SODIOM - Surface Ocean Density Influence On Mineralisers

Lead Research Organisation: UNIVERSITY OF OXFORD
Department Name: Earth Sciences

Abstract

While the processes of current and future ocean warming are well-established, the impact of ocean dilution (i.e. the combined effect of persistent temperature rise and freshening due to ice melts) on marine life and ecosystems are hardly considered. Planktonic calcifiers play essential roles in different marine ecosystems including being basal contributors to the marine food web and the marine carbonate pump. Besides the first order control on a large number of their physiological processes including calcification, temperature also affects their swimming behaviour. The thermal expansion of the ocean reduces its density, while seawater expands further by freshening. Under warmer climates the ocean becomes even more diluted by the glacial meltwater input due to shrinking continental ice sheets and this added dilution aggravates its acidification. The combined effects of lower ocean density and alkalinity may particularly influence non-motile calcifying plankton that solely depends on the physical properties of the water column for positioning, by affecting their flotation needs. The influence of future dilution on calcifying organisms may have a major impact on the modern carbon cycle and hence, rather than warming or acidification alone, it is oceanic dilution that might actually be a bigger challenge to the ecosystem and the society.

Although the effects of ocean chemistry on plankton are being extensively studied there is a lack in the literature about the effects of physical oceanic properties such as buoyancy or pressure, which very likely affect plankton physiology and morphology. Different calcifying species have different optimum living depth habitats, to which they adapt according to the oceanic inhomogeneity. These planktonic organisms are able to biosynthesize out of equilibrium with their ambient environment by maintaining chemical gradients however, as non-motile they must always retain equilibrium with the seawater to remain afloat. To inhabit certain depths, non-motile planktonic organisms should regulate their (cell) density to match that of the surrounding liquid in which they are immersed. Should this not be the case then the organisms must relocate until they reach a particular density horizon to equilibrate. It can thus be argued that plankton physiology is more sensitive to the physical rather than the chemical characteristics of seawater. To this extent the proposed research will help to elucidate the physical processes that control pelagic calcification, by investigating the control on planktonic foraminifera (PF) and coccolith shell mass changes of key environmental parameters such as ambient seawater density.
 
Description The mass of well-preserved calcite in planktonic foraminifera shells provides an indication of the calcification potential of the surface ocean. We have reported the shell weight of 8 different abundant planktonic foraminifera species from a set of core-top sediments along the Mid-Atlantic Ridge. The analyses showed that near the equator, foraminifera shells of equivalent size weigh on average 1/3 less than those from the middle latitudes. Deep dwelling species possess heavier tests than their surface-dwelling counterparts, suggesting that the weight of the foraminifera shells changes as a function of the depth habitat. Ambient seawater carbonate chemistry of declining carbonate ion concentration with depth cannot account for this interspecies difference. The results suggest a depth regulating function for plankton calcification, which is not dictated by water column acidity.
Exploitation Route Helpful for understanding density controls on marine biogenic calcite production.
Sectors Other

 
Title Tomographic data of Trilobatus trilobus shells from central Atlantic core-top sediment samples 
Description Understanding the controls behind the calcification and distribution of planktonic foraminifera in the modern ocean is important when using these organisms for paleoceanographic reconstructions. This study combines shell geochemistry, light microscopy, and X-ray micro-computed tomography to dissect various parameters of Trilobatus trilobus shells from surface sediments and explore the factors influencing their biometry. The goal is to understand which aspects of the marine environment are critical for the calcification and vertical distribution of this species. T. trilobus is found to produce larger, thinner and overall lighter shells in the equatorial regions compared to the subtropical gyre regions where the shells were up to 4% smaller, more than 60% thicker and approximately 45% heavier. The skeletal mass percentage together with other calcification metrics (shell weight, thickness) are found to depend primarily on ambient seawater salinity rather than carbonate chemistry. In line with their degree of calcification, based on geochemically reconstructed apparent calcification depths, this group of organisms is found shallower into the water column at the equator and the subtropical gyres, while its habitat deepens in between these regions at the extra-equatorial sites. Furthermore, it is demonstrated that T. trilobus, in the (central) Atlantic, occupies a density layer slightly below the salinity maximum isopycnal at various depths, presumably by adjusting its shell properties. 
Type Of Material Database/Collection of data 
Year Produced 2024 
Provided To Others? Yes  
Impact These are rare freely available X-ray tomographs of foraminifera fossil shells 
URL https://datadryad.org/stash/dataset/doi:10.5061/dryad.6t1g1jx6q
 
Description Press release 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This was a press release from the University of Oxford to EurekAlert.
Year(s) Of Engagement Activity 2024
URL https://www.eurekalert.org/news-releases/1066862