Richard H. Fillon

Role of glacial Arctic Ocean ice sheets in Pleistocene oxygen isotope and sea level records

Abstract A comparison of the oxygen isotope signal in deep-sea benthic foraminifera with the record of glacio-eustatic sea level for the last 160,000 years reveals that the amplitude of the benthic δ 18 O records predicts more continental ice volume than appears to be reflected in lowered sea level stands. These differences between the benthic δ 18 O ice volume estimates and radiometrically-dated records of eustatic sea level are consistent with the presence of a large floating Arctic Ocean ice mass during glacial intervals. The presence of an Arctic Ocean ice sheet during glacial intervals may account for the two climatic modes observed in oxygen isotope records which span the entire Pleistocene. The early Pleistocene (1.8 to 0.9 Myr B.P.) interval is characterized by low-amplitude, high-frequency δ 18 O fluctuations between glacial and interglacial periods, while the late Pleistocene (0.9 Myr B.P. to present) is characterized by large-amplitude, low-frequency δ 18 O changes. These two climatic modes can be explained by the initiation of earth orbital conditions favoring the co-occurrence of glacial period Arctic Ocean ice sheets and large continental ice sheets approximately 900,000 years before present.

Dynamics of meltwater discharge from Northern Hemisphere ice sheets during the last deglaciation

During the last ice age, ice sheets covered large portions of North America, Europe, Asia, and the Arctic Ocean. The ice sheets were composed of meteoric water, enriched in the light isotope of oxygen (16O), leaving oceanic seawater correspondingly enriched1,2 in 18O. The volume of past continental ice sheets during the last glacial maximum 18,000 yr ago has been estimated from terrestrial evidence of ice limits3, glaciological modelling4, the δ18O of foraminifera from deep sea cores5,6, and sea-level evidence from raised coral terraces7,8. Oxygen isotope studies of planktonic foraminifera reveal that δ18O shifts during deglaciations in areas near sources of ice meltwater, like the Gulf of Mexico and Labrador Sea/Baffin Bay, are often larger than the corresponding average global oceanic shift9–11 and that these variations reflect the timing of ice decay without the ocean-mixing lag contained in both distal and benthic δ10O variations12. We show here that geographically-distinct planktonic δ18O gradients can be related to the volume of meltwater discharged through specific meltwater outlets and thus to the initial volume of the ice sheets at the commencement of deglaciation and to the subsequent dynamics of ice-sheet decay.

Glacial evolution of the plio-pleistocene: Role of continental and arctic ocean ice sheets

Abstract An empirical method is developed for assessing long-term trends in the growth of both mid-latitude Northern Hemisphere continental and high-latitude Arctic Ocean ice sheets during the last 5 million years. The growth of Arctic Ocean Ice Sheets is assumed to be modulated principally by caloric insolation variations related to obliquity, while variations in the extent of Northern Hemisphere continental ice sheets are controlled by precession-dominated caloric insolation variations. In developing empirical equations for the tendency of Arctic Ocean Ice Sheet growth (A) and the tendency for continental ice sheet growth (C), we have also assumed that there would be strong feedback effects between the two ice masses contingent upon North Atlantic surface water heat transport. A clear correspondence between high A values and cold paleotemperatures recorded in North Atlantic deep-sea cores spanning the last 2.0 m.y. is taken as evidence of a long-term linkage with the North Atlantic. Alternatively, strong similarities were found to exist between the predicted record of C and the classical terrestrial glacial/interglacial record back to about 3.2 m.y. B.P. A marked coincidence was also found between the combined tendency of continental and Arctic Ocean Ice Sheet growth (A + C) with the deep-sea δ18O record. A dramatic increase in the amplitude and period of δ18O variations beginning at about 1.0 m.y. B.P. is reflected by a sharp increase in the number of A + C values greater than the long-term mean. Phase relationships between the earths orbital parameters and North Atlantic sea surface temperatures have apparently played a major role in the terrestrial and marine glacioclimatic records for at least the last 3.2 million years.

