DNA with Noah Gluschankoff
Noah Gluschankoff, an undergraduate student from University of California Santa Barbara is sampling for microbial DNA during this cruise. The extent of microbes from vents is not well described, so he is trying to study how microbial communities mediate the geochemical composition of vent fluids and plumes to better understand global biogeochemical cycles. To do so, his DNA analyses focuses on the 16s ribosomal RNA gene. Every prokaryote has this gene and it is well conserved over evolutionary time scale. Small differences in this gene help distinguish different microbes. For his sampling, Noah Gluschankoff is taking 2 to 4L samples in the water column mostly concentrated bellow, in and above the plume. On-board he pre-filteres the samples on 3µm filters followed by 0,2µm filters. The pre-filteres are used to get out any larger particles that could negatively impact DNA analysis and the 0,2µm is used to collect the microbial DNA. Both filters are frozen and will be analysed at the University of California Santa Barbara. Nitrate isotopes are also collected in parallel on the Titanium rosette and will be analysed in Woods Hole Oceanographic Institute. Nitrate is a nutrient source for many microbes. Using nitrate isotope data with DNA analysis will contribute to the understanding of biologically mediated nitrate utilization and, thus, nitrate fractionation.
During this cruise Noah Gluschankoff hopes to assist in determining:
– The extent of microbes around vents and how their potential metabolic characteristics influence a suite of geochemical fluxes in iron, nitrate, sulfide and other key nutrients
– How nitrate isotopes can be used to describe both biotic and abiotic nitrogen isotope fractionation.
Ba and Cd isotopes with Allison Bryan
Allison Bryan, PhD student at Oxford University, is looking during this cruise at the concentration and isotopic composition of barium and cadmium in the dissolved and particulate fraction in and around vents sites. Hydrothermal activity acts as a source or sink for many elements. For Ba and Cd, a very high concentration is observed in the dissolved and particulate phase close to the hydrothermal fluids. But we still don’t fully understand the global mass balance of these elements in the ocean. The isotopes of Ba and Cd can help us understand the importance of hydrothermal vents to the global mass balance of Ba and Cd. To collect her samples, Allison Bryan used different sampling tools:
– From the stainless-steel rosette, she is in charge of the dissolved rare earth element and dissolved barium sampling (around 500ml needed). From this rosette and away from the plume she also takes large volume of water (around 50L) to do offline filtering for barium particulate (background signal).
– From the SAPS (Stand Alone Pumps) she collects particulate Ba and Cd using filters that pumped 300 to 1000 L of sea water in-situ at set depths in and around the plume.
– From the Titanium Rosette, dissolved cadmium is collected by the trace metal team because cadmium is contamination sensitive.
– From the FISH, she collects water (around 160L) to get after offline filtering the particulate Cd signal in shallow water.
After the sampling, ratio measurement (∂138/134 Ba and ∂114/110 Cd) will be done on mass spectrometers at the Oxford University. Thanks to this upcoming data, Allison Bryan expects to:
– Better understand if the Ba and Cd signal is localised around the plume or if it can be tracked far away and have a global signal.
– To understand the hydrothermal vents impact on the modern oceanic mass balance of Cd and Ba and so get a better idea of the role of hydrothermal activity in the oceanic cycle.
Th and Pa isotopes with Haley Spaid
Haley Spaid, MS student from the University of Southern Mississippi is looking during this cruise at dissolved thorium isotopes (DTh230 and DTh 232) and dissolved protactinium isotope (DPa231) in the vents sites and fracture zones. Th230 is an isotope generated by the decay of uranium. As uranium is conservative in sea water, Th230 is produce uniformly in the water column but we can normally observe an increase of DTh230 with depth. This is due to the fact that Th230 is quickly stuck onto the particles that sink in the water column (scavenging), producing a disequilibrium between the two fractions (dissolved and particulate) of Th230. As a consequence, the sinking particle will deliver Th230 to the bottom, which then comes off of the particle, producing a higher concentration of DTh230 at the bottom. But in hydrothermal vents, particle density is so high that almost all the Th230 available is scavenged to the particle fraction, producing lower concentration of DTh230 in vents. Off the vents we can still see a low DTh230 concentration especially in fracture zones. In some areas, away from vents where there is low DTh but no evidence of particle resuspension to scavenge DTh, the hypothesis of Haley’s group is that it could be due to low Th water flowing from the vents to areas like the fracture zones creating this anomaly. To confirm that theory, samples are collected above, on and off hydrothermal vents and on fracture zones. Haley Spaid also studying another thorium isotope Th232 isotope, a tracer of lithogenic/dust deposition input. This isotope concentration decrease with depth but still can be measured above and in the plume. The dissolved protactinium isotope (DPa231) is also measured and used to complement thorium data. Indeed, Pa acts like Th, sticking to particles and sinking with them but it is more soluble than Th. The Pa/Th ratio can be then used as a proxy for ocean circulation.
During this cruise Haley Spaid expects to see:
– How hydrothermal vents affects Pa and Th scavenging and how it will impact fraction zone Th concentrations and concentrations in sediment cores taken near hydrothermal vents.
– How Th and Pa behave in the ocean.
– How to correct proxies of the modern ocean (overturning, ocean circulation of the past vs now) with a better understanding of this hydrothermal area in sediment records.