Abstract:
The generation of sea surface paleotemperature estimates using planktic foraminiferal census data from ODP Site 1125 and DSDP Site 594 has enabled a reconstruction of the paleoceanographic history of surface water circulation east of southern New Zealand in the late Pleistocene. Planktic foraminiferal census counts were compiled for 78 samples from ODP Site 1125, located on the northern slopes of the Chatham Rise. Site 1125 was chosen primarily as a counterpart for Site 594, located south of the Chatham Rise, to investigate paleoceanographic changes across the Suptropical Front (STF) that is currently locked to the crest of the rise. An age model for the 9.9 m core section was constructed by correlating the reflectance curve of the studied interval with the tuned reflectance curve at nearby Site 1123. The 78 samples that were analysed in the composite section ensured a resolution between samples of 4000 to 10000 years. Sea surface temperature (SST) estimates were generated for each sample depth at Site 1125 using the Modern Analogue Technique and the combination of these results with SST data from Weaver et al. allowed the construction of a 600 kyr paleotemperature record. Distinct changes in the planktic foraminiferal assemblages are evinced between the two sites, with the transitional and subpolar species G. inflata and N. pachyderma (dextral) dominating at Site 1125, and the polar species N. pachyderma (sinistral) most prominent in the Site 594 census counts. Site 1125 sediment is dominated by mud (<63 micro-m), with the highest mud percentage values occurring during the late Pleistocene interglacial periods. The highest rates of carbonate dissolution, as recorded by the Fragmentation Index of Le and Shackleton, and the benthic/planktic foraminiferal ratio also occur during the interglacial periods, particularly Marine Isotope Stage (MIS) 7. SST estimates of between 17 and 19 C are recorded in each of the full interglacial periods at Site 1125, with SST differences of <2 C indicating that paleoceanographic conditions have remained relatively stable on the northern slopes of the Chatham Rise in the warmer periods of the Late Pleistocene. A greater variation of up to 5 C is evidenced in the glacial periods at Site 1125 however, with colder SST estimates recorded during the later glacial stages of the late Pleistocene in MIS 8, 6, 4 and 2. At Site 594 extreme changes in the SST estimates occurred between glacial and interglacial periods, especially between MIS 12 and 11, and MIS 6 and 5, with differences of c.14 C over double what is recorded in the corresponding intervals at Site 1125. The DCCA ordination identified two main environmental parameters, the age within the studied interval and sea surface temperature, which exerted the strongest influence on the faunal distribution patterns at Site 1125. Large differences in SST of >10 C between Site 1125 and Site 594 are evident in the late Pleistocene glacial periods, confirming that the intense temperature gradient previously reported in MIS 2 was repeated in the earlier glacial stages back to MIS 14. The very cold SST estimates recorded at Site 594 during the glacial stages may have resulted from the northwards migration of the Antarctic Polar Front and subsequent incursion of polar waters through the Pukaki Saddle and into the Bounty Trough. The cooler surface water paleotemperatures recorded during the later glacial stages of the late Pleistocene at Site 1125 is suggestive of the possible intrusion of colder subantarctic waters along the northern slopes and across the crest of the rise or alternatively an increase in the upwelling of intermediate waters. Temperature differences of <2 C are recorded between 1125 and 594 during the warmest interglacial periods of the late Pleistocene in MIS 11, 5.5 and 1, and may be indicative of the expansion and migration southwards of the STF during these periods. SST data from other sites worldwide record similar warm temperatures during MIS 11, indicating that the greatest poleward expansion of the STF in the last 0.5 Ma occurred during this interval.