As a trained “geologist” I am sometimes exasperated when people summarize my work by saying, “oh, so you study rocks.” Most of my research is focused on unconsolidated (loose) marine sediments and my interests lie in the stories they preserve about past oceanographic conditions and climate.
Many of you will have heard about this year’s extraordinary marine heatwaves (Fig 1) that have plagued the North Atlantic and Arctic, now almost annual occurrences superimposed on a background of long term warming that is the undeniable result of anthropogenic greenhouse gas emissions. In addition to the impacts on marine life, warm sea surface temperatures contribute to ice melt and sea level rise, and increase the likelihood of dangerous storms.
Fig 1. Daily global sea surface temperature (°C) averaged over the 60°S–60°N domain plotted as a time series for each year from 1 January 1979 to 31 July 2023. The years 2023 and 2016 are shown with thick lines shaded in bright red and dark red, respectively. Other years are shown with thin lines and shaded according to the decade, from blue (1970s) to brick red (2020s). Data: ERA5. Credit: C3S/ECMWF.
One of our cruise objectives is to monitor how the Arctic is changing, including by documenting physical and chemical changes to the water column. A key instrument for this work is CTD which stands for Conductivity (salinity), Temperature, and Depth (creative name, we know). A standard CTD measures these properties continuously as it is lowered and raised through the water column and our instrument also measures fluorescence (a proxy for photosynthetic activity), and oxygen (a proxy for biological activity and water mass changes) (Fig 2). These properties and their variations with depth and from site to site give us invaluable baseline information about the Arctic and Nordic Seas. In turn, these observations can help us understand how oceanographic circulation patterns are changing and what the implications might be for global deep-water circulation.
Fig 2. A sample CTD profile from a recent station. Depth is along the y axis with the deepest part of the ocean at the bottom of the screen. The color of each line corresponds to the axis, for example temperature is in red. This CTD profile is quite a bit noisier than usual (see the bumps) because of very rough sea conditions (3-4m seas).
In addition to the water column profiles we obtain from the CTD, we also use the bottles on the instrument to take samples of the water column from discrete depths. I work with the winch operator to deploy the CTD, help Chief Scientist Dr. Morley choose the sample depths, and fire (close) the bottles at the right depths. When the CTD is resting safely on deck, a flurry of activity commences (video) to subsample the big bottles into smaller vials that are measured for isotopes (δ18O and δ13C), alkalinity and dissolved inorganic carbon (DIC) (to constrain the carbonate system), trace elements, and nutrients.
Most of our climate reconstruction work is reliant upon proxies. You might be familiar with this word from voting by ‘proxy’ which means to send someone in your stead to represent your preference. I like to use an analogy to illustrate how this works in the climate context: If you look outside and see people carrying umbrellas, what do you know intuitively? It’s raining! So we might think that umbrellas are a proxy for rain. To make sense of the geochemical information preserved in geologic archives, climate scientists must work hard to make sure they understand the systems and what they are telling us. If you see a pile of umbrellas in a landfill (the sedimentary record) what might you infer? Your first thought might be that it had rained earlier that week. But do you throw away your umbrella the minute it stops raining? Or only after a storm, when the umbrella has been damaged by wind and is beyond repair? Geochemists must diligently assess whether a proxy is a pure recorder of one environmental variable (rain) or whether it is subject to the influence of a secondary variable (strong wind).
The snapshots of ocean properties we obtain from the CTD bottles are critical for our geochemical work on microfossils and sediments. By measuring the environmental variables in the water column and measuring the signals recorded in modern foraminifera and sediments (like the multi cores we are collecting) we can improve our understanding of how proxies work and what paleoclimate information they record.
For example, Dr. Morely and her group are interested in trace element incorporation into planktonic (surface-dwelling) foraminifera. They are working diligently to quantify how trace elements like magnesium (Mg) are incorporated into calcite as a function of temperature, carbonate chemistry, and other variables. A second step is to understand how this primary geochemical signal may be altered by sedimentary processes as the foraminifera become part of the geologic record.
This is the difficult, but critical work of geochemists. Only with a clear understanding of past variability can we accurately predict and prepare for future changes in our climate system.
Video. CE23011 Scientists sampling the CTD.