Geochemistry of slow-growing corals : reconstructing sea surface temperature, salinity and the North Atlantic Oscillation
Geochemistry of slow-growing corals : reconstructing sea surface temperature, salinity and the North Atlantic Oscillation
Date
2007-06
Authors
Goodkin, Nathalie F.
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Location
32°N,64°W
Bermuda
Bermuda
DOI
10.1575/1912/1814
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Keywords
Climatic changes
Corals
Corals
Abstract
A 225-year old coral from the south shore of Bermuda (64°W, 32°N) provides a record of
decadal-to-centennial scale climate variability. The coral was collected live, and sub-annual
density bands seen in x-radiographs delineate cold and warm seasons allowing for precise dating.
Coral skeletons incorporate strontium (Sr) and calcium (Ca) in relative proportions inversely to
the sea surface temperature (SST) in which the skeleton is secreted. δ18O of the coral skeleton
changes based on both temperature and the δ18O of sea water (δOw), and δOw is proportional to
sea surface salinity (SSS).
Understanding long-term climate variability requires the reconstruction of key climate
parameters, such as sea surface temperature (SST) and salinity, in records extending beyond the
relatively short instrumental period. The high accretion rates, longevity, and skeletal growth
bands found in coral skeletons make them an ideal resource for well-dated, seasonal climate
reconstructions. Growing between 2 and 6 mm/year and reaching more than 1m in length, slow-growing
corals provide multi-century records from one colony. Additionally, unlike the fast
growing (10-20 mm/year) species Porites, slow-growing species are generally found in both
tropical and sub-tropical locations greatly expanding the geographical location of these records.
A high resolution record (HRR, ~11 samples per year) was drilled for the entire length of
the coral record (218 years). Samples were split and Sr/Ca, δ18O, and δ13C were measured for
each sample. Sr/Ca was used to reconstruct winter time and mean-annual SST. Oxygen isotopic
measurements were used to determine directional salinity changes, in conjunction with Sr/Ca
based SST reconstructions. Winter-time and mean annual SSTs show SSTs ~1.5 °C colder
during the end of the Little Ice Age (LIA) relative to today. Simultaneously, SSS is fresher
during that time.
Sr/Ca based climate reconstructions from coral skeletons have been met with some
skepticism because some reconstructions show temperature changes back in time that are 2-4
times greater than the reconstructions of other marine proxies. In this study, we show that when
using bulk-sampled, slow-growing corals, two steps are critical to producing accurate
reconstructions: 1) incorporating growth rate into multi-variant regressions with SST and Sr/Ca
and 2) using multiple colonies that grew at the same time with varying average growth rates and
Sr/Ca. Application of these novel methods over the period of the instrumental record from
Hydrostation S (monthly since 1954, 32º10'N, 64º30'W) reduces the root mean square of the
residuals between the reconstructed SST and the instrumental SST by as much as 1.52°C to
0.46°C for three coral colonies.
Winter-time SSTs at Bermuda are correlated to phases of the North Atlantic Oscillation
(NAO), a meridional oscillation in atmospheric mass. Much uncertainty remains about the
relationship between the NAO and the ocean, and one critical outstanding question is whether
anthropogenic changes are perturbing the system. Using winter Sr/Ca as a proxy for temperature,
we show strong coherence to the NAO at multi-decadal and inter-annual frequencies. These coral
records show significant changes in variance in the NAO during the late 20th century compared to
the cooler LIA, but limited changes in the mean phase (positive or negative) of the NAO,
implying that climate change may be pushing the NAO to extremes but not to a new mean
position.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2007
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Citation
Goodkin, N. F. (2007). Geochemistry of slow-growing corals : reconstructing sea surface temperature, salinity and the North Atlantic Oscillation [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/1814