One way to get at the answer is by studying the organisms that are present in ocean sediments. As with most ocean floor sediment, the top few centimeters of the sediment in cores we are collecting (the modern sea floor sediment) is teaming with these organisms, two types of which are particularly helpful in our paleoclimate reconstructions: foraminifera and ostracoda.
Foraminifera, typically called forams, are single-celled amoeboid protists that make intricate calcite skeletons. Forams live their lives either as planktonic (floating in the surface waters) or benthonic (living on the seafloor) organisms.
Ostracoda, commonly called ostracods, are tiny crustaceans. Their bodies are flattened from side to side and protected by a hinged bivalved "shell." In modern oceans they are found from the shoreline to the abyss. They are most often benthonic, living on the sea floor.
The hard parts (the shells) of these organisms are very distinctive and they can be identified with relative ease (and lots of practice!) in both modern and ancient sediments. And, the chemical make-up of the shells tells us about the water chemistry and water temperature at the time the shell was formed.
Cruise co-PI Gary Dwyer will spearhead the study of these organisms in the modern-day environment (in the top few centimeters of the box cores and multicores) in order to “calibrate” our studies of the older (deeper) sediments in our cores. Just like we have to “zero” our bathroom scales before we weigh ourselves (or else risk having an erroneous weigh-in), paleoclimate scientists must make sure that the tools they are using to measure ancient water temperature have been properly “ground truthed” – calibrated.
By studying the forams and ostracods found in the uppermost centimeters of the modern ocean, we can determine how the chemistry of their shells varies with normal changes in water temperature. For example, the temperature of ocean water that is 4000 meters deep is colder than that of the water that is only 35 meters. Not surprisingly, the organism may respond to this difference in water temperature by varying the chemistry off their shells.
Gary will carry out these calibration studies, using both forams and ostracods from core-top samples. He is especially interested in studying the differences in modern shell chemistry along our two deep-to-shallow coring transects – where we have a series of ocean floor sediment samples from as deep as 3500 meters to as shallow as 35 meters.Gary is an expert in paleoenvironmental proxy development – calibration studies that help us determine which organisms, and which chemical characteristic of those organisms, are best used as proxies for past climates. He studies Ba/Ca, Mg/Ca, and δ18O in forams and ostracods – all of which can be used as proxies for past environmental conditions.
The Mg/Ca ratio in planktonic forams can be used as a proxy indicator of paleotemperature. By combining Mg/Ca measurements and δ18O analysis of the same sample, we can also estimate paleosalinity. And, by determining the Ba/Ca ratios in forams we can learn about river runoff (because river water has a higher Ba/Ca ratios than sea water).
One of the main goals of the research that will be done on the sediments collected during this cruise is to determine past changes in ocean temperature. We know that sea surface temperature (SST, as it is called) has varied greatly over geologic time and that changes in SST have a large impact on overall Earth climate.
What we don’t know well is exactly how SST has varied in this part of the Atlantic. Understanding this variability will allow us to evaluate the importance of the past changes in the tropical Atlantic to changes in the climate of the Amazon Basin.
Gary’s calibration studies – and the numeous short cores to get the seafloor surface samples they require – are essential to this understanding. Without them, we would be stepping onto the scales without resetting to zero!
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