists reconstruct seasonal and annual patterns of climate using paleoclimate proxies. In ocean sediments, these proxies include the remnants of microscopic organisms such as foraminifera, coccolithophorids and diatoms that lived in the surface ocean and are preserved in ocean sediments when they die and sink to the seafloor. Modern observations of these organisms in the ocean show that some species have well-defined differences in seasonal preference. Researchers from the USGS have developed a method to reconstruct winter temperatures in the ocean surface using a species of foraminifera that grows almost exclusively during the winter. This advance allows paleoceanographers to reconstruct records of wintertime surface temperature extending back tens of thousands of years in the geologic record.

USGS scientists conduct research on planktic foraminifera, single-celled organisms living in the open ocean that build shells out of calcium carbonate. In addition to being a critical sink for atmospheric carbon, planktic foraminifera provide a means for paleoclimate researchers to document past changes in ocean temperature and salinity. The USGS deployed a sediment trap in the Gulf of Mexico to better define the depth and seasonal habitat of these geologically important organisms, as well as the factors that influence their chemical composition. The sediment trap, deployed since 2008, collects sinking particles in sample cups every 1–2 weeks. Those samples are recovered from the deep ocean every nine months and brought back to the lab to investigate the flux of different species of foraminifera (as well as other proxy recorders), and to analyze the chemical composition of their calcium carbonate shells.

diagram of sediment trap

A. Depth profile at the sediment trap site. The non-encrusted G. truncatulinoides calcify in the surface mixed layer, the crust is added below the thermocline, and all the dead specimens fall into the sediment trap. Also plotted in red is an average temperature profile. Note: Mg/Ca increases with temperature, shown in red dots on the foraminifera.

B. Scanning electron microscope (SEM) images of non-encrusted G. truncatulinoides (top two panels) and the encrusted form (bottom two panels).

C. Schematic of the entire sediment trap mooring.

(Credit: Caitlin Reynolds, USGS. Public domain.)

One species of foraminifera, Globorotalia truncatulinoides, was found to have a very specific preference for winter conditions, with 92% of specimens living in January–March. Globorotalia truncatulinoides has traditionally been classified by paleoceanographers as a deep-dwelling species, preventing its use as a proxy for temperatures in the surface ocean. Using isotopic (d 18O and d 13C) and trace metal (Mg/Ca) analyses, USGS scientists demonstrated that one form (the non-encrusted form) of this species lives within the surface mixed-layer of the ocean. The trace-metal analyses were conducted using a technique called laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS). This approach allowed scientists to explicitly link the growth of primary calcium carbonate of the non-encrusted form of G. truncatulinoides to the surface ocean. Using this same technique, they were able to show that another form of this species (encrusted form) adds most of its calcium carbonate shell in deep water, below the thermocline, down to depths greater than 500 meters.

Ultimately, this study shows that when researchers avoid using specimens that have formed a crust, the Mg/Ca of the other (non-encrusted) species can be used to reconstruct winter sea surface temperature throughout the tropical and subtropical oceans, allowing scientists to reconstruct changes in seasonal climatic extremes in the geologic record.

This paper “Environmental controls on the geochemistry of Globorotalia truncatulinoides in the Gulf of Mexico: Implications for paleoceanographic reconstructions” was published in Marine Micropaleontology, and is available here: https://doi.org/10.1016/j.marmicro.2018.05.006

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