Antony J. Long

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.

Identifying coseismic subsidence in tidal‐wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Tidal marsh stratigraphy, sea-level change and large earthquakes, i: a 5000 year record in washington, U.S.A.☆

Abstract Many of the estuaries of the Pacific Northwest of the U.S.A. and Canada contain stratigraphic sequences typified by alternating peat-mud couplets. Recent studies in this region interpret such couplets as the product of repeated large (magnitude S or 9) earthquakes on the Cascadia subduction zone. The resultant pattern of land-level movements is described by a model, the ‘earthquake deformation cycle’, of coseismic land subsidence followed by land uplift during interseismic strain accumulation. However, peat-mud couplets similar to those recorded in the Pacific Northwest are found on other less tectonically active temperate-latitude coasts, such as northwest Europe and the Atlantic coast of the U.S.A., where they have been interpreted as the product of non-seismic coastal processes. In this paper we apply the methods and scientific framework common to sea-level investigations in northwest Europe to a sequence of peat-mud couplets recorded in the lower Johns River, an estuary in southern Washington, to provide a test of the ‘earthquake deformation cycle’. Stratigraphic investigations of the intertidal sediments along the lower Johns River, using lithological, pollen, diatom and foraminiferal data, show evidence for eight coastal submergence events during the last 5000 years. To evaluate the ‘earthquake deformation cycle’ we assess the lateral extent of peat-mud couplets, the synchroneity of submergence, the presence of tsunami deposits accompanying submergence, and the suddenness and amount of submergence. Each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the ‘earthquake deformation cycle’, demonstrating the continued intermittent seismic activity of the Cascadia subduction zone throughout the mid and late-Holocene. Quantitative analyses of contemporary and fossil biostratigraphic data, using TWINSPAN and Detrended Correspondence Analysis, enable us to estimate the magnitude of submergence accompanying each peat-mud couplet. One event was accompanied by submergence of about 1.5 m or more, four events by intermediate submergence of about 1±0.5 m, and a further three events by submergence of

Isolation basin stratigraphy and Holocene relative sea‐level change on Arveprinsen Ejland, Disko Bugt, West Greenland

This paper presents the results of an investigation into Holocene relative sea-level (RSL) change, isostatic rebound and ice sheet dynamics in Disko Bugt, West Greenland. Data collected from nine isolation basins on Arveprinsen Ejland, east Disko Bugt, show that mean sea level fell continuously from ca. 70 m at 9.9 ka cal. yr BP (8.9 ka 14C yr BP) to reach a minimum of ca. −5 m at 2.8 ka cal. yr BP (2.5 ka 14C yr BP), before rising to the present day. A west–east gradient in isostatic uplift across Disko Bugt is confirmed, with reduced rebound observed in east Disko Bugt. However, RSL differences (up to 20 m at 7.8 ka to 6.8 ka cal. yr BP (7 ka to 6 ka 14C yr BP)) also exist within east Disko Bugt, suggesting a significant north–south component to the area’s isostatic history. The observed magnitude and timing of late Holocene RSL rise is not compatible with regional forebulge collapse. Instead, RSL rise began first in the eastern part of the bay, as might be expected under a scenario of crustal subsidence caused by neoglacial ice sheet readvance. The results of this study demonstrate the potential of isolation basin data for local and regional RSL studies in Greenland, and the importance of avoiding data compilations from areas where the isobase orientation is uncertain. Copyright

Late Devensian and Holocene relative sea-level changes in northwestern Scotland: New data to test existing models

Abstract Pollen, diatom, lithostratigraphic and radiocarbon data from five sites in northwestern Scotland provide new data from an area previously devoid of reliable and precise information on Late Devensian and Holocene sea-level changes. The sites cover a range of palaeoenvironments, indicative of diversity in coastal evolution since deglaciation. For each site and palaeoenvironment the reference water (tide) level, indicative range, age and tendency of sea-level movement of all sea-level index points are quantified to enable correlation of the diverse coastal environments. The data record patterns of relative sea-level change and tendencies of sea-level movement from 12 ka BP to 1 ka BP. This is the longest and most comprehensive published record of relative sea-level change from the area. The information is used to test the accuracy of existing models of relative sea-level change. The results are only broadly consistent with a quantitative rebound model, and there is significant disagreement with empirical models during the Late Devensian and the early Holocene.

New observations on the relative sea level and deglacial history of Greenland from Innaarsuit, Disko Bugt

Relative sea level (RSL) data derived from isolation basins at Innaarsuit, a site on the south shores of the large marine embayment of Disko Bugt, West Greenland, record rapid RSL fall from the marine limit (ca. 108 m) at 10,300–9900 cal yr B.P. to reach the present sea level at 3500 cal yr B.P. Since 2000 cal yr B.P., RSL rose ca. 3 m to the present. When compared with data from elsewhere in Disko Bugt, our results suggest that the embayment was deglaciated later and more quickly than previously thought, at or slightly before 10,300 cal yr B.P. The northern part of Disko Bugt experienced less rebound (ca. 10 m at 6000 cal yr B.P.) compared with areas to the south. Submergence during the late Holocene supports a model of crustal down-warping as a result of renewed ice-sheet growth during the neoglacial. There is little evidence for west to east differences in crustal rebound across the southern shores of Disko Bugt.

