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Effect of early marine diagenesis on coral reconstructions of surface‐ocean 13 C/ 12 C and carbonate saturation state
Author(s) -
Müller Anne,
Gagan Michael K.,
Lough Janice M.
Publication year - 2004
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2003gb002112
Subject(s) - aragonite , coral , diagenesis , reef , coral reef , oceanography , carbonate , geology , saturation (graph theory) , seawater , ocean acidification , calcite , paleontology , chemistry , mathematics , organic chemistry , combinatorics
Recent research suggests that future decreases in the carbonate saturation state of surface seawater associated with the projected build‐up of atmospheric CO 2 could cause a global decline in coral reef‐building capacity. Whether significant reductions in coral calcification are underway is a matter of considerable debate. Multicentury records of skeletal calcification extracted from massive corals have the potential to reconstruct the progressive effect of anthropogenic changes in carbonate saturation on coral reefs. However, early marine aragonite cements are commonly precipitated from pore waters in the basal portions of massive coral skeletons and, if undetected, could result in apparent nonlinear reductions in coral calcification toward the present. To address this issue, we present records of coral skeletal density, extension rate, calcification rate, δ 13 C, and δ 18 O for well preserved and diagenetically altered coral cores spanning ∼1830–1994 A.D. at Ningaloo Reef Marine Park, Western Australia. The record for the pristine coral shows no significant decrease in skeletal density or δ 13 C indicative of anthropogenic changes in carbonate saturation state or δ 13 C of surface seawater (oceanic Suess effect). In contrast, progressive addition of early marine inorganic aragonite toward the base of the altered coral produces an apparent ∼25% decrease in skeletal density toward the present, which misleadingly matches the nonlinear twentieth century decrease in coral calcification predicted by recent modeling and experimental studies. In addition, the diagenetic aragonite is enriched in 13 C, relative to coral aragonite, resulting in a nonlinear decrease in δ 13 C toward the present that mimics the decrease in δ 13 C expected from the oceanic Suess effect. Taken together, these diagenetic changes in skeletal density and δ 13 C could be misinterpreted to reflect changes in surface‐ocean carbonate saturation state driven by the twentieth century build‐up of atmospheric CO 2 .