Comment on “Anomalous radiocarbon ages for foraminifera shells” by W. Broecker et al.: A correction to the western tropical Pacific MD9821‐81 record

Abstract

[1] The effort to obtain high-resolution paleoceanographic data often leads to continental margin settings where the flux of clastic sediments produces higher sediment accumulation rates. However, on continental margins the possibility for down-slope transport of reworked sediment is a concern. This is particularly true in tectonically active settings such as the western Pacific, which includes the maritime islands of the Philippines and the adjacent archipelago. The continental margins of the western tropical Pacific are strategically located within the Pacific Warm Pool, and detailed paleoceanographic information from this region is a valuable asset in efforts to improve our understanding of what factors contribute to large and sometimes abrupt changes in global climate. The acquisition of data sets such as sea surface temperatures from the tropical Pacific warm pool through the last glacial cycle has indeed added measurably to a growing array of data from lowlatitude locations. Certainly, there is much left to be learned from the high deposition rate records located in continental margin settings. However, over the past 2 years, there have been concerns that one of the high deposition rate sequences in the western tropical Pacific, R/V Marion Dufresne core MD98-2181, is composed of down-slope transport of reworked material or slumping [Broecker et al., 2006]. Because this site has provided one of the highest-resolution data sets of sea surface temperature and salinity information for the Pacific warm water pool through the last glacial cycle [Stott et al., 2002, 2004], it is important to assess the stratigraphic integrity of the sequence and to identify where there may be problems with the record. [2] The MD98-2181 core was collected from a depth of 2114 m on the continental slope south of Mindanao Island. During the last glacial and Holocene sediments accumulated at this site at rates of between 50 and 80 cm/kyr making this an ideal location to collect high-resolution paleoceanographic information from the Pacific Warm Pool [Stott et al., 2004]. We have worked on the MD98-2181 core for the past eight years, conducting stable isotope and Mg/Ca measurements on planktonic and benthic foraminifers in an effort to document the timing and the magnitude of sea surface temperature and salinity changes in the western tropical Pacific Warm Pool through the last glacial cycle [Stott et al., 2002, 2004]. In doing so we have obtained C ages for 42 planktonic foraminifer samples from the Holocene and late glacial portions of the core (Table 1 and Figure 1). The majority of our MD98-2181 samples were taken from u-channel subcores. These subcores were sectioned continuously at 2 cm increments in Stott’s laboratory and the samples were washed in buffered DI water over a 63 mm sieve to remove fine clays. The planktonic Globigeriniodes ruber used for stable isotope and Mg/Ca measurements and also for C determinations was picked from the >150 mm fraction whereas Globigerinoides sacculifer was picked from the >250 mm fraction. In some instances other more robust planktonic species have also been C dated including, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, which were picked from the >250 mm fractions. M. Rincon, who has been Stott’s laboratory manager for the past 15 years, picked each sample used in these studies. The G. ruber dO stratigraphy for MD98-2181 is shown in Figure 2. [3] The late glacial to Recent dO stratigraphy from this core matches other high-resolution sequences from this region with the exception of a 20 cm interval between 941 and 961 cm. The samples from the 941–961 cm interval exhibit dO values that are clearly higher than adjacent intervals immediately above and below (Figure 2). The interval centered at 950 cm would have been deposited during the last deglaciation and the apparent timing of the anomalously high dO values would put it close to the time of the Younger Dryas. For this reason it was not initially clear whether these values represented the presence of reworked glacial age material or if the values were indicative of a brief return to glacial-like conditions during the deglaciation. The intervals directly below 961 cm do not exhibit such high dO values as those at 950 cm and the isotope stratigraphy appears to closely match that of other high-resolution records from the western tropical Pacific region [Stott et al., 2004]. [4] In 2003, W. Broecker approached Stott about a collaboration that involved C dating of planktonic and PALEOCEANOGRAPHY, VOL. 22, PA1211, doi:10.1029/2006PA001379, 2007 Click Here for Full Article