Lawrence A. Krissek

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment

A 1284.87-m-long sediment core (AND-1B) from beneath the McMurdo sector of the Ross Ice Shelf provides the most complete single section record to date of fluctuations of the Antarctic Ice Sheets over the last 13 Ma. The core contains a succession of subglacial, glacimarine, and marine sediments that comprise ∼58 depositional sequences of possible orbital-scale duration. These cycles are constrained by a chronology based on biostratigraphic, magnetostratigraphic, and 40 Ar/ 39 Ar isotopic ages. Each sequence represents a record of a grounded ice-sheet advance and retreat cycle over the AND-1B drill site, and all sediments represent subglacial or marine deposystems with no subaerial exposure surfaces or terrestrial deposits. On the basis of characteristic facies within these sequences, and through comparison with sedimentation in modern glacial environments from various climatic and glacial settings, we identify three facies associations or sequence “motifs” that are linked to major changes in ice-sheet volume, glacial thermal regime, and climate. Sequence motif 1 is documented in the late Pleistocene and in the early Late Miocene intervals of AND-1B, and it is dominated by diamictite of subglacial origin overlain by thin mudstones interpreted as ice-shelf deposits. Motif 1 sequences lack evidence of subglacial meltwater and represent glaciation under cold, “polar”-type conditions. Motif 2 sequences were deposited during the Pliocene and early Pleistocene section of AND-1B and are characterized by subglacial diamictite overlain by a relatively thin proglacial-marine succession of mudstone-rich facies deposited during glacial retreat. Glacial minima are represented by diatom-bearing mudstone, and diatomite. Motif 2 represents glacial retreat and advance under a “subpolar” to “polar” style of glaciation that was warmer than present, but that had limited amounts of subglacial meltwater. Sequence motif 3 consists of subglacial diamictite that grades upward into a 5- to 10-m-thick proglacial retreat succession of stratified diamictite, graded conglomerate and sandstone, graded sandstone, and/or rhythmically stratified mudstone. Thick mudstone intervals, rather than diatomite-dominated deposition during glacial minima, suggest increased input of meltwater from nearby terrestrial sources during glacial minima. Motif 3 represents Late Miocene “subpolar”-style glaciation with significant volumes of glacially derived meltwater.

Onset of Mediterranean outflow into the North Atlantic

The when of Mediterranean water outflow The trickle of water that began to flow from the Mediterranean Sea into the Atlantic Ocean after the opening of the Strait of Gibraltar turned into a veritable flood by the end of the Pliocene 2 to 3 million years ago. It then began to influence large-scale ocean circulation in earnest. Hernández-Molina et al. describe marine sediment cores collected by an ocean drilling expedition (see the Perspective by Filippelli). The results reveal a detailed history of the timing of Mediterranean outflow water activity and show how the addition of that warm saline water to the cooler less-salty waters of the Atlantic was related to climate changes, deep ocean circulation, and plate tectonics. Science, this issue p. 1244; see also p. 1228 Mediterranean outflow water began to enter the Atlantic and influence global ocean circulation by the late Pliocene. [Also see Perspective by Filippelli] Sediments cored along the southwestern Iberian margin during Integrated Ocean Drilling Program Expedition 339 provide constraints on Mediterranean Outflow Water (MOW) circulation patterns from the Pliocene epoch to the present day. After the Strait of Gibraltar opened (5.33 million years ago), a limited volume of MOW entered the Atlantic. Depositional hiatuses indicate erosion by bottom currents related to higher volumes of MOW circulating into the North Atlantic, beginning in the late Pliocene. The hiatuses coincide with regional tectonic events and changes in global thermohaline circulation (THC). This suggests that MOW influenced Atlantic Meridional Overturning Circulation (AMOC), THC, and climatic shifts by contributing a component of warm, saline water to northern latitudes while in turn being influenced by plate tectonics.

Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica

In 2007, the Antarctic Geological Drilling Program (ANDRILL) drilled 1138.54 m of strata ~10 km off the East Antarctic coast, includ ing an expanded early to middle Miocene succession not previously recovered from the Antarctic continental shelf. Here, we pre sent a facies model, distribution, and paleoclimatic interpretation for the AND-2A drill hole, which enable us, for the fi rst time, to reconstruct periods of early and middle Miocene glacial advance and retreat and paleo environmental changes at an ice-proximal site. Three types of facies associations can be recognized that imply signifi cantly different paleoclimatic interpretations. (1) A diamictite-dominated facies association represents glacially dominated depositional environments, including subglacial environments, with only brief intervals where ice-free coasts existed, and periods when the ice sheet was periodically larger than the modern ice sheet. (2) A stratifi ed diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-infl uenced depositional environments and is more consistent with a very dynamic ice sheet, with a grounding line south of the modern position. (3) A mudstone-dominated facies association generally lacks diamictites and was produced in a glacially infl uenced hemipelagic depositional environment. Based on the distribution of these facies associations, we can conclude that the Antarctic ice sheets were dynamic, with grounding lines south of the modern location at ca. 20.1‐19.6 Ma and ca. 19.3‐18.7 Ma and during the Miocene climatic optimum, ca. 17.6‐15.4 Ma, with ice-sheet and sea-ice minima at ca. 16.5‐16.3 Ma and ca. 15.7‐15.6 Ma. While glacial minima at ca. 20.1‐19.6 Ma and ca. 19.3‐18.7 Ma were characterized by temperate margins, an increased abundance of gravelly facies and diatomaceous siltstone and a lack of meltwater plume deposits suggest a cooler and drier climate with polythermal conditions for the Miocene climatic optimum (ca. 17.6‐15.4 Ma). Several periods of major ice growth with a grounding line traversing the drill site are recognized between ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma, with evidence of cold polar glaciers with ice shelves. The AND-2A core provides proximal evidence that during the middle Miocene climate transition, an ice sheet larger than the modern ice sheet was already present by ca. 14.7 Ma, ~1 m.y. earlier than generally inferred from deep-sea oxygen isotope records. These fi ndings highlight the importance of high-latitude ice-proximal records for the interpretation of far-fi eld proxies across major climate transitions.

Regional patterns of Pleistocene ice‐rafted debris flux in the North Pacific

IRD mass accumulation rate (MAR) records are presented for Deep Sea Drilling Project site 580 and Ocean Drilling Program sites 882 and 887 and are combined with results from previous IRD MAR studies to provide a synthesis of North Pacific Pleistocene ice rafting. Important regional differences in the ice rafting histories of the NW and NE Pacific are highlighted by this synthesis and include (1) significant variations in IRD fluxes across the North Pacific, which identify two major IRD sources, coastal Alaska and coastal Siberia/Kamchatka Peninsula, and (2) significant variations in the timing of IRD supply across the North Pacific, particularly in response to global-scale climatic forcing. These regional differences indicate that local and regional controls on iceberg input, transport, and melting exert a major influence on ice rafting throughout the North Pacific. This suggests that a single generalized ice rafting record for the North Pacific does not adequately describe the complexity of this basins Pleistocene history. Data used to calculate LSRs and IRD MARs for sites 380, 882, and 887 are available on diskette or via anonymous FTP from kosmos.agu.org, directory APEND (Usemame = anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800-966-2481;

The record of Ontong Java Plateau: Main results of ODP Leg 130

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Neogene glacial record from the Sirius Group of the Shackleton Glacier region, central Transantarctic Mountains, Antarctica

