Coarse-sand ice-rafted debris components and mass accumulation rates of ODP Site 152-918

To understand the late Cenozoic glacial history of the Northern Hemisphere, continuous long-term proxy records from climatically sensitive regions must be examined. Ice-rafted debris (IRD) from Ocean Drilling Program (ODP) Site 918, located in the Irminger Basin, is one such record. IRD in marine sediments is a direct indicator of the presence of glacial ice extending to sea level on adjacent landmasses, and, therefore, is an important paleoclimatic signal from the mid- to high latitudes. The IRD record at Site 918 is the first long-term ice-rafting record available for southeast Greenland, a region that may have been a key nucleation area for widespread glaciation during the late Cenozoic (Larsen et al, 1994, doi:10.2973/odp.proc.ir.152.1994). This data report presents the results of coarse sand-size IRD mass accumulation rate (MAR) analyses for Site 918 from the late Miocene through the Pleistocene. In addition, a preliminary analysis of IRD compositions is included. Detailed discussions of the local, regional, and global paleoclimatic implications of this data, and of the companion Site 919 Pleistocene IRD MAR data (Krissek, 1999, doi:10.2973/odp.proc.sr.163.118.1999), are in preparation. Such future work will include comparisons of these IRD MAR data sets to the Site 919 oxygen isotope stratigraphy developed by Flower (1998, doi:10.2973/odp.proc.sr.152.219.1998).

Paleomagnetism of DSDP Leg 80 holes

Paleomagnetic results from sediments acquired from the continental margin at DSDP Sites 548, 549, 550, and 551 are described. Where possible, the results were used to construct a polarity reversal stratigraphy for the sections sampled, thus enabling the biostratigraphic dating of the sediments to be refined. Several sections in this study were found to be suitable for magnetostratigraphic work, in particular the upper Paleocene to middle Eocene sediments from Site 549, which contained rich faunal assemblages. These sediments are underlain by a thick sequence of Cretaceous sediments that formed during the Long Cretaceous normal polarity interval. Sediments that formed during the later part of this magnetically quiet interval were also recovered at Site 550. Three short reverse polarity intervals were also recovered at this site; they lie directly over basement and are thought to represent a mixed-polarity interval of late Albian age. They may therefore provide important evidence concerning the age of the earliest sediments at this site. In addition, measurements of the magnetic susceptibility and intensity of remanent magnetism proved to be of interest. A significant decrease in the susceptibility and intensity values close to the early/middle Eocene boundary was noted at Sites 548 and 549. This decrease may be correlated with the results from Holes 400A and 401, which were drilled on DSDP Leg 48 in the northeast Bay of Biscay. The decrease may represent an abrupt reduction in the supply of terrigenous material at the end of the early Eocene, reflecting, perhaps, a change in sediment transport processes at that time

Petrographic data for samples from DSDP Holes 63-471 and 65-485

Detrital modes of middle Miocene sandstone recovered at Deep Sea Drilling Project Site 471 on the Magdalena Fan support the hypothesis that the fan has been displaced northward from a source hundreds of kilometers to the south near the present mouth of the Gulf of California. The modes are dissimilar to those of modern sand derived from onshore outcrops of Miocene volcanic and volcaniclastic rocks, Neogene sedimentary rocks, and Mesozoic subduction complex. They most closely match sand associated with the mouth of Gulf of California. The overall stratigraphy, sand composition, and diagenesis at Site 471 are consistent with deposition of the Magdalena Fan on young oceanic crust near a spreading ridge at a triple junction.

Diatom abundance in ODP Leg 178 holes

This paper documents the biostratigraphic distribution and abundance of diatoms from sites drilled during Ocean Drilling Program Leg 178, off the Pacific margin of the Antarctic Peninsula. Drift sediments cored on the continental rise at Sites 1095, 1096, and 1101 have good recovery and a well-defined paleomagnetic record. Well-preserved diatoms are present throughout the upper Miocene to middle Pliocene and in the upper Quaternary section of these sites. The stratigraphic occurrence of diatom species through these intervals defines numerous datum levels. Diatom events are given absolute age estimates through direct correlation to the established paleomagnetic stratigraphy of Sites 1095, 1096, and 1101. Leg 178 diatom biostratigraphic results enable the development of a regional stratigraphic framework for the Pacific sector of the Southern Ocean and record the interaction of open-ocean and shelf-margin diatom floras.

