T.C. Moore

THE EVOLUTION OF PLEISTOCENE CLIMATE: A TIME SERIES APPROACH

Abstract In the later part of the Pleistocene, variations in global ice volume have been dominated by an approximate 100,000-year cycle. Analysis of 2-Myr-long oxygen isotope record from an equatorial Pacific core indicates that this is true only for the last 900,000 years. Prior to this time the amplitude of the 100,000-year cycle is much reduced, as is the variance of all oscillations with periods greater than 60,000 years. Based on results of time series analysis of this 2-Myr-long record, the Pleistocene glacial cycles can be divided into three sections: (1) the late Pleistocene (0–900 kyr B.P.) where the variations in the isotope record are dominated by the 100,000-year cycle; (2) the middle Pleistocene (900–1450 kyr B.P.) in which low-frequency components are not as important as in the later period of the Pleistocene, and (3) the early Pleistocene/late Pliocene (1450–2000 kyr B.P.) where general reductions of importance at all frequencies is seen as compared to the later intervals. Recent modeling efforts which describe variations in global ice volume show that the dominant low-frequency component observed in the late Pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the Earths orbital parameters. It is hypothesized that during the early Pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice-cap formation to below sea level. When the ice caps became marine-based, more rapid decay of the ice became possible.

High resolution stratigraphic correlation of benthic oxygen isotopic records spanning the last 300,000 years

Abstract In order to compare the response of different oceanographic regions to global climate change, very detailed stratigraphic techniques are required. The global signal of ice volume changes recorded in the oxygen isotopic composition of foraminifera can provide the tool for developing the necessary high resolution stratigraphy. In order to evaluate the resolution of a stratigraphy based on detailed isotopic records, two techniques are used to correlate a set of benthic oxygen isotope records from seven piston cores taken in the North and South Atlantic, the Indian, and the equatorial and North Pacific oceans. The first technique was modified from the graphic correlation procedure of Shaw (1964). This procedure requires the identification of isotopic events that are correlated from core to core. Detailed correlations for intervals of the cores between events are provided by a series of straight line segments connecting all common events. The second technique developed by Martinson et al. (1982) uses inverse procedures to define a continuous non-linear mapping function that correlates the isotopic records. The mapping function maximizes the correlation coefficient between data sets being compared. The techniques are independent in that they rely on different criteria for correlating the data series. Stratigraphic correlations obtained by these procedures are in excellent agreement. The mean difference between the correlations is on the order of the sampling intervals of each core and, when corrected for sedimentation rates, suggests that benthic isotope records from the suite of seven cores can be correlated to a resolution of 2000 to 4000 yrs.

Depositional and tectonic framework of the rift basins of Lake Baikal from multichannel seismic data

Recent multichannel seismic reflection data from Lake Baikal, located in a large, active, continental rift in central Asia, image three major stratigraphic units totaling 3.5 to 7.5 km thick in four subbasins. A major change in rift deposition and faulting between the oldest and middle-rift units probably corresponds to the change from slow to fast rifting in early Pliocene time inferred from on-land studies. A minor modification of fault patterns characterizes the youngest unit. A brief comparison of the basins of Lake Baikal with those of the East African rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

The reconstruction of sea surface temperatures in the Pacific Ocean of 18,000 B.P.

Abstract All the major microfossil groups were used to reconstruct the summer and winter sea-surface temperatures of an ice-age Pacific Ocean. The use of these four groups was necessary because of the varying degrees of preservation of siliceous and carbonate-rich sediments in the Pacific. Their use also permits comparisons of temperature estimates for samples in which more than one group is preserved. The standard error of estimate for the transfer function equations used in this study average about ± 1.5° C for the summer temperature estimates, and about ± 1.9° C for winter estimates. Laboratory (counting) errors result in an average error of estimate of about 0.6° C. Most of the individual estimates using different equations on the same samples agree within their pooled standard error. The reconstructions of ice-age temperature patterns show cooling in the subarctic region by about 4°C in both August and February. The equatorial region is 2–4° C cooler only in the winter season (August). Seasonality (August minus February temperatures) is stronger in the western Subarctic and Transition Zones at 18,000 B.P. than it is at present. Such changes in seasonality result primarily from increased winter cooling and equatorward shifts in the frontal zones. Temperatures within the centers of the subtropical gyres at 18,000 B.P. are generally as warm as, or warmer than, modern sea-surface temperatures. In particular, the Southern Hemisphere shows little or no cooling in tropical and subtropical latitudes except along the equator and in the eastern boundary current. The distribution of biotic assemblages and the derived temperature patterns indicate an intensified circulation at 18,000 B.P. Temperature gradients are steeper in the boundary currents, and divergence at the equator is increased. The “spin-up” of the subtropical gyres associated with this intensified flow appear to contain the warm tropical waters within the gyre centers, rather than allowing their dispersal to subpolar regions. These relatively warm gyre centers may act as loci for low pressure systems in the atmosphere which draw moisture away from the land masses.

