Indexes for carbonate dissolution were examined in lower Paleogene sediment (65–33.7 Ma) recovered from two boreholes (1209A and 1211A) on Shatsky Rise. The two best measures for carbonate dissolution are probably FRAG and BENTH, which indicate the relative abundance of foraminifer test fragmentation and benthic foraminifers, respectively. These proxies display good covariance across the sample set and similar downhole patterns in Holes 1209A and 1211A.
For Holes 1209A and 1211A, all indexes suggest two significant early Paleogene dissolution episodes. The first of these occurred in the early late Paleocene at ~59–58 Ma (lower part of planktonic foraminifer Zone P4) and coincides with a recently identified biotic event. We suspect that the biotic event and accompanying seafloor carbonate dissolution relate to a change in deep ocean circulation. The second dissolution episode begins in the middle Eocene at ~45 Ma (Zone P11) but reaches a maxima during the late Eocene at ~37–33.7 Ma (Zones P15–P16). This is probably the most prominent interval of carbonate dissolution recorded in Paleogene sediments from Shatsky Rise and may represent a time of shallow lysoclines and CCDs throughout the oceans. Other intervals of enhanced carbonate dissolution occurred in the late Paleocene at ~56 Ma (upper Zone P4) and in the late early Eocene at ~51 Ma (Zone P7). The IETM interval was not really investigated in this study, but the lysocline may have first shoaled then deepened.