Late Eocene-earliest Miocene Sr isotopic reference section of DSDP Site 73-522

We present a revised calibration of Sr isotopes to the geomagnetic polarity timescale (GPTS) using closely spaced (~0.15 m.y. resolution) samples from the classic uppermost Eocene through lowermost Miocene section at Site 522, eastern South Atlantic. The Sr isotopic data are fit with two linear segments with a sharp change in slope at circa 27.5 Ma from 0.000038/m.y. (27.5 to 34.4 Ma) to 0.000051/m.y. (23.8 to 27.5 Ma). Regression analysis indicates that stratigraphic resolution ranges from ±1 m.y. (for one analysis) to ±0.6 m.y. (for three analyses) for the younger interval and ±1.2 m.y. (for one analysis) to ±0.7 m.y. (for three analyses) for the older interval, representing an increase in resolution from previous studies of ±1-2 m.y. The paleoceanographic significance of this change in slope is unclear. It occurs during an interval of intermittent Antarctic glaciation, between the Oi2a and Oi2b glaciations. The subsequent interval from circa 27 to 24 Ma appears to be an interval of minimal glaciation. Thus this observation does not support previous suggestions that increases in rates of Sr isotopic change are directly associated with the frequency of Antarctic glaciations. Rather, the increase in slope may be related to increased weathering associated with the "mid-Oligocene" glaciation.

Nannofossil assemblages and flux rates during the late Oligocene-early Miocene

This study investigates abundance variations in Noelaerhabdaceae assemblages during the late Oligocene-early Miocene at three subtropical sites in the Atlantic and Pacific oceans (DSDP Sites 516, 608 and 588). At these three sites, nannofossil assemblages were characterized by the successive high proportion of Cyclicargolithus, Dictyococcites and Reticulofenestra. Local paleoceanographic changes, such as the input of nutrient-poor water masses, might explain shifts in ecological prominence within the Noelaerhabdaceae at DSDP Site 516 (South Atlantic). But the similar timing of a decline in Cyclicargolithus at the three studied sites more likely corresponds to a global process. Here, we explore possible causes for this long-term taxonomic turnover. A global change in climate, associated with early Miocene glaciations, could have triggered a decline in fitness of the taxon Cyclicargolithus. The ecological niche made vacant because of the decrease in Cyclicargolithus could then have been exploited by Dictyococcites and Reticulofenestra that became prominent in the assemblages after 20.5 Ma. Alternatively, this global turnover might reflect a gradual evolutionary succession and be the result of other selection pressures, such as increased competition between Cyclicargolithus and Dictyococcites/Reticulofenestra. A diversification within Dictyococcites/Reticulofenestra, indicated by an expansion in the size variation within this group since ~ 20.5 Ma, may have contributed to the decreased fitness of Cyclicargolithus.

Stable foraminifer isotope composition and age of ODP Sites 184-1147 and 184-1148, South China Sea

Sites 1147 (18°50.11'N, 116°33.28'E; water depth = 3246 m) and 1148 (18°50.17'N, 116°33.94'E; water depth = 3294 m) are located on the lowermost continental slope off southern China near the continent/ocean crust boundary of the South China Sea Basin. Site 1147 is located upslope ~0.45 nmi west of Site 1148. Three advanced piston corer holes at Site 1147 and two extended core barrel holes at Site 1148 were cored and combined into a composite (spliced) stratigraphic section, which provided a relatively continuous profile for the lower Oligocene to Holocene (Wang, Prell, Blum, et al., 2000, doi:10.2973/odp.proc.ir.184.2000; Jian, et al., 2001, doi:10.1007/BF02907088) for studying stratigraphy and paleoceanography. A total of 1047 planktonic foraminifers stable isotope measurements were performed on 975 samples covering the upper 409.58 meters composite depth (mcd) at ~42-cm intervals (Tables T1, T2), and a total of 1864 benthic foraminifers measurements were performed on 1650 samples in the upper 837.11 mcd at ~51-cm intervals (Tables T3, T4). We significantly improved the time resolution of the benthic stable isotope record in the upper 476.68 mcd by reducing the average sample spacing to ~29 cm. This translates into an average sampling resolution of ~16 k.y. for the Miocene sequence and ~8 k.y. for the Pliocene-Holocene interval, assuming a change in sedimentation rates from ~1.8 to ~3.5 cm/k.y., as suggested by shipboard stratigraphy. These data sets provide the basis for upcoming studies to establish an oxygen isotope stratigraphy and examine the Neogene evolution of deep and surface water signatures (temperature, salinity, and nutrients) in the South China Sea.

Planktonic foraminifer stratigraphy of late Pliocene sediments from ODP Hole 167-1021C

This report is an initial investigation of the planktonic foraminifers from the late Pliocene of Ocean Drilling Program Hole 1021C. Planktonic foraminifer assemblages show fair preservation in many samples. Assemblages are quantitatively dominated by Globigerina bulloides, dextral coiling Neogloboquadrina atlantica, and Globorotalia inflata and alternate between cool temperate and subarctic conditions.

Late Cretaceous palynofloras of ODP Hole 120-748C

Pollen, spore, and dinoflagellate cyst floras of Late Cretaceous age were found at Sites 748 (120-748C-62R through -79R) and 750 (120-750B-11W) of Ocean Drilling Program Leg 120 to the Kerguelen Plateau area in the Southern Indian Ocean. The ranges of dinocyst and sporomorph species indicate ages between the Cenomanian and Coniacian (to possibly the early Santonian). The ratio of marine/terrestrial flora elements is extremely variable, showing a trend from highly terrestrial (up to -70%) in the late Cenomanian to highly marine (up to 90%) in the Coniacian/early Santonian. Low sedimentation rates of about 3-5 cm/1000 yr were calculated for the glauconitic sediments of Turonian and Coniacian age at Site 748 (lithologic Subunit IIIB).

