Implications of coral reef buildup for the controls on atmospheric CO₂ since the Last Glacial Maximum

Abstract

[1] We examine the effect on atmospheric CO 2 of the occurrence of increased shallow water carbonate deposition and regrowth of the terrestrial biosphere following the last glacial. We find that contrary to recent speculations that changes in terrestrial carbon storage were primarily responsible for the observed $20 ppmv late Holocene CO 2 rise, a more likely explanation is coral reef buildup and other forms of shallow water carbonate deposition during this time. The importance of a responsive terrestrial carbon reservoir may instead be as a negative feedback restricting the rate of CO 2 rise possible in the early stages of the deglacial transition. This separation in time of the primary impacts of regrowth of the terrestrial biosphere and increased shallow water carbonate deposition explains the occurrence of an early Holocene carbonate preservation event observed in deep-sea sediments. We demonstrate that their combined influence is also consistent with available proxy estimates of deep ocean carbonate ion concentration changes over the last 21 kyr. Accounting for the processes that act on the carbonate chemistry of the ocean as a whole then allows us to place strong constraints on the nature of the remaining processes that must be operating at the deglacial transition. By subtracting the net CO 2 effect of coral reef buildup and terrestrial biosphere regrowth from recent high-resolution ice core data, we highlight two periods, from 17.0 to 13.8 kyr and 12.3 to 11.2 kyr BP characterized by sustained rapid rates of CO 2 increase (>12 ppmv kyr À1 ). Because these periods are coincident with Southern Hemisphere ''deglaciation,'' we argue that changes in the biogeochemical properties of the Southern Ocean surface are the most likely cause.

Holocene carbon cycle dynamics

“…We prescribed a constant accumulation of carbonates on the tropical shelves of 15.5 Tmol/a, produced by coral reef growth and general accumulation on the shelves, for example through shelf bioherms. The magnitude of this forcing is in line with the estimate by Milliman [1993] and the model value by Ridgwell et al [2003], but higher by about 5 Tmol/a than the estimate used by Kleypas [1997].…”

Coral reefs and changing seawater carbonate chemistry

“…The "coral reef hypothesis" [Berger, 1982;Opdyke and Walker, 1992] states that episodic flooding of continental shelves during postglacial sea level rise led to dramatic increases in CaCO 3 production, which released significant amounts of CO 2 to the atmosphere (by a process explained in section 2.1). Recent modeling efforts attest to the importance of this mechanism [Ridgwell et al, 2003]. …”

Modelled interglacial carbon cycle dynamics during the Holocene, the Eemian and Marine Isotope Stage (MIS) 11

“…Using the forward modelling approach, Ridgwell et al (2003) used estimates of deep ocean carbonate ion con-centrations to constrain the carbon cycle. They found that the observed trend in atmospheric CO 2 during the last 8000 years can best be explained by the build-up of coral reefs and other forms of shallow-water carbonate deposition.…”