How well would modern‐day oceanic property distributions be known with paleoceanographic‐like observations?

Abstract

Compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins, mid-ocean ridges, and bathymetric highs, where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells. Here we design an idealized experiment to test our predictive ability to reconstruct modern-day seawater properties by considering paleoceanographic-like data. We attempt to reconstruct the known, modern-day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics. When a modern-like suite of observations (Θ, practical salinity, seawater 18 O, 13 C DIC , PO 4 , NO 3 , and O 2 ) is used from the sparse paleolocations, the state estimate is consistent with the withheld data at all depths below 1500 m, suggesting that the observational sparsity can be overcome. Physical features, such as the interbasin gradients in deep 13 C DIC and the vertical structure of Atlantic 13 C DIC , are accurately reconstructed. The state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales (at least 10,000 km horizontally and 1500 m vertically) and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used. When the sparse observations are more realistically restricted to the paleoceanographic proxy observations of 13 C, 18 O, and Cd/Ca, however, the large-scale property distributions are no longer recovered coherently. At least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large-scale property distributions of the Last Glacial Maximum.

Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

“…Relevant details of the water mass decomposition technique are emphasized next, but the reader is referred to previous publications for more details (Gebbie, ; Gebbie et al, , ). The output of the inversion includes more than the water mass geometry, as this knowledge is also used to reconstruct 3‐D global distributions of multiple properties simultaneously.…”

Global deepwater circulation between 2.4 and 1.7 Ma and its connection to the onset of Northern Hemisphere Glaciation

“…The limitation of these approaches is due to the dependence of a nonconservative trace alone. Since the deepwater masses have at least two end‐members, NADW and AABW, event under stringent constraint of mass conservation a conservative tracer must be used alongside with δ 13 C in order to separate the mixing component from the remineralization effect, or multiple interrelated nonconservative tracers are required [ Gebbie and Huybers , ; Gebbie et al ., ; Huybers et al ., ; LeGrand and Wunsch , ]. Benthic δ 18 O would be the obvious choice of a conservative tracer, given both temperature and seawater δ 18 O are conservative properties.…”

Why estimates of deglacial ice loss should be biased low

“…To translate to Absolute Salinity, the modern-day lookup table provided by the Gibbs seawater toolbox is not applicable to the glacial ocean, as the carbon cycle was likely in a different state. Instead we use an empirical relationship between phosphate and the nonconservative part of salinity (see Appendix A1, Gebbie et al, 2016) that captures about half of the variance in the carbon cycle effect.…”