Andrea Dutton

Ice Volume and Sea Level During the Last Interglacial

More Melting The last interglacial period, around 125,000 years ago, was 1° to 2°C warmer than the present, and the sea level was thought to be 4 to 6 meters higher. However, Dutton and Lambeck (p. 216), now suggest that sea level was possibly as much as 10 meters above current levels. Such a large excess of seawater would mean that the Greenland and Antarctic ice sheets melted much more than previously assumed, which has implications for how much sea-level rise we should expect with anthropogenic climate warming. Global average sea level during the last interglacial period was 6 to 10 meters higher than it is today. During the last interglacial period, ~125,000 years ago, sea level was at least several meters higher than at present, with substantial variability observed for peak sea level at geographically diverse sites. Speculation that the West Antarctic ice sheet collapsed during the last interglacial period has drawn particular interest to understanding climate and ice-sheet dynamics during this time interval. We provide an internally consistent database of coral U-Th ages to assess last interglacial sea-level observations in the context of isostatic modeling and stratigraphic evidence. These data indicate that global (eustatic) sea level peaked 5.5 to 9 meters above present sea level, requiring smaller ice sheets in both Greenland and Antarctica relative to today and indicating strong sea-level sensitivity to small changes in radiative forcing.

Sea-level rise due to polar ice-sheet mass loss during past warm periods

Warming climate, melting ice, rising seas We know that the sea level will rise as climate warms. Nevertheless, accurate projections of how much sea-level rise will occur are difficult to make based solely on modern observations. Determining how ice sheets and sea level have varied in past warm periods can help us better understand how sensitive ice sheets are to higher temperatures. Dutton et al. review recent interdisciplinary progress in understanding this issue, based on data from four different warm intervals over the past 3 million years. Their synthesis provides a clear picture of the progress we have made and the hurdles that still exist. Science, this issue 10.1126/science.aaa4019 Reconstructing past magnitudes, rates, and sources of sea-level rise can help project what our warmer future may hold. BACKGROUND Although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level (GMSL) rise over the last century, mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions to GMSL rise under future warming. To better constrain polar ice-sheet response to warmer temperatures, we draw on evidence from interglacial periods in the geologic record that experienced warmer polar temperatures and higher GMSLs than present. Coastal records of sea level from these previous warm periods demonstrate geographic variability because of the influence of several geophysical processes that operate across a range of magnitudes and time scales. Inferring GMSL and ice-volume changes from these reconstructions is nontrivial and generally requires the use of geophysical models. ADVANCES Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise. Advances in our understanding of polar ice-sheet response to warmer climates have been made through an increase in the number and geographic distribution of sea-level reconstructions, better ice-sheet constraints, and the recognition that several geophysical processes cause spatially complex patterns in sea level. In particular, accounting for glacial isostatic processes helps to decipher spatial variability in coastal sea-level records and has reconciled a number of site-specific sea-level reconstructions for warm periods that have occurred within the past several hundred thousand years. This enables us to infer that during recent interglacial periods, small increases in global mean temperature and just a few degrees of polar warming relative to the preindustrial period resulted in ≥6 m of GMSL rise. Mantle-driven dynamic topography introduces large uncertainties on longer time scales, affecting reconstructions for time periods such as the Pliocene (~3 million years ago), when atmospheric CO2 was ~400 parts per million (ppm), similar to that of the present. Both modeling and field evidence suggest that polar ice sheets were smaller during this time period, but because dynamic topography can cause tens of meters of vertical displacement at Earth’s surface on million-year time scales and uncertainty in model predictions of this signal are large, it is currently not possible to make a precise estimate of peak GMSL during the Pliocene. OUTLOOK Our present climate is warming to a level associated with significant polar ice-sheet loss in the past, but a number of challenges remain to further constrain ice-sheet sensitivity to climate change using paleo–sea level records. Improving our understanding of rates of GMSL rise due to polar ice-mass loss is perhaps the most societally relevant information the paleorecord can provide, yet robust estimates of rates of GMSL rise associated with polar ice-sheet retreat and/or collapse remain a weakness in existing sea-level reconstructions. Improving existing magnitudes, rates, and sources of GMSL rise will require a better (global) distribution of sea-level reconstructions with high temporal resolution and precise elevations and should include sites close to present and former ice sheets. Translating such sea-level data into a robust GMSL signal demands integration with geophysical models, which in turn can be tested through improved spatial and temporal sampling of coastal records. Further development is needed to refine estimates of past sea level from geochemical proxies. In particular, paired oxygen isotope and Mg/Ca data are currently unable to provide confident, quantitative estimates of peak sea level during these past warm periods. In some GMSL reconstructions, polar ice-sheet retreat is inferred from the total GMSL budget, but identifying the specific ice-sheet sources is currently hindered by limited field evidence at high latitudes. Given the paucity of such data, emerging geochemical and geophysical techniques show promise for identifying the sectors of the ice sheets that were most vulnerable to collapse in the past and perhaps will be again in the future. Peak global mean temperature, atmospheric CO2, maximum global mean sea level (GMSL), and source(s) of meltwater. Light blue shading indicates uncertainty of GMSL maximum. Red pie charts over Greenland and Antarctica denote fraction (not location) of ice retreat. Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise from polar ice-sheet loss during past warm periods. Accounting for glacial isostatic processes helps to reconcile spatial variability in peak sea level during marine isotope stages 5e and 11, when the global mean reached 6 to 9 meters and 6 to 13 meters higher than present, respectively. Dynamic topography introduces large uncertainties on longer time scales, precluding robust sea-level estimates for intervals such as the Pliocene. Present climate is warming to a level associated with significant polar ice-sheet loss in the past. Here, we outline advances and challenges involved in constraining ice-sheet sensitivity to climate change with use of paleo–sea level records.