Grain-size variations in North Atlantic non-carbonate sediments and sources of terrigenous components

Abstract Fine-silt to medium-sand grain-size spectra of the carbonate-free fractions of 115 sea bed samples from the northern North Atlantic, the Labrador Sea and Baffin Bay can be explained as mixtures of five distinct end-members. The five end-members resemble combinations of grain-size modes which are characteristic of till-matrix (Dreimanis and Vagners, 1972). Therefore the ultimate source of most of the terrigenous deep-sea sediments in the study area is probably the veneer of glacial comminution products on the surrounding continents. Three sand-rich end-members exhibit patterns of distribution that are largely compatible with the modern transport of ice-rafted debris as inferred from iceberg drift observations. The best sorted of those sandy end-members however also appears to be concentrated in regions characterized by high-energy turbidity and contour currents and so may also represent hydraulically winnowed sediment. The distribution of two lutite end-members appears in general to reflect the transport of fines by moderate to low energy thermohaline currents; although, in Baffin Bay and the northeastern North Atlantic the lutite could have accumulated primarily in turbidites. Ice-rafting by icebergs calved from Greenland glaciers, the reworking of glacigenic sediments on the Iceland—Faeroes Ridge and injection of turbid glacial meltwater into deep Baffin Bay from West Greenland fiords are suggested as the principal means of introduction of clastic particles into the sediment distribution (redistribution) budget of the deep North Atlantic.

Chronostratigraphy of the Expanded Lower Paleogene Wedge in Southern Louisiana: An Identity Crisis for the Wilcox

ABSTRACT The Wilcox Group is a reservoir-prone system containing over 3,000 ft of dominantly sandy section that records the rapid seaward progradation of a siliciclastic shelf/slope wedge onto a foundering Cretaceous surface. It is locally divided into Upper and Lower Wilcox units by an interval called the Big Shale. Biostratigraphic work has placed the upper part of the wedge in the Lower Eocene and the lower part in the Upper Paleocene. However, our examination of detailed planktonic foraminiferal and calcareous nannofossil data has shown that in some locales the entire nominal Wilcox section lies within the Eocene. The occurrence of Hantkenina spp. and planktonics characteristic of Zone P9, throughout the log-correlated upper and lower wedge, specify a Middle Eocene age. Nannofossils indicating Middle Eocene Zone NP14 (Carrizo equivalent age) confirm this assessment. Lower Eocene planktonics, no older than Zone P8, occur below the nominal Wilcox within a shaley interval containing several minor sands which may be equivalent to type Upper Wilcox (Sabinian). Because Discoaster binodosus was not seen in place, we were unable to confirm the presence of the Wilcox age nannofossil Zone, NP13. Paleocene microfossils were encountered only in a condensed marly Zone immediately overlying the Cretaceous. Our work suggests that, locally, the nominal Wilcox of southern Louisiana might be an expanded, down-dip equivalent of the Carrizo Formation, perhaps a canyon fill unit associated with the 49.5 MY sequence boundary.

Abstract: A Comparison of Two Late Pleistocene Shelf-Edge Deltas (Indonesia and Gulf of Mexico)- Stratigraphic Architecture, Systems Tracts, Bounding Surfaces, and Reservoir Potential

Abstract Thousands of kilometers of high-resolution seismic data have been collected over two late Pleistocene shelf-edge deltas in very different settings, the northern Gulf of Mexico and the eastern shelf of Borneo in Indonesia. Both deltas have been constructed by falling-to-lowstand deposition associated with the latest Pleistocene glacial maximum, the former by the temperate Mobile River, the latter by the equatorial Mahakam River. Four cores provide detailed stratigraphic control for the Mobile River delta while one long boring and numerous piston and vibracores provide stratigraphic control on the Mahakam delta. Systems tracts and key bounding surfaces have been related to the eustatic sea level curve in both settings over ca. 125 years. Sequence architectures differ significantly, an important consequence of different depositional settings. The tropical Mahakam shelf is tectonically active and has low wave energy, strong oceanic currents, upwelling, and a mixed siliciclastic - carbonate depositional system. The resulting falling-to-lowstand clinoforms downlap a highly irregular surface of isolated carbonate bioherms built above a transgressive surface that formed during the preceding sea level rise. The northeastern Gulf of Mexico is relatively stable, also has low wave energy, but is dominated by siliciclastic sedimentation. Falling-to-lowstand progradation of the Mobile River delta has occurred in numerous overlapping and spatially offset lobes incised by a complex channel network. Clinoforms downlap an isotope stage 5 interglacial condensed section. The Mobile depocenter has migrated from east to west; eastern lobes show evidence of wave reworking while the western flank is fluvially dominated. Both the Mahakam and Mobile deltas are composed of sand-rich clinoforms and channel deposits that possess excellent potential reservoir properties.