Alkenones and coccolithophorids in late Quaternary sediments from the Walvis Ridge

Sea surface temperatures (SSTs) derived from the alkenone UK'37) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a UK'37-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310). To assess possible effects of past species changes on the UK'37-temperature signal, we have analyzed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the d18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began to increase systematically in relative abundance since the stage 5/4 transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the UK'37-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported UK'37-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman. S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38, alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404). Our results suggest that UK'37-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa.

Neogene calcareous nannofossils in ODP Leg 116 holes

The three sites (717, 718, and 719) drilled on the distal Bengal Fan during ODP Leg 116 cored turbidite sediments almost exclusively. Calcareous nannofossils were recovered sporadically and, although all of them probably have been redeposited, it is possible to date the sediments at all three sites with reasonable confidence. Site 717 penetrated the uppermost middle Miocene Catinaster coalitus highest occurrence datum and represents the most nearly continuous succession of turbidites. Site 718 penetrated the lower Miocene, well below the Helicosphaera ampliaperta highest occurrence datum, but this site contains a major late Pliocene to mid-Pleistocene hiatus. Site 719, the shallowest hole, penetrated only into the upper Miocene. Identification of several critical lowest occurrence datums allows using the poorly constrained but more numerous highest occurrence datums for comparison with the model succession (zonal markers) and thereby to derive a reasonably accurate time framework for the sediments.

Calcareous nannofossils in DSDP Leg 87 holes

Drilling at three DSDP drill sites on the western margin of the Pacific Ocean off the coast of Japan yielded thick sequences of hemipelagic muds and clays generally depleted of calcareous nannofossils. Operations at Sites 582 and 583 recovered dominantly Quaternary sediments. The Pliocene/Pleistocene boundary was reached near the bottom of Hole 582B. At both sites, preserved coccolith populations contained generally few to common nannoliths. The effects of reworking were evident throughout most sections at these two sites. Drilling at Site 584 in the Japan Trench recovered Holocene to Miocene sediments. Populations of nannofossils from this site were generally more depleted than those from the two Nankai Trough sites. Reworking within these sections appears to be much less severe than in samples from the more southern sites.

Pliocene-Pleistocene calcareous nannofossils from the Iberian Abyssal Plain

During Ocean Drilling Program Leg 149, five sites were drilled on the Iberia Abyssal Plain, west of the Iberian Peninsula. Five holes (Holes 897A, 897C, 898A, 899A, and 900A) yielded Pliocene-Pleistocene sediments, which consist mainly of turbidites. Among these, Holes 897C and 898A yielded significant Pliocene-Pleistocene sediments that provided a high-resolution nannofossil biostratigraphy essential for locating paleomagnetic polarity events and for interpreting the age and frequency of turbidite sedimentation in the Iberia Abyssal Plain. Pliocene-Pleistocene nannofossils recovered during Leg 149 are generally abundant and well to moderately preserved. Although reworking is evident in most samples, the Pliocene-Pleistocene nannofossils proved quite reliable for dating the sediments. Most Pleistocene zonal boundaries proposed by S. Gartner in 1977 and the Pliocene standard zonal boundaries proposed by E. Martini in 1971 were easily recognized. In addition, several other nannofossil events proposed by D. Rio et al. in 1990 and by T. Sato and T. Takayama in 1992 were recognized and proved valuable for improving the resolution of Pliocene-Pleistocene nannofossil biostratigraphy. The Pliocene-Pleistocene nannofossil biostratigraphic results of Holes 897C and 900A coincide rather well with the discerned paleomagnetic polarity events. As a result, the combination of nannofossil biostratigraphic and paleomagnetic studies provides important information for fulfilling the second objective of this leg: to determine the history of turbidite sedimentation in the Iberia Abyssal Plain. The general trend of sedimentation rates inferred by nannofossil biostratigraphy indicates that sedimentation rates increase from the continental margin to the deep sea along with increasing water depth.

Cenozoic calcareous nannofossils in DSDP Leg 77 holes

Three of the six DSDP Leg 77 sites drilled in the western approaches to the Straits of Florida yielded thick sequences of Cenozoic sediment rich in calcareous nannofossils. Hiatuses are prominent in each of these continuously cored intervals. A prominent upper Oligocene hiatus, observed at each of these three sites, can be correlated to a large-scale "global" regression event. Other disconformable horizons present in the study area cannot be positively related to sealevel fluctuations and may be caused by a number of factors including local tectonic activity. Paleogene sections are generally marked by thick accumulations within the upper Oligocene Sphenolithus ciperoensis Zone and by a pronounced braarudosphaerid-holococcolith bloom recorded in the lower Oligocene and upper Eocene. This bloom is particularly well developed at Site 540. All samples examined contain abundant nannofossils. Preservation fluctuates throughout the sections from good to poor.

Calcareous nannofossils from ODP Leg 178

Sites 1096 and 1101, two hemipelagic sediment drift sites on the continental rise off the northwestern Pacific margin of the Antarctic Peninsula, contained calcareous nannofossils in the upper intervals of each site (downhole to 168.37 meters below seafloor [mbsf] in Hole 1096B and 121.1 mbsf in Hole 1101A). The occurrences were sporadic and observed to be confined to fine-grained intervals. These intervals were interpreted on board to be interglacial and often contained foraminifers as well. Calcareous nannofossils exhibited a reliable stratigraphy during an interval when other fossils groups were absent, quite rare, or reworked. In total, nine events and three zones were recognized in this study. The base of the Pleistocene was not recorded with calcareous nannofossils, the oldest datum being the first occurrence of medium Gephyrocapsa spp. at 1.69 Ma. All events have been correlated to the paleomagnetic record for each site.

Coccolithophore taxa and PCA factor analysis of ODP Site 1233

We provide high-resolution sea surface temperature (SST) and paleoproductivity data focusing on Termination 1. We describe a new method for estimating SSTs based on multivariate statistical analyses performed on modern coccolithophore census data, and we present the first downcore reconstructions derived from coccolithophore assemblages at Ocean Drilling Project (ODP) Site 1233 located offshore Chile. We compare our coccolithophore SST record to alkenone-based SSTs as well as SST reconstructions based on dinoflagellates and radiolaria. All reconstructions generally show a remarkable concordance. As in the alkenone SST record, the Last Glacial Maximum (LGM, 19-23 kyr B.P.) is not clearly defined in our SST reconstruction. After the onset of deglaciation, three major warming steps are recorded: from 18.6 to 18 kyr B.P. (~2.6°C), from 15.7 to 15.3 kyr B.P. (~2.5°C), and from 13 to 11.4 kyr B.P. (~3.4°C). Consistent with the other records from Site 1233 and Antarctic ice core records, we observed a clear Holocene Climatic Optimum (HCO) from ~8-12 kyr B.P. Combining the SST reconstruction with coccolith absolute abundances and accumulation rates, we show that colder temperatures during the LGM are linked to higher coccolithophore productivity offshore Chile and warmer SSTs during the HCO to lower coccolithophore productivity, with indications of weak coastal upwelling. We interpret our data in terms of latitudinal displacements of the Southern Westerlies and the northern margin of the Antarctic Circumpolar Current system over the deglaciation and the Holocene.