End-Cretaceous extinction in Antarctica linked to both Deccan volcanism and meteorite impact via climate change

The cause of the end-Cretaceous (KPg) mass extinction is still debated due to difficulty separating the influences of two closely timed potential causal events: eruption of the Deccan Traps volcanic province and impact of the Chicxulub meteorite. Here we combine published extinction patterns with a new clumped isotope temperature record from a hiatus-free, expanded KPg boundary section from Seymour Island, Antarctica. We document a 7.8±3.3 °C warming synchronous with the onset of Deccan Traps volcanism and a second, smaller warming at the time of meteorite impact. Local warming may have been amplified due to simultaneous disappearance of continental or sea ice. Intra-shell variability indicates a possible reduction in seasonality after Deccan eruptions began, continuing through the meteorite event. Species extinction at Seymour Island occurred in two pulses that coincide with the two observed warming events, directly linking the end-Cretaceous extinction at this site to both volcanic and meteorite events via climate change.

Stable carbon isotopes (δ13C) of total organic carbon and long-chain n-alkanes as proxies for climate and environmental change in a sediment core from Lake Petén-Itzá, Guatemala

Sediment core PI-6 from Lake Petén Itzá, Guatemala, possesses an ~85-ka record of climate and environmental change from lowland Central America. Variations in sediment lithology suggest large and abrupt changes in precipitation during the last glacial and deglacial periods, and into the early Holocene. We measured stable carbon isotope ratios of total organic carbon and long-chain n-alkanes from the core, the latter representing a largely allochthonous (terrestrial) source of organic matter, to reveal past shifts in the relative proportion of C3–C4 terrestrial biomass. We sought to test whether stable carbon isotope results were consistent with other paleoclimate proxies measured in the PI-6 core, and if extraction and isotope analysis of n-alkanes is warranted. The largest δ13C variations are associated with Heinrich Events. Carbon isotope values in sediments deposited during the last glacial maximum indicate moderate precipitation with little fluctuation. The deglacial was a period of pronounced climate variability, e.g. a relatively warm and moist Bølling–Allerød, but a cool and dry Younger Dryas. Arid periods of the deglacial were inferred from samples with high δ13C values in total organic carbon, which reflect times of greater proportions of C4 plants. These inferences are supported by stable isotope measurements on ostracod shells and relative abundance of grass pollen from the same depths in core PI-6. Similar trends in carbon stable isotopes measured on bulk organic carbon and n-alkanes suggest that carbon isotope measures on bulk organic carbon in sediments from this lake are sufficient to infer past climate-driven shifts in local vegetation.

