David M. Harwood

Bio: David M. Harwood is an academic researcher from University of Nebraska–Lincoln. The author has contributed to research in topics: Ice sheet & Antarctic ice sheet. The author has an hindex of 40, co-authored 130 publications receiving 5177 citations. Previous affiliations of David M. Harwood include Ohio State University.

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Abstract: 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 ( approximately 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, approximately 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 approximately 3 degrees C warmer than today and atmospheric CO(2) concentration was as high as approximately 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 CO(2).

Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton

Abstract: Recycled Cretaceous and Cenozoic marine microfossils have been recovered from samples of the Pliocene Sinus Formation. Samples were collected from outcrops in the Reedy, Beardmore, and Ferrar glacier areas of the Transantarctic Mountains between lat 77° and 86°S. The glaciogene sediments contained diatoms, foraminifera, calcareous nannoplankton, silicoflagellates, radiolarians, sponge spicules, palynomorphs, and ostracodes of Late Cretaceous, Paleocene, Eocene, late Oligocene, late Miocene, and Pliocene age. This suggests the presence of open marine basins on the East Antarctic craton during late Mesozoic and Cenozoic time. The apparent absence of early Oligocene and early through middle and earliest late Miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times. The high-elevation setting of Sirius Formation outcrops suggests one of two hypotheses for their origin: (1) They are in situ Pliocene glaciomarine deposits that were uplifted 1,750–2,500 m with the Transantarctic Mountains to their present elevation; (2) the Sirius Formation deposits are a mixture of derived sediments stripped from sub-ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the Transantarctic Mountains. We favor the second hypothesis, with transport to sites sometime within the past 3 m.y.

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

Abstract: 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 today1, the Antarctic ice sheets may have been unstable2, 3, 4, 5, 6, 7. 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 cycles8, 9, 10. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets11. 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 event5).

Palynomorphs from a sediment core reveal a sudden remarkably warm Antarctica during the middle Miocene

Abstract: An exceptional triple palynological signal (unusually high abundance of marine, freshwater, and terrestrial palynomorphs) recovered from a core collected during the 2007 ANDRILL (Antarctic geologic drilling program) campaign in the Ross Sea, Antarctica, provides constraints for the Middle Miocene Climatic Optimum. Compared to elsewhere in the core, this signal comprises a 2000-fold increase in two species of dinoflagellate cysts, a synchronous five-fold increase in freshwater algae, and up to an 80-fold increase in terrestrial pollen, including a proliferation of woody plants. Together, these shifts in the palynological assemblages ca. 15.7 Ma ago represent a relatively short period of time during which Antarctica became abruptly much warmer. Land temperatures reached 10 °C (January mean), estimated annual sea-surface temperatures ranged from 0 to 11.5 °C, and increased freshwater input lowered the salinity during a short period of sea-ice reduction.

The impact of climate change on the world's marine ecosystems.

Abstract: Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed. Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation. The impacts of anthropogenic climate change so far include decreased ocean productivity, altered food web dynamics, reduced abundance of habitat-forming species, shifting species distributions, and a greater incidence of disease. Although there is considerable uncertainty about the spatial and temporal details, climate change is clearly and fundamentally altering ocean ecosystems. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.

Air pressure and cosmogenic isotope production

Abstract: The cosmic ray flux increases at higher altitude as air pressure and the shielding effect of the atmosphere decrease. Altitude-dependent scaling factors are required to compensate for this effect in calculating cosmic ray exposure ages. Scaling factors in current use assume a uniform relationship between altitude and atmospheric pressure over the Earth's surface. This masks regional differences in mean annual pressure and spatial variation in cosmogenic isotope production rates. Outside Antarctica, air pressures over land depart from the standard atmosphere by ±4.4 hPa (1σ) near sea level, corresponding to offsets of ±3–4% in isotope production rates. Greater offsets occur in regions of persistent high and low pressure such as Siberia and Iceland, where conventional scaling factors predict production rates in error by ±10%. The largest deviations occur over Antarctica where ground level pressures are 20–40 hPa lower than the standard atmosphere at all altitudes. Isotope production rates in Antarctica are therefore 25–30% higher than values calculated by scaling Northern Hemisphere production rates with conventional scaling factors. Exposure ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides at levels close to saturation, may be millions of years younger than published estimates.

Tectonic forcing of late Cenozoic climate

Abstract: Global cooling in the Cenozoic, which led to the growth of large continental ice sheets in both hemispheres, may have been caused by the uplift of the Tibetan plateau and the positive feedbacks initiated by this event. In particular, tectonically driven increases in chemical weathering may have resulted in a decrease of atmospheric C02 concentration over the past 40 Myr.

The Neogene Period

Abstract: An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.

Contribution of Antarctica to past and future sea-level rise

Abstract: Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics-including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs-that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.