Recovering sediment with the Multi-corer. This device allows us to bring up 8 tubes of sediment (mostly mud) from the seabed. Sediments accumulate on the sea floor in each year in layers, so the longer the core we recover, the further back in time we can study. Photo credit to Robyn Von Swank, instagram: @vonswank.
Napoleon once said he made all his generals out of mud. Here on the RV Thomas G. Thompson future chief scientists are elbows deep in mud. Today we head back towards shore with an intense coring plan for San Pedro Basin, the region between Catalina Island and the Long Beach area. Although this large (approximately 15 x 25 miles) depression in the seafloor is not the deepest of the region, it has the potential to contain water with very low oxygen content near the bottom. Due to a shallower sill depth of around 800m, the oxygen minimum zone (OMZ) we have been observing in our CTD casts (seen from 650-800m deep) may be trapped at the bottom. This zone occurs because microorganisms living in the water “breathe” oxygen similar to us, in the process of respiration, depleting the finite supply within the water. Previous research from this area has shown that the bottom waters of this basin do sometimes seasonally exhibit this characteristic.
For the coring group aboard, this would provide the opportunity to examine some interesting features of the seabed. Dr. Josh Williams (Virginia Institute of Marine Science) will use a combination of radioisotopes to date the intact sediments we recover. Some of these radioisotopes are naturally occurring (234Th, 7Be, 210Pb, 226Ra), and decay within the sediments over a range of time scales from a few months to a century. Others were introduced with nuclear bomb testing in the 1950-60’s (137Cs, 239+240Pu). Based on the depth and amount present of these radioisotopes in the cores, we can determine the rate at which sediments are accumulating on the seafloor, and therefore assign an approximate age to each specific depth. With lower oxygen waters, there are fewer organisms that are adapted to live on the seafloor in those conditions, so the amount of sediment mixed (bioturbated) will be less, and the thin layers of sediment accumulating will be better preserved. In San Pedro Basin, the sediment accumulating is sourced from a combination of those produced by phytoplankton in the water column (marine) and river run off (terrigenous). The relative input on any given year of these two sources is determined by a multitude of factors, including the amount of coastal rainfall (increases the sediment delivered by rivers) and upwelling conditions (increases the amount of phytoplankton). Evaluating changes in these inputs over the last century, we may be able to see the impacts of humans (e.g., building dams on rivers) and changing climatic conditions recorded in the sediments.
Preparing to process the sediment cores in the ship’s staging bay. Before we extrude (push out of the tube) the sediment, we evaluate the amount of sediment recovered (i.e., the depth the tube penetrated the seafloor) and the condition it is in to distribute among the coring group of scientists based on our individual scientific goals. Photo credit to Robyn Von Swank, instagram: @vonswank.
Dr. Molly Patterson (UMass Amherst) will examine the remnants of organic material deposited on the surface of the sea floor. This organic material is used by paleo(ancient)ceanographers to reconstruct past changes in sea surface temperature (SST) from sediment archives spanning millions of years. These SST reconstructions combined with modeling experiments is widely used to assess forcings, feedbacks and the surface temperature response of past climates. While several analytical techniques exist and have been successfully applied to various environmental settings each has it’s own set of uncertainties. SST reconstructions derived from the degree of unsaturation of C37 alkenones (UK’37) provide the basis for some of the most reliable estimates of past changes in mean annual SST. Whereas the relatively new proxy based on archaeal tetraether lipids, the TEX86 (tetraether index of tetraethers consisting of 86 carbon atoms) proxy is considered and argued to be less reliable in areas of oceanic upwelling and from sediment records underlying OMZs. Molly will be using the sediments recovered from San Pedro Basin to assess the relationship between these two proxies.
Ph.D. candidate Emily Osborne (South Carolina University) studies the geochemistry of microscopic plankton (planktonic foraminifera) that are deposited and preserved on the seafloor to understand how the chemistry of the ocean has changed since the onset of the Industrial Revolution. As atmospheric CO2 concentrations have increased exponentially, mainly due to fossil fuel consumption, one-third of that total concentration is incorporated in to the surface ocean. The incorporation of CO2 in the ocean triggers a series of reactions that cause the pH of the ocean to decrease; this phenomenon is known as Ocean Acidification.
Dr. Chris Lowery (University of Texas, Austin) is a foraminiferal micropaleontologist whose research focuses on the response of marine organisms to climatic and oceanographic perturbations, particularly changes in dissolved oxygen. As the modern ocean warms, its ability to hold dissolved gases like oxygen will decrease. (Kind of like how a cold beer is nice and fizzy, while a warm beer is more flat.) Therefore, it is vitally important to study changes in dissolved oxygen both in the modern ocean and in past environments where oxygen concentrations were more extreme than can be observed today. Chris’s research on this cruise is focused on the former, which will hopefully improve the study of the latter. To put it another way, we have to study the modern ocean to improve our knowledge of oceans of the past, so that we can better predict the future. Makes sense? No? OK.
Chris’s research on the Thompson is focused on two avenues: 1) Studying the assemblage of benthic foraminifera that are currently living in a variety of oxygen conditions and 2) with collaborators, using those living foraminifera from known oxygen concentrations to test a new geochemical proxy for ancient changes in dissolved oxygen. To do this, Chris takes mud from the upper few centimeters of the seafloor, adds a biological stain that marks living foraminiferal protoplasm, and then sticks his sample in the refrigerator for at least 12 hours to incubate. Then he adds ethanol to kill everything in the mud, puts it back in the fridge, and forgets about it until he can return to his lab in Texas to begin the long task of sieving his samples and picking out the foraminifera that were stained.
Multi-core tubes with a recovery of approximately 6 inches of sediment. Although this does not seem like a lot, with a sediment accumulation rate of around 1/16 -1/8 inch per year, these sediments have likely recorded at least the last 50 years. Once back to land, I will use a combination of different radioisotopes to determine these rates more precisely. Photo credit to Robyn Von Swank, instagram: @vonswank.
Posted by Molly, Emily, Josh, and Chris