Direct stratigraphic dating of India-Asia collision onset at the Selandian (middle Paleocene, 59 ± 1 Ma)

The collision of India with Asia had a profound influence on Cenozoic topography, oceanography, climate, and faunal turnover. However, estimates of the time of the initial collision, when Indian continental crust arrived at the Transhimalayan trench, remain highly controversial. Here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when Asian-derived detritus was first deposited onto India in the classical Sangdanlin section of the central Himalaya, which preserves the best Paleocene stratigraphic record of the distal edge of the Indian continental rise. Deep-sea turbidites of quartzarenite composition and Indian provenance are replaced upsection by turbidites of volcano-plutoniclastic composition and Asian provenance. This sharp transition occurs above abyssal cherts yielding radiolaria of Paleogene radiolarian zones (RP) 4–6 and below abyssal cherts containing radiolaria of zone RP6 and calcareous shales with nannofossils of the Paleocene calcareous nannofossil zone (CNP) 7, constraining the age of collision onset to within the middle Paleocene (Selandian). The youngest U-Pb ages yielded by detrital zircons in the oldest Asia-derived turbidites indicate a maximum depositional age of 58.1 ± 0.9 Ma. Collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 ± 1 Ma. This age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern Himalaya, and suggests that, within the resolution power of current methods, the India-Asia initial collision took place quasi-synchronously in the western and central Himalaya.

Possible role of oceanic heat transport in early Eocene climate

Increased oceanic heat transport has often been cited as a means of maintaining warm high-latitude surface temperatures in many intervals of the geologic past, including the early Eocene. Although the excess amount of oceanic heat transport required by warm high latitude sea surface temperatures can be calculated empirically, determining how additional oceanic heat transport would take place has yet to be accomplished. That the mechanisms of enhanced poleward oceanic heat transport remain undefined in paleoclimate reconstructions is an important point that is often overlooked. Using early Eocene climate as an example, we consider various ways to produce enhanced poleward heat transport and latitudinal energy redistribution of the sign and magnitude required by interpreted early Eocene conditions. Our interpolation of early Eocene paleotemperature data indicate that an approximately 30% increase in poleward heat transport would be required to maintain Eocene high-latitude temperatures. This increased heat transport appears difficult to accomplish by any means of ocean circulation if we use present ocean circulation characteristics to evaluate early Eocene rates. Either oceanic processes were very different from those of the present to produce the early Eocene climate conditions or oceanic heat transport was not the primary cause of that climate. We believe that atmospheric processes, with contributions from other factors, such as clouds, were the most likely primary cause of early Eocene climate.

Method Of Randomly Distributing Grains For Microscopic Examination

ABSTRACT A technique has been devised by which fine, hydrodynamically heterogenous grains may be mounted on a microscope slide in such a way that their distribution on the slide is random. The grains are settled from a well-mixed suspension onto the microscope slide where they are held in place by a thin film of gelatin. This technique allows the investigator to use any part of the slide as a representative subsample, and obviates the need for repeated splitting of samples and the counting of a very large number of grains.

Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific

The Antarctic Circumpolar Current (ACC) is a key feature of the Southern Ocean. Its development may have helped cool Antarctica and initiate Southern Hemisphere glaciation. The deep circulation of the ACC must have been established after both the Tasman gateway (between Antarctica and Australia) and the Drake Passage (between South America and Antarctica) opened. However, estimates for ACC initiation range over 20 m.y., from the middle Eocene to early Miocene. A new piston core of upper Oligocene to Holocene sediments from the South Pacific has allowed us to delimit the formation of the ACC to the late Oligocene (ca. 25–23 Ma). Upper Oligocene, current-worked sediments and a hiatus to the upper Miocene result from the beginning of the modern ACC flow; i.e., when strong currents and mixing throughout the water column were established. Previously published Nd isotope data date the first intrusion of Pacific water into the Atlantic much earlier. The discrepancy with our results can be reconciled by the different methods measuring different flow regimes. Tracer methods such as Nd are sensitive to relatively small and shallow incursions of water, whereas pelagic erosional regimes require vigorous deep flow.

The distribution of radiolarian assemblages in the modern and ice-age Pacific

Abstract Data on radiolarian abundances from several recent regional studies of the Pacific have been combined with new data from the temperate and tropical area to provide an ocean-wide view of radiolarian distributions. A Q -mode factor analysis of these data identified seven factors which have their areas of dominance in the following regions: tropics, western Pacific, subarctic, Antarctic, transitional zone, temperate regions, and the eastern central water masses. The distributions of these factors tended to follow those of surface water masses and major ocean currents. A more detailed analysis of the temperate and tropical region better delineated the complex flow and counter flow in this area. The relationship between the distribution of modern radiolarian assemblages and the surface circulation of the Pacific can be used to deduce the nature of oceanographic changes which occurred in the past. The modern radiolarian distributions are compared with those mapped at the 18, 000 B.P. level and reveal two major differences in the ice-age Pacific. The Tropical Factor, restricted primarily to the eastern half of the ocean in modern times, ranged across the entire ocean at 18, 000 B.P. and extended into the area of the western boundary currents. The Subarctic Factor, now found mainly in the western subarctic, expanded to the east and south at 18, 000 B.P. and had strong similarities to an assemblage found in the subantarctic area. The expansion of these two dominant assemblages was at the expense of the Western Pacific, Temperate, and Transitional Factors. These differences in the 18, 000 B.P. distributions are thought to be caused by an increase in the influence of arctic air masses in the North Pacific and by a general increase in the wind-driven zonal flow.