Alexandra Haase-Schramm

Sr/Ca ratios and oxygen isotopes from sclerosponges: Temperature history of the Caribbean mixed layer and thermocline during the Little Ice Age

We investigate aragonitic skeletons of the Caribbean sclerosponge Ceratoporella nicholsoni from Jamaica, 20 m below sea level (mbsl), and Pedro Bank, 125 mbsl. We use d18O and Sr/Ca ratios as temperature proxies to reconstruct the Caribbean mixed layer and thermocline temperature history since 1400 A.D. with a decadal time resolution. Our age models are based on U/Th dating and locating of the radiocarbon bomb spike. The modern temperature difference between the two sites is used to tentatively calibrate the C. nicholsoni Sr/Ca thermometer. The resulting calibration points to a temperature sensitivity of Sr/Ca in C. nicholsoni aragonite of about -0.1 mmol/mol/K. Our Sr/Ca records reveal a pronounced warming from the early 19th to the late 20th century, both at 20 and 125 mbsl. Two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the Maunder and Dalton sunspot minima, respectively. Another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum. The temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century. This is interpreted as a long-term deepening and subsequent shoaling of the Caribbean thermocline. The major trends of the Sr/Ca records are reproduced in both specimens but hardly reflected in the d18O records.

Annual to interannual temperature variability in the Caribbean during the Maunder sunspot minimum

We reconstruct Caribbean seawater temperatures from sclerosponge Sr/Ca ratios using a specimen of Ceratoporella nicholsoni that grew at 20 m below sea level in a reef cave at Jamaica. We sample the time interval from 1620 to 1745 A.D. with almost monthly resolution. This interval includes the Maunder sunspot minimum, one of the coldest periods of the Little Ice Age. Reconstructed annual temperature amplitudes are on the order of about 1°C. The mean growth rate calculated from the annual Sr/Ca variations corresponds perfectly with U-Th-based growth rates. We find that the interannual climate variability is determined by El Nino–Southern Oscillation and by a decadal signal, most likely originating from the tropical North Atlantic. On a multidecadal timescale the Maunder Minimum is characterized by a 1°–2°C cooling and reduced amplitudes of the interannual and decadal temperature variations.