Expedition News: SO289 South-Pacific GEOTRACES Cruise
Valparaiso, Chile – Noumea, New Caledonia
CREATE Participants: Sarah Moriarty and Chris Galley
The SO289 research cruise aboard R/V Sonne, which involves the trans-Pacific leg of the GEOTRACES program, is a 48-day expedition consisting of 48 planned survey stations taken at approximately 1-3° intervals all the way from the coast of Chile to the South Fiji Basin (Figure 1).
Figure 1. Map of the SO289 cruise track, with station positions marked by white and orange circles. More sampling is performed at super-station than regular stations. Bathymetric data used was taken from GEBCO Compilation Group (2021) GEBCO 2021 Grid.
iMAGE-CREATE had two members (Sarah Moriarty and Chris Galley) participate on the cruise, who made up the ship’s physical oceanography/geology team. Of their many tasks on board they were primarily responsible for overseeing and processing the EM122 bathymetric data that the ship collects. Prior to the cruise there only existed satellite-derived 1 minute (~1.8km) resolution bathymetric data along the sailed stretched of the South-Pacific, but with the collected EM122 ship bathymetry data we could enhance that resolution to 25m (Figure 2).
Figure 2. The multibeam data collected during the SO289 cruise, coloured, plotted against grey-scale GEBCO bathymetry data (GEBCO Compilation Group, 2021). Four example sections of the transect have been zoomed in on: a) the EPR spreading center; b) the Chilean Through subduction zone; c) the hydrothermally active Monowai volcanic cone; and d) the Kermadec Trench subduction zone. The cruise’s stations are marked by white circles, and depths are in meters below sea-level.
With 25m resolution data we can now map many morphological features that the satellite-derived data is simply too coarse to see, but are still important to note. Features such as seafloor volcanos, many of which are smaller than ~1.8km across and can be missed by satellite mapping. Gaining a better understanding on the extent and density of volcanism across the Pacific will allow us to quantify how variable events of major magmatic input are across the past 100 Ma.
In addition to collecting and processing the bathymetry data, Sarah and Chris performed surveys of the water column using Conductivity, Temperature, Depth (CTD) sensors. Two types of CTDs were used on this cruise, the first being a “clean CTD” with a frame constructed from titanium and all other metal components, such as the cable, enclosed in plastic/rubber to reduce trace-metals contaminants introduced into the water column from our equipment (Figure 3). The second was a stainless-steel CTD, used for collecting water samples where trace-metal contamination isn’t a concern. Both CTDs had numerous sensors attached to their base, which are used to measure parameters such as conductivity (used to interpret salinity), oxygen concentration, pressure (used to derive the CTD’s depth), chlorophyl levels, etc.
Figure 3. CTD operations. a) The Ti “clean” CTD being hoisted on deck. The CTD frame support 24 bottles, as well as the CTD sensor at its base. Note the yellow plastic casing to the CTD’s cable, as well as the lack of metal in the winch and pulley system to limit contamination to the trace-metal water samples. b) the control room for the Ti-CTD, where the bottle firing controls and data read-outs are displayed on the left monitor, and the CTD winch controls are on the right. (Photo: Lea Blum [a], Sarah Moriarty [b]).
Additionally, each of the CTDs contained a rosette sampler (i.e., 24 Niskin bottles attached in a ring around their frame that could all be closed separately on command from the control room), which is what allows us to collect water at specific depth. Each bottle can contain slightly over 10 L of fluid.
Bottles were “fired” (i.e., closed) when the CTDs were raised back up through the water column (called the upcast). During the downcast (initial lowering of the CTD), water property data was collected and displayed live for to be interpreted. Before the cast the station chiefs tried to best predict the location of the South Pacific’s water masses at that location, but we didn’t actually know their depths until the downcast data is displayed (Figure 4). The water masses at depth are determined primarily by the salinity and oxygen concentration anomalies in the water column.
While surveying directly over and near the East Pacific Rise (EPR), the turbidity parameter was able to determine the depth the CTD had passed through a hydrothermal plume. The EPR is an active seafloor spreading center that forms new crust upon spreading. The spreading is driven in part by a subseafloor magmatic heat source that, in addition to the volcanogenic influence, also drives seawater convection within the crust. The progressive heating of infiltrated subseafloor seawater causes chemical exchange to occur between the altered seawater, or hydrothermal fluid, and basaltic oceanic crust. Where hydrothermal fluids are discharged at the seafloor, the change in temperature and redox conditions reduces the solubility for certain minerals, causing the precipitation of chimney-like hydrothermal vent structures and mounds. Upon coming into contact with the cooler seawater the hydrothermal fluids can become dark and cloudy as the solutes it picked up from the crust begin precipitating. These dark, underwater clouds can be measured in the water column by their ability to block light created in the CTD’s turbidity sensor. A high turbidity signal is observed when the CTD is within a hydrothermal plume (Figure 5).
Figure 5. A summary figure of our station over a hydrothermal vent field. a) the uncalibrated CTD data, where the oxygen and salinity data can be used to identify water masses and the turbidity data clearly shows the depth and thickness of the hydrothermal plume. b) a map showing the location of the station, along the East Pacific Rise, just above the Juan Fernandez Microplate. c) a vertically exaggerated cross-section of the seafloor across our cruise path.
During the cruise Sarah collected ~ 5-20 mL seawater samples from the stainless-steel CTD, creating a sampling array comprised of water column depth profiles at numerous stations for later S isotope analysis at MUN, Lehigh University, and MIT (Figure 6). The goal for these analyses is to better constrain the average seawater sulfate S isotope composition with respect to d34S and ∆33S, as well as to delineate potential water-mass associated isotopic variances in the water column. These S isotope analyses will provide critical insight into global seawater sulfate cycling on modern timescales, and specifically test the idea that global ocean sulfate is homogenous with respect to S isotope compositions. These analyses may also help to provide important context to more high priority GEOTRACES investigations, as water column S isotope inhomogeneities (or lack thereof) may be related to the ecological biogeochemical environment from which they were collected.
In addition to S-isotope sampling, Chris collected dissolved inorganic carbon (DIC) water samples from the CTD casts. From these samples the total inorganic carbon can be measured from a number of levels through the water column, across the South Pacific Ocean. As the amount of carbon dioxide (CO2) gas dissolved in the samples is a component of the DIC, it was important to collect the samples such that they are free of any air within the bottle. They were then poisoned with mercuric chloride (HgCl2) to ensure no organic processes would produce any more CO2 gas before the samples could be analyzed on land. This data is collected regularly across Earth’s oceans to provided information on the amount of anthropogenic carbon being added to the system.
Figure 6. Water samples being collected from the stainless-steel CTD. a) Sarah and Chris collecting their sampled, with the sulfur isotope samples shown in b) and the DIC samples in c). (Photos: Sarah Moriarty[a], Chris Galley [a, b-c]).