The carbonate compensation depth (CCD) — the ocean depth below which calcium carbonate dissolves faster than it is buried — integrates volcanic outgassing, chemical weathering on land, and the ocean's biological carbonate and organic-carbon pumps. A 53-million-year carbonate accumulation record from the equatorial Pacific (Pälike et al. 2012) shows that, superimposed on a long-term deepening of the CCD from roughly 3.0–3.5 km in the early Cenozoic to the present 4.6 km, the middle–late Eocene (about 46–34 million years ago) was punctuated by seven carbonate accumulation events (CAE-1 to CAE-7): episodes of 0.25–1 million years of deep CCD and high carbonate accumulation interrupted by rapid shoalings of about 0.5–1 km. Earth-system modelling with the GENIE coupled ocean–sediment model identified net weathering solute supply and the labile/refractory partitioning of particulate organic carbon as the only viable primary drivers among the mechanisms tested, but left open whether the events are global and synchronous, whether that attribution survives alternative weathering-feedback parameterizations, and whether deep-ocean ventilation or surface carbonate production changes can be excluded. Later records from the northwestern Pacific, the Atlantic and the Southern Ocean show the fluctuations are widespread, yet the literature disagrees on their synchrony and on their cause. The problem asks for the geographic expression and phase structure of these events across ocean basins and palaeo-depths, and for a quantitative, parameterization-robust identification of the mechanism that produced them.

Background

The carbonate compensation depth (CCD) is the ocean depth at which calcium carbonate (CaCO₃) supply to the seafloor equals its dissolution, so that sediments below it are essentially carbonate-free; the zone of increasing dissolution above it is the lysocline. Because the ocean removes excess alkalinity by shifting the balance between carbonate burial and seafloor dissolution ('carbonate compensation'), the CCD integrates the geological carbon cycle: volcanic and metamorphic CO₂ outgassing, the weathering of silicate and carbonate rocks on land, and the rain of biogenic carbonate and organic matter into the deep sea. On million-year timescales atmospheric CO₂ is thought to be regulated by the silicate-weathering feedback — the tendency of a warmer, wetter climate to accelerate the chemical erosion of silicate rocks, which consumes atmospheric CO₂ — but the strength, nonlinearity, and division of this feedback between silicate and carbonate weathering are poorly constrained for past warm climates.

The equatorial Pacific scientific ocean-drilling transects — the Deep Sea Drilling Project (DSDP), the Ocean Drilling Program (ODP, Leg 199) and the Integrated Ocean Drilling Program (IODP, Expeditions 320/321), together the 'Pacific Equatorial Age Transect' — provide the depth-stratified sediment columns needed to trace the CCD through time. From them, Pälike et al. (2012) assembled a 53-million-year carbonate accumulation record along a backtracked palaeo-depth transect (past water depths at each site reconstructed by correcting for the thermal subsidence of the aging oceanic crust): the equatorial Pacific CCD deepened from about 3.0–3.5 km near 55 million years ago (Ma) to 4.6 km today, broadly tracking Cenozoic cooling, and large fluctuations are superimposed on this trend during the middle and late Eocene — the reconstruction resolves five to seven CCD deepenings and carbonate accumulation events (CAE-1 to CAE-7, roughly 46–34 Ma), each lasting about 0.25–1 million years (Myr), in which intervals of deep CCD and high carbonate accumulation are interrupted by rapid shoalings of roughly 0.5–1 km; the five major fluctuations have amplitudes of 200–900 m.

To interpret these events, the authors ran steady-state ensembles with the GENIE Earth-system model (a gridded model of ocean circulation, biogeochemistry and sediments) under Eocene boundary conditions, with transient checks in the LOSCAR carbon-cycle box model. They found: (i) changes in the net deep-ocean supply of weathering-derived solutes (Ca²⁺ and HCO₃⁻) can shift the CCD by several hundred metres to more than a kilometre; (ii) when the silicate-weathering temperature feedback is enabled, co-varying atmospheric CO₂ and weathering flux can leave the steady-state CCD nearly invariant — a strong buffering result that depends on the assumed feedback parameterization; and (iii) changing the initial fraction of labile (rapidly oxidized) versus refractory particulate organic carbon (POC) delivered to sediments by a factor of two to four shifts the regional CCD by roughly 300–600 m, whereas changes in the surface export rain ratio of CaCO₃ to organic carbon, in shelf-to-basin carbonate fractionation, and in deep-water temperature and seawater Mg/Ca were all too small to explain the events. The paper's closing discussion states what remains unresolved: whether the CAEs are global and synchronous (continuous, well-dated pelagic records at multiple palaeo-depths outside the equatorial Pacific did not exist — the equatorial Atlantic ODP Site 929E showed the presence of the events, but with only shipboard biostratigraphic age control, and other candidate extra-Pacific sections were affected by condensed intervals and hiatuses, so the North Atlantic test sections the authors called for are still missing); how robust the model conclusions are to alternative weathering parameterizations; that the labile-organic-matter mechanism had no direct palaeobiogeochemical test; that ventilation-driven explanations (changes in deep-water formation and circulation) could not be fully excluded; and that transient behaviour on sub-Myr timescales was only partially explored, orbital effects on weathering fluxes being explicitly left untested.

Problem Statement

Taking the equatorial Pacific record of Pälike et al. (2012) as the reference, determine the geographic expression and the causal mechanism of the middle–late Eocene fluctuations of the carbonate compensation depth (the CCD, the ocean depth below which calcium carbonate dissolves faster than it is buried) during the interval of the named carbonate accumulation events (CAE-1 to CAE-7, approximately 46–34 Ma; the reference reconstruction resolves five to seven such CCD deepenings and accumulation events, interrupted by rapid shoalings). Concretely, two linked questions must be answered. (1) Geographic expression and phasing: establish which of these carbonate accumulation events and their bounding dissolution intervals are recorded in pelagic sections beyond the equatorial Pacific — including the Atlantic, at as many palaeo-depth positions as the available records permit — and whether they are synchronous expressions of a single carbon-cycle signal (the hypothesis of Pälike et al. (2012) that the CAEs are global features, tentatively supported by ODP Site 929E in the equatorial Atlantic) or regionally diachronous, with any inferred phase relationships assessed against the age-model and palaeo-depth uncertainties of the records actually used. (2) Mechanism: identify the mechanism, or combination of mechanisms — net deep-ocean supply of weathering-derived solutes, changes in the labile/refractory partitioning of particulate organic carbon delivered to sediments (the split between rapidly oxidized and more resistant organic matter reaching the seafloor), deep-ocean ventilation changes (changes in deep-water formation and circulation), or surface CaCO₃ production — that quantitatively accounts for the reconstructed event amplitudes (200–900 m for the five major fluctuations, with interrupting shoalings of about 0.5–1 km) and durations (roughly 0.25–1 Myr), where the attribution must be shown to be robust to the strength, nonlinearity and silicate/carbonate partitioning assumed for the silicate-weathering feedback. Because the candidate mechanisms imply different spatial patterns — weathering-solute forcing drives broadly synchronous CCD and lysocline changes across basins, whereas a changed labile/refractory partitioning leaves a deeper CCD in the eastern equatorial Pacific and Atlantic with smaller changes in the western Pacific — the attribution may be a single mechanism operating throughout the interval or may differ among events and regions, and whichever form is claimed has to be supported by the geographic pattern of the evidence.

References

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