Holocene relative sea-level changes and coastal vegetation history at Kentra Moss, Argyll, northwest Scotland

Abstract A late-Holocene fall in relative sea level in northwest Scotland, from ca. 1.3mm yr−1 to ca. 1.0 mm yr−1, is interpreted from lithostratigraphic, biostratigraphic, chronostratigraphic and numerical analyses of fossil tidal marsh and acidic peat bog communities elevated by isostatic uplift. Pollen, diatom and stratigraphic data from contemporary depositional environments are used to define the indicative range (±0.2 m) and reference water level (mean high water of spring tides or highest astronomical tide) of thirteen dated sea-level index points. No Holocene intertidal sediments are recorded above + 7.7 m OD and all sea-level index points are younger than ca. 4 kyr B.P. In parts of Kentra Moss, beyond the limit of Holocene intertidal clastic sedimentation, raised bog communities were established by at least 8.3 kyr B.P. These age and altitude parameters differ from those interpolated for the “Main Postglacial Shoreline”, but support a regional model in which isostatic uplift continues at present in the Kentra Moss area.

Holocene sea-level change and coastal evolution in the Humber estuary, eastern England: an assessment of rapid coastal change

New stratigraphic data collected from six sites in the Humber estuary establish a record of Holocene relative sea-level (RSL) change, and enable testing of four possible causes of rapid coastal change: sea-level rise, changes in sedimentation, storm-surge history, and human impact. Mean high water of spring tides (MHWST) in the Humber rose from c. 9 m OD at 7500 cal. yrs BP to 0 m OD by 4000 cal. yrs BP, at an average long-term rate of c. 3.9 mm yr-1. After this, the rate of rise gradually decreased to c. 1 mm yr’. Discrete episodes of rapid RSL rise are not identified although their absence may reflect limited data availability. However, we do observe two episodes of rapid coastal change in the Humber estuary. The first occurs between c. 3200 and 1900 cal. yrs BP, as marine conditions expand to their Holocene maximum and then contract. This pattern of coastal development differs from that in the East Anglian Fenlands, suggesting local processes control sedimentation at one or both of these sites. The second period of rapid change relates to a well-documented episode of increased storm surge activity in the Humber estuary and elsewhere in the UK and the North Sea region between c. 700 and 500 cal. yrs BP. Coastal development during this period varies considerably with erosion, accretion and flooding in different parts of the estuary system. Finally, we examine evidence for accelerated sediment delivery to the Humber estuary due to woodland clearance and prehistoric agriculture from 5700 cal. yrs BP onwards. Maximum sediment input is likely at c. 3200 to 1900 cal. yrs BP; a period which tentatively correlates with an episode of estuary infilling and shoreline advance.

Stratigraphic architecture, relative sea-level, and models of estuary development in southern England: new data from Southampton Water

Abstract This paper presents the results of an investigation into the Holocene depositional history of Southampton Water, southern England. A three phase history of estuary development is proposed. Between c. 7500 and 5000 bp (8200 to 5700 cal. a bp), mean sea-level rose rapidly from c. −9m to −4 m od. During this interval thin basal peats which developed in present outer estuary locations were inundated and the area of intertidal and subtidal environments within the estuary expanded. Relative sea-level (RSL) rise began to slow between 5000 and 3000 bp (5700 and 3200 cal. a bp) and a phase of saltmarsh and freshwater peat accumulation occurred. In this interval freshwater peat-forming communities extended outwards and seawards across former saltmarsh and mudflat environments and caused a reduction in the extent of the intertidal area within the estuary. During the late Holocene there was a switch to renewed minerogenic sedimentation as most of the freshwater coastal wetlands of Southampton Water were inundated. This tripartite model is broadly applicable to the Thames and the Severn estuaries, suggesting that regional processes have controlled their macroscale evolution. RSL change and variations in sediment supply emerge as key controls during the first two phases of estuary development. The late Holocene demise of the estuary wetlands probably reflects a propensity for increased sediment reworking and unfavourable conditions for the accumulation and preservation of organogenic deposits due to reduced rates of long-term RSL and watertable rise.

Rapid coastal change during the mid- to late Holocene: the record of barrier estuary sedimentation in the Romney Marsh region, southeast England

A common problem facing sea-level researchers lies in determining the cause of rapid changes observed in coastal stratigraphic sequences. Such changes are commonly ascribed to the interaction of processes that operate over differing temporal and spatial scales, i.e. rapid local and regional sea-level trends and storm magnitude/frequency. At least some of the difficulty in distinguishing between these processes lies in the often limited stratigraphic database upon which palaeoenvironmental reconstruction is based. Here, we present the results of detailed morphostratigraphic and micropalaeontological investigations from a series of sites located at the interface between the protective gravel barrier complex of Dungeness and the barrier estuary sediments of Romney Marsh, England. A period of rapid coastal regression is identified between c. 4500 and 3000 cal. yrs BP, during which time the sea level continued to rise but was outpaced by sediment accretion and peat accumulation. A subsequent acceleration in the rate of rise led to a reversal of this excess of sediment supply over sea-level rise, and to rapid inundation of the entire barrier estuary between c. 2800 and 1900 cal. yrs BP. At the local level, coastal development reflects an interdependence between storms and long-term relative sea-level (RSL) rise; processes which lie at opposite ends of the magnitude/frequency spectrum. However, when viewed from a holistic perspective encompassing the Holocene development of Romney Marsh as a whole, storms are of secondary importance in controlling coastal evolution. Rather, our study suggests that at this macro-scale coastal evolution responds primarily to the combined effects of RSL rise and sediment supply