The drilling campaign of ODP Leg 130 on Ontong Java Plateau resulted in the recovery of complete Neogene sections at several depths, providing materials for detailed biostratigraphic and paleoceanographic studies in the western equatorial Pacific. The acquisition of extensive logging records and high-resolution physical-property data allow detailed correlation from hole to hole and from site to site and provide the basis for a paleoceanographic interpretation of acoustic reflectors. We drilled 16 holes at 5 sites on the north-eastern flank of the plateau (Sites 803 through 807). All sites are close to the equator, at water depths ranging from 2,500 m to 3,900 m. Sites 803 and 807 penetrated into basement (26 m and 149 m, respectively). The K/T boundary was recovered at both of these sites. Neogene sedimentation rates decrease with depth, as expected, but this decrease is much greater than calculated from carbonate content, under the assumption that carbonate dissolution is the sole cause of the decrease. At any one site, sedimentation rates vary by a factor of more than two, with a striking maximum in the latest Miocene to early Pliocene, and strong minima in late early to early middle Miocene and in the Pleistocene. Many acoustic reflectors correlate between sites, within the limits of stratigraphic resolution. This suggests paleoceanographic events as a cause, generating changes in physical properties of sediments at the time of deposition. Many of the reflectors occur at carbonate reduction events (CREs). Some apparently are the product of diagenetic enhancement of property changes, as, for example, within the ooze/chalk transition (which is diachronous). The interval corresponding to the Cretaceous/Tertiary (K/T) transition in the area is characterized by the presence of a deep CCD. The sequence at one site is calcareous; that at the other, is not. The fact that the two K/T sections recovered occur in sequences with major hiatuses suggests special conditions for preservation during the transition. We propose early cementation caused by high silicate concentrations in an ocean with greatly reduced productivity. The basalt cored at Sites 803 and 807 is predominantly aphyric to sparsely olivine or plagioclase phyric; the last flows are Albian to Aptian in age. At Site 807, pillow lavas buried sediments. One very thick flow (∼28 m) was penetrated here, possibly a flood basalt, indicative of massive outpourings on Ontong Java Plateau during the middle Cretaceous.

Zooplankton and Nekton: Natural Barriers to the Seaward Transport of Suspended Terrigenous Particles off Peru

Neogene glacigenic strata, collectively referred to as the Sirius Group, are widely distributed throughout the Transantarctic Mountains. The group is particularly well exposed near the head of Shackleton Glacier (858109 to 858409S) on Roberts Massif and at Bennett Platform. These deposits are critical for examining the nature of former ice flow from the East Antarctic Ice Sheet into the Ross Embayment. The Sirius Group rests on a glacially grooved and striated pavement named herein the ‘‘Shackleton erosion surface.’’ The sub‐Sirius Group surface on Roberts Massif was of low relief, and glaciogenic sediment was deposited on it as a sheet of uniform thickness. At Bennett Platform, stratigraphic sections attain a thickness of 110 m thick and are subdivided into the Shackleton Glacier Formation (maximum thickness 98 m) and the overlying Bennett Platform Formation (44 m), separated by an unconformity. A third, older, lithified diamictite containing wood fragments occurs as clasts within these formations and as boulders in the modern lateral moraines of Shackleton Glacier. The dominant facies, massive diamict, is interpreted primarily as lodgement till. Other facies indicate glaciofluvial and glaciolacustrine deposition. Facies associations suggest deposition either by sliding temperate or by polythermal glaciers, under much warmer conditions than those of today. Widespread large- and small-scale faulting has affected the Sirius Group and underlying rocks to the extent that inland exposures are over 500 m higher than those to the north, over a distance of 30 km. Thus, at the time of deposition of the Sirius Group, the mountains were probably lower, and the ice sheet was much thinner.

Development of the Efficacy Beliefs for Conceptual Change Learning Questionnaire

Some of the strongest biogenic signals of coastal upwelling reported in Recent marine sediments (e.g., organic carbon content >20 wt. %) are found in a mud facies on the inner continental margin off Peru, particularly between 10° and 14°S. Terrigenous sediments found there are anomalously fine grained and have overall grain-size distributions which are similar to those of deep-sea sediments collected seaward of the Peru Trench. To explain the observed dispersal patterns of fine silts and clays on the inner continental margin, rapid vertical transport of large aggregates (fecal pellets) from a wind-driven surface layer is required.