After nearly 40 days at sea, the I09N GO-SHIP cruise (short for “Global Ocean Ship-based Hydrographic Investigations Program”) aboard the R/V Thomas G. Thompson arrived in Phuket, Thailand on April 27th, successfully completing its mission surveying both the Indian Ocean and Bay of Bengal. After departing from Fremantle, Australia on March 21st, the international team of researchers spent weeks collecting data essential for investigating global changes in ocean physics, chemistry, and biology.
U.S. GO-SHIP is part of the international GO-SHIP network, which carries out a systematic and global re-occupation of select hydrographic sections. This cruise focused on the I09N line which crosses the Indian Ocean and was previously sampled in 1995, 2007, and 2016. However, for the first time, the I09N transect extended northward to 18°N into the Bay of Bengal off the coast of Bangladesh, achieving a full coast-to-coast transect across the Indian Ocean when combined with the 2024 I08S section from the southern Indian Ocean to the Antarctic shelf. With these cruises, it is important to maintain the same cruise tracks (i.e. “transects”) and return to predetermined stations with marked coordinates where scientists can repeatedly collect samples in the same area, tracking major changes in ocean physics and chemistry over years and decades. Data gathered on GO-SHIP cruises continue to address critical questions regarding ocean circulation and the ocean’s uptake of heat and carbon dioxide.
As part of this international effort, two scientists at AOML participated in the cruise. Leah Chomiak, a Cooperative Institute for Marine and Atmospheric Studies (CIMAS) Postdoctoral Associate and Oceanographer, served as Co-Chief Scientist while Chuck Featherstone, a NOAA AOML scientist, served as a Dissolved Inorganic Carbon (DIC) analyst.
At the beginning of the cruise, while monitoring the weather, the team noticed the emergence of a tropical depression in the southeastern Indian Ocean. This depression developed into a tropical storm and eventually transformed into category 4 Cyclone Courtney (cyclones are what hurricanes are called in the Indian Ocean). Luckily, the cruise experienced a slight delay before departure which ultimately proved to be beneficial and kept the cruise away from a direct encounter with Courtney. The ship held its location for 3 days to avoid crossing paths with Courtney.
During the mission, the science team successfully completed 98 full-depth CTD casts along the transect, including 9 new stations within Bangladesh waters which had not been previously sampled in the prior I09N occupations. CTD, which stands for “Conductivity, Temperature, and Depth,” refers to a package of electronic instruments that measure these properties from near the surface to just off the seafloor. Often times, CTDs are attached to a much larger metal frame called a rosette, which holds water-sampling bottles, known as Niskin bottles, that are used to collect water at different depths, as well as other sensors that can measure additional physical or chemical properties continuously as the CTD travels from surface to seafloor. A 36-bottle rosette permitted seawater samples from a maximum of 36 depths to be collected at each station. A total of 2,179 Niskin bottles from various depths were collected along the I09N line.
With each station along the transect, seawater samples collected with the niskin bottles were analyzed to measure a plethora of ocean chemistry parameters such as dissolved oxygen, nutrients, and dissolved inorganic carbon (DIC), in addition to others. In a container van outfitted as a full-functioning carbon analysis laboratory on the ship’s aft deck, Chuck Featherstone performed this crucial step running DIC analysis while the ship was underway.
“Dissolved Inorganic carbon (DIC) is a major component of the global carbon cycle, representing a significant portion of carbon in the ocean. Monitoring DIC through time helps scientists understand the carbon sources and sinks, and carbon fluxes in the ocean. It also helps scientists understand the interactions between carbon cycling and other biogeochemical processes in the ocean,” says AOML Oceanographer Chuck Featherstone.
On this cruise, a second CTD cast was performed at select stations to collect seawater samples for biological analysis. These biological parameters analyzed included particulate organic carbon, flow cytometry, persistent organic pollution, and environmental RNA and DNA (i.e. “eDNA”, the slough of genetic material shed by marine organisms from phytoplankton to whales into the environment).
As part of the emerging BIO-GO-SHIP program, these “Bio casts” are relatively shallow casts, reaching down to 1000 meters. By analyzing the various biological parameters collected across the transect, scientists can understand the diversity of planktonic communities at the base of open-ocean ecosystems and species at higher trophic levels that exist in a given area. They also aid scientists in understanding the role of marine organisms in the cycling of key elements – carbon, oxygen, nitrogen – and the impacts that changing environmental conditions have on diversity of marine life found in the open ocean.
During the cruise, the ship passed over the equator and went from the Indian Ocean to the Bay of Bengal. As they approached the Bay of Bengal, the team was able to see the water getting fresher and the oxygen drastically decreasing below the surface—real-time oceanography.
“Crossing the equator was more than switching hemispheres – from a science perspective, it came with stark measurable changes in ocean salinity with the furthering influence of fresh-Indonesian Throughflow waters, diminished dissolved oxygen content as we crossed into one of the largest oxygen minimum zones in the world [Bay of Bengal], and exhilarating subsurface currents characteristic of equatorial dynamics. From a handful of polywog’s perspectives, it concluded with a refreshing bucket shower of seawater on the deck — much appreciated by everyone, given the hot weather,” said Leah Chomiak, Co-Chief scientist and CIMAS Oceanographer.
Beyond performing CTD casts and collecting seawater samples, the team also deployed 12 Core Argo and 7 Biogeochemical (BGC) Argo floats in support of the Argo Program.
All Argo floats measure temperature and salinity to depths of more than a mile below the ocean’s surface, but BGC Argo floats also measure properties like chlorophyll, nitrate, and oxygen to holistically assess the cycling of key elements in the open ocean that fuel entire ecosystems. The ship also deployed 6 Electromagnetic Autonomous Profiling Explorer (EM APEX-SQUID) floats designed to measure internal waves, and 20 surface drifters from the NOAA Global Drifter Program to assess and monitor surface ocean currents.
The Global Drifter Program and the Argo Program both have global arrays around the world to provide real-time data for weather prediction and ocean state estimation. Therefore, having GO-SHIP cruises like this one deploy floats and drifters in the Indian Ocean, an undersampled region, is mutually beneficial for tracking major changes in ocean biogeochemistry, advancing forecasts of major storms, and assessing the status of open-ocean ecosystems – in some of the most remote areas of the world.
These GO-SHIP cruises crossing entire oceans have been performed by scientists across different countries since the 1980s, monitoring major environmental changes across decades and providing crucial insight to ocean conditions driving weather patterns. As the team onboard the I09N cruise return back to their home institutions and analyze the vast data collected over five weeks, others are preparing for future GO-SHIP voyages returning to transects in the South Atlantic later this year, continuing the global mission of monitoring some of the Earth’s most remote places.