Detlef A. Warnke

A 1.0 Myr Record of Glacial North Atlantic Intermediate Water Variability from ODP Site 982 in the Northeast Atlantic

Changes in the intermediate water structure of the North Atlantic were reconstructed using benthic foraminiferal δ13C at Ocean Drilling Program (ODP) site 982 for the past 1.0 Myr. During most terminations of the late Pleistocene, melting of icebergs and low-salinity surface waters caused production of Glacial North Atlantic Intermediate Water to cease, resulting in decreased ventilation of the middepth North Atlantic. Poor ventilation of intermediate water masses lasted well into some interglacial stages until upper North Atlantic Deep Water (NADW) production resumed under full interglacial conditions. The magnitude of benthic δ13C minima and ice-rafted debris maxima at terminations at site 982 generally match the degree of glacial suppression of NADW inferred from site 607. These processes may be related and controlled by the spatial and seasonal extent of sea ice cover during glaciations in the Nordic Seas.

Climatic evolution of the Southern Ocean during the Pliocene epoch from 4.8 to 2.6 million years ago

Abstract We studied the paleoclimatic history of the Southern Ocean during the Pliocene, before the intensification of northern hemisphere glaciation at 2.4 Ma, from isotopic and sedimentologic data from ODP Leg 114 sites (subantarctic South Atlantic). Relatively warm climatic conditions prevailed in the Southern Ocean during the early early Pliocene (between ∼4.6 and 4.2 Ma) as evidenced by benthic and planktonic δ 18 O values that were generally less than the Holocene. Despite this warmth, significant ice-rafting episodes are recorded at several drill sites and piston-core locations throughout the Southern Ocean. This interval correlates with a eustatic highstand and represents a climatic optimum for the Southern Ocean when low- and mid-latitude species migrated into the Antarctic and surface waters were warmer than at present. Although it is generally agreed that this interval was warm in the southern hemisphere, divergent opinions exist as to whether this warming led to the partial decay or growth of the Antarctic ice-sheet. A distinct mid-Pliocene cooling occurred between ∼3.5 and 3.2 Ma, as evidenced by an increase in δ 18 O values of planktonic and benthic foraminifers in Hole 704A, and by faunal changes indicative of progressive cooling of Antarctic surface waters and development of biotic provinciality in the Southern Ocean. During the late Pliocene (3.25-2.6 Ma), planktonic and benthic δ 18 O values commonly exceeded those of the Holocene and several distinct cooling and/or ice volume events occurred during the late Gauss Chron (most notably at 2.9 and 2.63 Ma). Faunal studies suggest several northward advances of the Polar Front Zone and expansion of sea ice in the Antarctic during this period. These events preceded the major climatic transition at ∼2.4 Ma when the PFZ moved to the north, sea ice expanded in the Antarctic, and glaciation intensified in both hemispheres.

Granulometric study of the Hanaupah Fan, Death Valley, California

We applied new granulometric techniques to the various surfaces of the Hanaupah Fan, Death Valley, California, namely the Q1 surface, with an estimated age of 800–490 ka, the younger Q2 (170–105 ka) and Q3 (50–14 ka) surfaces, the <14 ka deposits of the incised channel, and to a (c. 14 ka) Lake Manly shoreline deposit at the northern periphery of the fan. We used these techniques to generate quantitative information on surface clast grain-size distributions, clast sphericity, roundness, and clast orientation to provide a data set that could be used to define fan-segment surfaces, and to help interpret fan genesis. Grain-size analyses were carried out by photo-sieving of 139 surface pictures, by petrographic identification of samples taken in the incised channel, and by identification and measuring of the largest clasts (1452 measurements) on the Q3 surface. The results show that all fan-segment surfaces, regardless of age, have similar size distributions, with a well-defined gravel mode of −2·3 to −3·0 phi, and are poorly to moderately sorted. Samples from the incised channel have distributions that are very similar to each other, regardless of distance from the apex, but display reduced sorting compared to the fan surfaces (which largely lack fines, perhaps from winnowing by secondary overland flow). Only the shoreline deposit is different from the other elements, showing a much narrower, well-defined gravel mode (−3·0 phi), and is moderately well sorted. Sphericity and roundness of clasts on all surfaces show only minor differences, similar to the other sedimentary parameters, indicating a remarkable homogeneity of the surfaces of the sediment body. In addition, measurements of the largest clasts (>100 cm long axis) on the Q3 surface showed no discernible trend either with radial distance or with rock type. These data suggest large depositional episodes that produce extensive sedimentary units without differentiation relative to distance from the source. Of the examined parameters, clast orientation is the best predictor of relative age of fan surfaces. Clast orientation in the main channel is bimodal, i.e. the long axes of clasts are either at right angles or parallel to transport direction. This bimodality disappears with increasing age, and the preferred orientation becomes unimodal (long clast axes normal to transport direction) on the Q1 surface. Although the causes of this change are still in debate, use of this parameter as a relative-age dating tool seems possible.

