2nd international workshop on clumped isotopes, London, August 10-12 2011
Past Ocean Temperatures and Coupled U/Th and 14C
Measurements from Deep-Sea Corals
Nivedita Thiagarajan1*, Jess Adkins1, John Eiler1
1) Caltech 1200 E California Blvd Pasadena, CA 91125
* Corresponding author: nivedita@caltech.edu
Deep-sea corals are a unique archive in paleoceanography. They have large banded skeletons
that allow for high resolution records and have a high uranium content allowing for accurate calendar
ages independent of radiocarbon age measurements. One problem with using deep-sea corals for long
records is that it is difficult to date a large numbers of corals accurately and precisely. Unlike sediment
cores, fossil fields of corals have no inherent stratigraphy and each individual coral must be separately
dated.
Here we present the calibration of the carbonate clumped isotope thermometer in modern
deep-sea corals. Our calibration is based on 11 specimens of three species of deep-sea corals and one
species of a surface coral spanning a temperature range of 2-25°C. Analytical precision for
measurements made over the course of this study was typically in the range 1-2°C (1σ). We find that
skeletal carbonate from deep-see corals shows the same relationship of Δ47 (the index of 13C-18O
ordering) to temperature as does inorganic calcite. In contrast, the δ18O values of these carbonates
(measured simultaneously with Δ47 for every sample) differ markedly from equilibrium with seawater;
i.e., these samples exhibit pronounced vital effects in their bulk isotopic compositions. We explore
several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence
of a vital effect despite having large conventional isotopic vital effects. Our results also indicate that
clumped isotope measurements in deep sea corals can be used to reconstruct deep ocean
paleotemperatures.
We will also present two temperature profiles (with coupled U/Th and 14C measurements in
the same deep-sea corals) from the North Atlantic (34-40oN, 60-68oW) during the Younger Dryas and
Heinrich 1. This study presents an example in which the degree-level precision of carbonate clumped
isotope temperatures has been achieved through extensive replication and mutual standardization of
samples to a common reference frame (interlaboratory standards and a modern deep-sea coral) over
the course of 18 months. We will also discuss various standardization methods used to achieve the
degree-level precision.
We find that the temperature profile of the ocean during both the YD and H1 coral population is
constant with depth. The average potential temperature of the Younger Dryas profile is 1.6 ± 0.5 oC while
the average potential temperature of the Heinrich 1 profile is 3.1±0.9 oC. If one outlier in the H1 profile is
removed the average temperature becomes 2.3±0.5 oC. We will discuss implications for salinity
gradients in the water column during these time periods as well changes in the circulation of the ocean.
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