Karst‐driven flexural isostasy in North‐Central Florida

Deformed marine terraces can be used to explore a region’s uplift history. Trail Ridge is a marine terrace in north Florida that is nearly 80 m above modern sea level and contains Quaternary marine fossils, a fact that is inconsistent with estimates of paleo-sea level history since the early Pleistocene. This implies that the terrace has experienced uplift since its formation, as well as nonuniform deformation recorded by the warping of its previously horizontal state. The Florida carbonate platform, located on the passive margin of eastern North America, is a setting where nontectonic influences (e.g. isostatic adjustment, dynamic topography) can be examined. We present a single-transect, numerical model of vertical displacement, derived from elastic flexure, to assess the influence of karst-driven isostatic uplift on present day topography of Trail Ridge in north Florida. Flexural modeling predicts elevations in central Florida not observed today, most likely because surface erosion and karst cavity collapse have obliterated this high topography. Older subsurface stratigraphic units, however, display the arched profile predicted from flexural modeling. Mass loss, calculated by differencing modeled topography and observed topography, was found to be 6.75 3 1012 kg, since emplacement of Trail Ridge. Uplift rates, assuming karst-driven flexural isostasy alone, using previously estimated ages of Trail Ridge of 0.125, 1.4, 3, or 3.5 Ma were found to be 0.535, 0.048, 0.022, and 0.019 mm/yr, respectively. A more likely explanation of uplift includes contributions from dynamic topography and glacial isostatic adjustment which should be further explored with more advanced geophysical modeling.

Karst-driven flexural isostasy in North-Central Florida: FLEXURAL ISOSTASY OF NORTH FLORIDA

Deformed marine terraces can be used to explore a regions uplift history. Trail Ridge is a marine terrace in north Florida that is nearly 80 meters above modern sea level and contains Quaternary marine fossils, a fact that is inconsistent with estimates of paleo-sea level history since the early Pleistocene. This implies that the terrace has experienced uplift since its formation, as well as non-uniform deformation recorded by the warping of its previously horizontal state. The Florida carbonate platform, located on the passive margin of eastern North America, is a setting where non-tectonic influences (e.g. isostatic adjustment, dynamic topography) can be examined. We present a single-transect, numerical model of vertical displacement, derived from elastic flexure, to assess the influence of karst-driven isostatic uplift on present day topography of Trail Ridge in north Florida. Flexural modeling predicts elevations in central Florida not observed today, most likely because surface erosion and karst cavity collapse have obliterated this high topography. Older subsurface stratigraphic units, however, display the arched profile predicted from flexural modeling. Mass loss, calculated by differencing modeled topography and observed topography, was found to be 6.75 × 1012 kg, since emplacement of Trail Ridge. Uplift rates, assuming karst-driven flexural isostasy alone, using previously estimated ages of Trail Ridge of 0.125, 1.4, 3, or 3.5 Ma were found to be 0.535, 0.048, 0.022, and 0.019 mm/yr, respectively. A more likely explanation of uplift includes contributions from dynamic topography and glacial isostatic adjustment which should be further explored with more advanced geophysical modeling.

Late Holocene inter-annual temperature variability reconstructed from the δ18O of archaeological Ostrea angasi shells

ABSTRACT Two multi-year oxygen isotope (δ18O) records were obtained from archaeological Ostrea angasi shells, confirming the potential of this species to provide valuable environmental records for the late Holocene period in southeastern Australia. High-resolution δ18Oshell samples from the O. angasi clearly display a seasonal variability, offering insight into past climate conditions in a region where such information is presently limited. The oxygen isotope record in O. angasi reflects a combined temperature–salinity signal. Observations of δ18Oshell data from modern specimens are used as a point of reference to assist in decoupling these two influences, with the two archaeological samples compared with the δ18Oshell profile of four modern O. angasi. Assuming similar paleo- δ18Owater values at the collection sites, data from these archaeological shells present a record of temperatures during the period of their growth that are consistently lower than modern day, with mean annual temperatures ∼2°C cooler.