Organic geochemistry of lower Cretaceous sediments at DSDP Hole 77-535

At Site 535, the four lithologic units of Cretaceous age are controlled by two types of sedimentologic facies: (1) the massive light-colored limestones or marly limestones in which the total organic carbon (TOC) content is low and the organic matter more or less oxidized and (2) laminated dark facies in which the TOC content is higher and associated with a well-preserved organic matter of Type II origin. Very little typical Type III organic matter occurs in the whole series from late Berriasian to Aptian and Cenomanian. Fluctuations from oxidizing to reducing environments of deposition are proposed to account for the variations in properties of the Type II organic matter between the different facies. Dark laminated layers are good but immature potential source rocks: petroleum potential is often higher than 2 kg HC/t of rock.

Preliminary ichthyolith biostratigraphy and age profiles at DSDP Sites 91-595 and 91-596

Seventy meters of Cenozoic and Mesozoic pelagic clay cored at DSDP Sites 595 and 596 provide the basis for a preliminary analysis of ichthyolith biostratigraphy in the southwest Pacific. A most likely order of the more reliable ichthyolith events is compared with a synthesis of ichthyolith biostratigraphy in the North Pacific and with dated composite ranges. The resultant preliminary ichthyolith stratigraphy suggests that the Cenozoic is represented by the upper 20 m at Site 596 and 16 to 22 m at Site 595. Mixing of taxa precludes a clear recognition of the Cretaceous/Tertiary boundary at Site 595. The occurrence of 13 newly described subtypes is recorded in Mesozoic sediments at Sites 595 and 596. These new subtypes and previously described Mesozoic forms may be useful for recognizing Mesozoic subdivisions when their occurrences in sequences dated by other microfossils are investigated.

Geochemistry, sediment components, and paleoproductivity reconstruction for the Miocene to early Pliocene sediments

We reconstruct paleoproductivity at three sites in the Atlantic Ocean (Ocean Drilling Program Sites 982, 925, and 1088) to investigate the presence and extent of the late Miocene to early Pliocene 'biogenic bloom' from 9 to 3 Ma. Our approach involves construction of multiple records including benthic foraminiferal and CaCO3 accumulation rates, Uvigerina counts, dissolution proxies, and geochemical tracers for biogenic and detrital fluxes. This time interval also contains the so-called late Miocene carbon isotope shift, a well-known decrease in benthic foraminiferal d13C values. We find that the timing of paleoproductivity maxima differs among the three sites. At Site 982 (North Atlantic), benthic foraminifera and CaCO3 accumulation were both at a maximum at ~5 Ma, with smaller peaks at ~6 Ma. The paleoproductivity maximum was centered earlier (~6.6-6.0 Ma) in the tropical Atlantic (Site 925). In the South Atlantic (Site 1088), paleoproductivity increased even earlier, between 8.2 Ma and 6.2 Ma, and remained relatively high until ~5.4 Ma. We note that there is some overlap between the interval of maximum productivity between Sites 925 and 1088, as well as the minor productivity increase at Site 982. We conclude that the paleoproductivity results support hypotheses aiming to place the biogenic bloom into a global context of enhanced productivity. In addition, we find that at all three sites the d13C shift is accompanied by carbonate dissolution. This observation is consistent with published studies that have sought a relationship between the late Miocene carbon isotope shift and carbonate preservation.

Early - Late Miocene palaeoclimate data and palaefloras of 48 samples from Southern China

Southern China, especially Yunnan, has undergone high tectonic activity caused by the uplift of Himalayan Mountains during the Neogene, which led to a fast changing palaeogeography. Previous study shows that Southern China has been influenced by the Asian Monsoon since at least the Early Miocene. However, it is yet not well understood how intense the Miocene monsoon system was. In the present study, 63 fossil floras of 16 localities from Southern China are compiled and evaluated for obtaining available information concerning floristic composition, stratigraphic age, sedimentology, etc. Based on such reliable information, selected mega- and micro-floras have been analysed with the coexistence approach to obtain quantitative palaeoclimate data. Visualization of climate results in maps shows a distinct spatial differentiation in Southern China during the Miocene. Higher seasonalities of temperature and precipitation occur in the north and south parts of Southern China, respectively. During the Miocene, most regions of Southern China and Europe were both warm and humid. Central Eurasia was likely to be an arid center, which gradually spread westward and eastward. Our data provide information about Miocene climate patterns in Southern China and about the evolution of these patterns throughout the Miocene, and is also crucial to unravel and understand the climatic signals of global cooling and tectonic uplift.