How do scientists know how the ocean and climate has varied in the past? Historical measurements of the ocean can take us back 70 years or so, but how do we find out how the ocean behaved in the longer-term past? The answer is in the mud at the bottom of the ocean.
Oceanographers are obsessed with measuring all facets of the ocean, from hydrographic properties such as temperature and salinity, to more dynamic processes such as the strength and direction of different ocean currents. Oceanographers even like to measure how much whale poop is in the ocean (believe it or not, whale poop can act as a significant carbon sink). And they’ve become quite adept at it, crafting a comprehensive toolkit of weird and wonderful techniques: enormous research vessels adorned with all manner of bells and whistles traverse the world’s oceans, high-tech moorings span ocean basins, and unmanned underwater gliders dive into the ocean’s depths, reaching otherwise unreachable places. In fact, one major goal of EPOC is to design an observing system to measure the Atlantic Meridional Overturning Circulation (AMOC).
Yet, for all their cutting edge technology, there’s a temporal limitation to oceanographers’ data collection, mainly spanning the last 30-70 years, which means it’s hard to contextualise any of these observations, e.g., are they part of a long-term trend, or do they just reflect natural variability? For example, the AMOC has been measured continuously since April 2004 by a mooring array that spans the Atlantic Ocean at ~26°N, revealing both a long-term decline up until ~2016, followed by a slight increase in AMOC strength. But while these observations yielded new insights into the AMOC on short timescales, the extent to which these changes are part of a long-term AMOC trend is unclear. The record is just too short.
So what can we do to solve this conundrum (apart from sitting around for 100 years)? The answer is hidden at the bottom of the ocean—in mud, or to use the correct paleoceanographic terminology, marine sediments.
Just like oceanographers, paleoceanographers are also interested in studying and measuring all facets of the ocean, except paleoceanographers (paleo, meaning old or ancient, especially with respect to the geological past) study the state of the ocean in the past. To achieve this, we paleoceanographers – unable to directly observe or measure the ocean in the past – rely on proxy evidence preserved in marine sediments to infer its past state. For example, past ocean temperatures are estimated by measuring the ratio of oxygen isotopes in the shells of tiny single-celled organisms called foraminifera, which once inhabited the ocean before sinking and being buried on the ocean floor. Alternatively, the determination of past deep ocean currents’ speeds involves measuring the size of sediment grains in mud, with larger grains indicating faster flow and vice versa – a concept reminiscent of a fast-flowing mountain streams littered with large rocks versus a slow-flowing lowland riverbed covered in fine-grained silts. Then, by deciphering when these proxies were living and/or deposited, often employing techniques such as radiocarbon dating, paleoceanographers can assign an age or date to their temperature or flow speed proxy, allowing them to determine the specific conditions of a particular oceanic region at a precise point in the past.
But while individual measurements are useful, what paleoceanographers are especially interested in is longer records made up of multiple measurements, ideally spanning the observational era and extending further back in time. To achieve this, they rely on marine sediment cores, i.e., long cylinders of mud, retrieved from the bottom of the ocean. Typically, this is done by lowering a plastic tube down to the ocean floor from the side of a ship (not quite as high tech as the oceanographers’ autonomous glider!). Once there, a weight is dropped, or a trigger is fired, causing the tube to penetrate into the underlying marine sediments, before being hauled back up.
The law of superposition states that in undisturbed sequences of sediments deposited in layers (such as those on the ocean floor), the youngest layers are at the top and the oldest layers are at the bottom. Thus, it is possible to develop a long-term record of past ocean temperature or flow speed by measuring temperature and flow speeds proxies at regular intervals throughout a marine sediment core. Then by dating the core, we can also date the proxy records, allowing us to infer past changes in ocean temperatures and flow speed.
While the above makes the paleoceanographic reconstruction process sound relatively straightforward, it’s also crucial to acknowledge the numerous caveats and complicating factors associated with reconstructing past ocean state from sediment cores.
Sediments cores are inherently messy – they are mud after all – and can undergo disturbance or ‘reworking’ due to various factors, ranging from burrowing worms to submarine landslides (the latter of which is evident in numerous cores from around Newfoundland due to the 1929 Grand Banks earthquake). These phenomena lead to younger sediments underlying older ones and, in extreme cases, result in the complete mixing of sedimentary layers. Needless to say, this reworking, complicates our interpretations, but it is possible to account for some disturbances using different techniques such as numerical modelling to unmix the cores or by simply omitting sections where there is evidence of submarine landslides.
Paleoceanographic proxies are also far from perfect. While the ratio of oxygen isotopes in foraminifera is controlled by temperature, a salinity component also affects oxygen isotope ratios, potentially obscuring the temperature signal. Yet, much like dealing with reworked sediments, it is also possible to account for the influence of secondary controls, such as salinity, by reconstructing them with other independent proxies. It’s also important to assess whether our proxy records are representative of the inferred hydrographic variable we are trying to reconstruct – i.e., is our proxy record of temperature actually a record of temperature? Typically this is done by comparing proxy data from the top of a sediment core with modern observations from the same location, and in cores with very high sedimentation rates, we can also compare proxy records with longer-term observational records—this latter approach is considered a gold standard within the field.
At UCL, we have employed these paleoceanographic techniques to reconstruct deep ocean flow speeds over the last 1000 years at multiple sites along the flow path of the Deep Western Boundary Current, with a particular focus on the last ~150 years. And now, we are attempting to make sense of the records, assessing wither they can aid us in contextualising of non-paleo-oceanographic colleagues’ observational data. We’ve also compiled various paleoceanographic proxy records from around the North Atlantic, and moving forward, will be evaluating their utility and robustness through a collaborative proxy and modelling investigation with colleagues at the University of Reading.
Spotlight article by Jack Wharton, UCL.
EPOC is funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.