Jerry M. Lloyd

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

Ice-stream retreat and ice-shelf history in Marguerite Trough, Antarctic Peninsula: Sedimentological and foraminiferal signatures

The timing, nature, and causes of grounded ice-sheet retreat following the Last Glacial Maximum (LGM) in Marguerite Trough, west Antarctic Peninsula, and subsequent early Holocene ice-shelf decay, are presented in this paper. We use sedimentological, foraminiferal, geotechnical, and accelerator mass spectrometer (AMS) radiocarbon data from marine cores from the mid-continental shelf, together with previously published AMS dates, to establish a sedimentological and chronological model. Initial ice-sheet retreat through the outer- and mid-shelf sectors of Marguerite Trough was under way by ca. 14 ka B.P., was rapid, and coincided with the sea-level rise of meltwater pulse 1a. An ice shelf formed during this retreat, and fine-grained, laminated muds reflecting meltwater-derived suspension settling and/or tidal pumping were deposited. During this time the ice sheet remained grounded on the inner shelf. Ice-shelf breakup and retreat of the calving front, from ca. 13.2 to 12.5 ka B.P., was slow (∼100 m a –1 ) across the outer- and mid-shelf, with calving bay conditions remaining for at least 3.5 ka. We interpret this ice-shelf decay to have been driven by an incursion of Weddell Sea Transitional Water onto the shelf. In contrast, grounding-line and ice-shelf retreat in the inner bay occurred from ca. 9.3 ka B.P. and was driven by Circumpolar Warm Deep Water encroaching onto the continental shelf. At this time the mid-shelf was an open-marine environment characterized by hemipelagic deposition. These findings highlight the importance of oceanographic controls in the breakup of Antarctic Peninsula ice shelves during the Holocene.

Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid-Pleistocene ice sheet growth

[1] Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9–0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (%C37:4 alkenone) and sea surface temperature (U37 ). Both sites experienced a secular expansion of subarctic waters from � 1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low %C37:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial %C37:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in %C37:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.

Interaction between subsurface ocean waters and calving of the Jakobshavn Isbræ during the late Holocene

A marine sediment core from Vaigat in Disko Bugt, West Greenland, has been analysed in terms of lithology, dinoflagellate cysts and foraminifera in order to evaluate the influence of oceanographic variability on West Greenland glacier stability. The data show that during the past 5200 years the Atlantic foraminiferal abundance in the subsurface waters of the West Greenland Current (WGC) episodically increased, indicating periods of increases in the inflow of subsurface warm Atlantic water at 2000—1500 cal. yr BP and 1300 cal. yr BP as well as periods of less pronounced increased bottom-water temperatures around 4700—4000 cal. yr BP, 3100—2800, 2600, 1000—800, 500—400, and at 200 cal. yr. The sedimentological and dinoflagellate cyst data indicate that these episodes with enhanced advection of Irminger Sea-derived waters are accompanied by increased iceberg rafting, which we link to increased iceberg calving in relation to destabilization of the Jakobshavn Isbrae. The long-term trend in the data documents the end of a late-Holocene Thermal Maximum between 5200 and 4300 cal. yr BP and a final onset of the Neoglaciation at 3500 cal. yr BP. Increased responses of the iceberg rafting after 3500 cal. yr BP, reflects a westward/seaward advance of the glacier margin in relation to onset of Neoglaciation and a development of the glacier into a floating tongue after 2000 cal. yr BP. A comparison of our record with a record from the eastern North Atlantic indicates that a NAO-like anomaly pattern between subsurface waters in West Greenland and atmospheric temperature in the Eastern North Atlantic may have been operating during most of the late Holocene. However, during the past 1000 years the NAO signal may have weakened as some other mode of climate variability overprints the anti-phase climate signal in this region.

Holocene coastal change in East Norfolk, UK: palaeoenvironmental data from Somerton and Winterton Holmes, near Horsey

We present palaeoenvironmental results from two cores, Winterton Holmes (GY2) and Somerton Holmes (GY3), south of Horsey, East Norfolk. The upper transgressive contact of the peat of cores GY2 and GY3 is dated to 2355–2742 cal. years bp and 2761–2949 cal. years bp, respectively. The litho- and biostratigraphical data show that both these peat contacts represent sea-level index points formed around mean high water spring tides during a positive tendency of sea-level movement, as saltmarsh indicators near the top of, and immediately above the peat show its deposition within intertidal environments. These sea-level index points when combined with other sea-level observations and predictions from East Norfolk, confirm an upward trend of Holocene relative sea-level change typical of an area at, or beyond, the margins of the last British ice sheet; rapid in the early Holocene then at a much reduced rate in the mid- and late Holocene. Late Holocene rates of relative uplift or subsidence are calculated for East Norfolk by subtracting a model of relative sea level from each index point and then calculating the best-fit linear trend. The late Holocene rate of relative sea-level rise (or land subsidence) is 0.67 ± 0.06 mm −1 which is c . 1 mm a −1 less than the twentieth-century mean sea-level trend estimated from Lowestoft tidal station, thus agreeing with the consensus of opinion that global sea levels have risen between 10 and 20 cm over the past century.

Lack of evidence for a substantial sea-level fluctuation within the Last Interglacial

During the Last Interglacial, global mean sea level reached approximately 6 to 9 m above the present level. This period of high sea level may have been punctuated by a fall of more than 4 m, but a cause for such a widespread sea-level fall has been elusive. Reconstructions of global mean sea level account for solid Earth processes and so the rapid growth and decay of ice sheets is the most obvious explanation for the sea-level fluctuation. Here, we synthesize published geomorphological and stratigraphic indicators from the Last Interglacial, and find no evidence for ice-sheet regrowth within the warm interglacial climate. We also identify uncertainties in the interpretation of local relative sea-level data that underpin the reconstructions of global mean sea level. Given this uncertainty, and taking into account our inability to identify any plausible processes that would cause global sea level to fall by 4 m during warm climate conditions, we question the occurrence of a rapid sea-level fluctuation within the Last Interglacial. We therefore recommend caution in interpreting the high rates of global mean sea-level rise in excess of 3 to 7 m per 1,000 years that have been proposed for the period following the Last Interglacial sea-level lowstand.Robust evidence for a previously proposed sea-level fall and rise during the Last Interglacial is lacking, according to a synthesis. This calls estimates of high rates of sea-level rise at the end of the Last Interglacial into question.