Early Marine Diagenesis In Corals and Geochemical Consequences For Sea Surface Temperature Reconstructions
Abstract
Sea surface temperatures (SSTs) are an important quantity for understanding past cli- mate dynamics, and estimates of SSTs are an essential boundary condition used in general circulation models of past and future climate. Large negative SST anomalies of 4 to 6.5rC have been reconstructed for the last deglaciation and the last glacial max- imum (LGM) using two supposedly independent paleothermometers based on d18O and Sr/Ca measurements in scleractinian corals. The tropical sea surface tempera- tures recorded from fossil coral for the LGM, however, are much lower than those recorded from other marine proxies. These proxies, which include foraminifera spe- ciation, foraminiferal oxygen isotopes and alkenone results, suggest a cooling of no more than 3rC. At present it is not clear if this difference reflects regional differences in the extent of cooling, or if one of the proxies is misleading. Another surprising finding is the large warming and/or freshening trends for the ocean surface over the last 200 years indicated by many recent coral d18O records. These long-term trends generally exceed those of the 20th century instrumental records and suggest that tracers in corals may overestimate cooling of the ocean in the past. Here we show that early marine di- agenesis, i.e. secondary precipitation of marine inorganic aragonite, can cause errors in past climate reconstructions from coral. We compare coral skeletal d18O and Sr/Ca data for two long coral cores spanning 1839-1994 AD at Ningaloo Reef, Western Aus- tralia, one of which includes significant secondary precipitation of marine inorganic aragonite. Long-term trends in reconstructed sea surface temperatures (SSTs) for the well preserved coral correlate strongly with instrumental SST records spanning the 20th century. In contrast, the d18O and Sr/Ca for the diagenetically altered coral give identical cool SST anomalies of 4-5rC, as a consequence of the addition of secondary aragonite enriched in 18O and Sr. Our results indicate that cross-checking of paleo- climate reconstructions with two supposedly independent paleothermometers may not be valid, and that coral records showing cooler SSTs in the past need to be interpreted with caution. Furthermore, modern coral records with long-term trends in d18O in- dicating recent warming and freshening of the ocean can be potentially explained by early marine diagenesis.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.3777M