Major deglaciation of east Antarctica during the early Late Pliocene? Not likely from a marine perspective

Abstract We have conducted an integrated study of ice-rafted debris (IRD) and oxygen isotopes (measured on Cibicides, Globigerina bulloides , and Neogloboquadrina pachyderma , using identical samples). We used samples from the early Late Pliocene Gauss Chron from ODP Site 114–704 on the Meteor Rise in the subantarctic South Atlantic. During the early Gauss Chron, the oxygen isotopic ratios are generally up to 0.5‰–0.6‰ less than their respective Holocene values. The lowest values in this record can accommodate a warming of about 2.5 °C OR a sea-level rise of about 50 m, but not both, and probably result from some warming and a small reduction in global ice volume. Starting with isotope stage MG2 [3.23 Ma on the Berggren et al. (1985) time scale; 3.38 on the Shackleton et al. (1995b) time scale] oxygen-isotopic values generally increase (and oscillate about a Holocene mean). The first significant IRD appears at the same time. There is a subsequent increase in IRD amounts upsection. In order to reach the site, this material must have been transported by large, tabular icebergs derived from Antarctic ice shelves or ice tongues, similar to occasional, large modern icebergs. This combined record suggests strongly that the Antarctic ice sheet was essentially intact; some warming at the drill site is indicated, but not a major reduction in ice-volume on Antarctica.

The Shape and Surface Texture of Loess Particles: Discussion

Very fine particles (< 2 microns) adhering to larger particles have been observed by scanning electron microscopy. Such fines, adhering to loess particles, were interpreted by Smalley and Cabrera (1970) as comminution debris, produced in the process of loess-material formation by glacial grinding. This is in contrast to some published information on the nature and composition of glacially produced material, and the subject remains open to debate. No positive proof was given by Smalley and Cabrera (1970) that the loess material depicted by them was glacially produced. Although these authors may be correct in their interpretation, other additional hypotheses on the origin of loess material cannot as yet be rejected. While there is an obvious connection between glaciation and many loess occurrences, the ultimate origin of much of the loess material has still to be demonstrated.

The Hanaupah-Fan shoreline deposit at Tule Spring, a gravelly shoreline deposit of Pleisticene Lake Manly, Death Valley, California, USA

The Hanaupah-Fan Shoreline Deposit (HSD) is an as yet undescribed occurrence of shoreline sediments of late Pleistocene Lake Manly in Death Valley, California. It is located in the southern part of Death Valley, at the northeastern periphery of Hanaupah Fan. The HSD is a gently sloping, WSW-ENE elongated ridge, about 600 m long, 165 m wide and 8 m high. Its surface extends from -12 to +28 m in elevation, i.e. it has a vertical range of 40 m. We interpret the deposit as a sediment body that extended from the Hanaupah Fan east into the lake. Rising lake level, and waves approaching both from the north and south eroded fan materials, and produced a sediment body with a complex architecture. Fetch for waves approaching from either direction was about 40 km. The sedimentary inventory consists of cross-stratified gravel beds of various size ranges, dipping towards the north, south, and east, and of horizontal berm gravel beds, and horizontal silt layers. A discordant gravel layer covers the entire surface of the deposit, probably produced by wave action during the last phase of lake regression. This uniform gravel layer forms a surface that is distinctly different from the surrounding fan surfaces. It is relatively fine grained, much better sorted, and densely packed. Rock varnish is very well developed, and imparts a dark color to the surface, which makes it easily recognizable on aerial photographs. No absolute age date is available as yet, but circumstantial evidence places the formation of the deposit at the peak of marine isotope stage 2 (Wisconsinan/Weichsellian glacial maximum)