Patterns Of Relative Sea-level Change Research Articles

Relative sea-level variability during the late Middle Pleistocene: New evidence from eastern England

Unravelling patterns of relative sea-level change during previous interglacials enhances our understanding of ice sheet response to changing climate. Temperate-latitude estuarine environments have the potential to preserve continuous records of relative sea level from previous interglacial (warm) periods. This is important because, currently, we typically only have snapshots of sea-level highstands from low-latitude corals and raised palaeoshoreline indicators while the (continuous) deep-sea oxygen isotope record only provides indirect evidence of sea-level changes. Here, we focus on the Nar Valley in eastern England, in which is preserved evidence of a late middle-Pleistocene marine transgression more than 20 vertical metres in extent. By applying a model of coastal succession and sea-level tendencies, as used in Holocene sea-level studies, we assess the mode (abrupt versus gradual) of sea-level change recorded by the interglacial Nar Valley sequences. Compiled palaeo-stratigraphic evidence comprising foraminifera, pollen and amino acid racemization dating, suggests that the mode of sea-level change in the Nar Valley interglacial sequence was gradual, with potentially two phases of regional transgression and relative sea-level rise occurring at two separate times. The first phase occurred during the latter part of marine Oxygen Isotope Stage (MIS) 11 from ∼8 to 18 m OD; and, the second phase potentially occurred during early MIS 9 from ∼-3 to 3 m OD (with long-term tectonic uplift included in these estimates). We cannot conclusively preclude an alternative MIS 11 age for these lower sediments. The lack of indicators for rapid sea-level oscillations in the Nar Valley adds weight to an argument for steady melt of the ice sheets during both MIS 11 and 9.

The solid Earth's influence on sea level

Because it lies at the intersection of Earth’s solid, liquid, and gaseous components, sea level links the dynamics of the fluid part of the planet with those of the solid part of the planet. Here, I review the past quarter century of sea-level research and show that the solid components of Earth exert a controlling influence on the amplitudes and patterns of sea-level change across time scales ranging from years to billions of years. On the shortest time scales (10 0 –10 2 yr), elastic deformation causes the ground surface to uplift instantaneously near deglaciating areas while the sea surface depresses due to diminished gravitational attraction. This produces spatial variations in rates of relative sea-level change (measured relative to the ground surface), with amplitudes of several millimeters per year. These sea-level “fingerprints” are characteristic of (and may help identify) the deglaciation source, and they can have significant societal importance because they will control rates of coastal inundation in the coming century. On time scales of 10 3 –10 5 yr, the solid Earth’s time-dependent viscous response to deglaciation also produces spatially varying patterns of relative sea-level change, with centimeters-per-year amplitude, that depend on the time-history of deglaciation. These variations, on average, cause net seafloor subsidence and therefore global sea-level drop. On time scales of 10 6 –10 8 yr, convection of Earth’s mantle also supports long-wavelength topographic relief that changes as continents migrate and mantle flow patterns evolve. This changing “dynamic topography” causes meters per millions of years of relative sea-level change, even along seemingly “stable” continental margins, which affects all stratigraphic records of Phanerozoic sea level. Nevertheless, several such records indicate sea-level drop of ∼230 m since a mid-Cretaceous highstand, when continental transgressions were occurring worldwide. This global drop results from several factors that combine to expand the “container” volume of the ocean basins. Most importantly, ridge volume decrease since the mid-Cretaceous, caused by an ∼50% slowdown in seafloor spreading rate documented by tectonic reconstructions, explains ∼250 m of sea-level fall. These tectonic changes have been accompanied by a decline in the volume of volcanic edifices on Pacific seafloor, continental convergence above the former Tethys Ocean, and the onset of glaciation, which dropped sea level by ∼40, ∼20, and ∼60 m, respectively. These drops were approximately offset by an increase in the volume of Atlantic sediments and net seafloor uplift by dynamic topography, which each elevated sea level by ∼60 m. Across supercontinental cycles, expected variations in ridge volume, dynamic topography, and continental compression together roughly explain observed sea-level variations throughout Pangean assembly and dispersal. On the longest time scales (10 9 yr), sea level may change as ocean water is exchanged with reservoirs stored by hydrous minerals within the mantle interior. Mantle cooling during the past few billion years may have accelerated drainage down subduction zones and decreased degassing at mid-ocean ridges, causing enough sea-level drop to impact the Phanerozoic sea-level budget. For all time scales, future advances in the study of sea-level change will result from improved observations of lateral variations in sea-level change, and a better understanding of the solid Earth deformations that cause them.

Relative sea-level rise around East Antarctica during Oligocene glaciation

During the middle and late Eocene (∼ 48-34 Myr ago), the Earth's climate cooled and an ice sheet built up on Antarctica. The stepwise expansion of ice on Antarctica induced crustal deformation and gravitational perturbations around the continent. Close to the ice sheet, sea level rose despite an overall reduction in the mass of the ocean caused by the transfer of water to the ice sheet. Here we identify the crustal response to ice-sheet growth by forcing a glacial-hydro isostatic adjustment model with an Antarctic ice-sheet model. We find that the shelf areas around East Antarctica first shoaled as upper mantle material upwelled and a peripheral forebulge developed. The inner shelf subsequently subsided as lithosphere flexure extended outwards from the ice-sheet margins. Consequently the coasts experienced a progressive relative sea-level rise. Our analysis of sediment cores from the vicinity of the Antarctic ice sheet are in agreement with the spatial patterns of relative sea-level change indicated by our simulations. Our results are consistent with the suggestion that near-field processes such as local sea-level change influence the equilibrium state obtained by an ice-sheet grounding line.

Nineteenth and twentieth century sea-level changes in Tasmania and New Zealand

Positive deviations from linear sea-level trends represent important climate signals if they are persistent and geographically widespread. This paper documents rapid sea-level rise reconstructed from sedimentary records obtained from salt marshes in the Southwest Pacific region (Tasmania and New Zealand). A new late Holocene relative sea-level record from eastern Tasmania was dated by AMS14C (conventional, high precision and bomb-spike), 137Cs, 210Pb, stable Pb isotopic ratios, trace metals, pollen and charcoal analyses. Palaeosea-level positions were determined by foraminiferal analyses. Relative sea level in Tasmania was within half a metre of present sea level for much of the last 6000yr. Between 1900 and 1950 relative sea level rose at an average rate of 4.2±0.1mm/yr. During the latter half of the 20th century the reconstructed rate of relative sea-level rise was 0.7±0.6mm/yr. Our study is consistent with a similar pattern of relative sea-level change recently reconstructed for southern New Zealand. The change in the rate of sea-level rise in the SW Pacific during the early 20th century was larger than in the North Atlantic and could suggest that northern hemisphere land-based ice was the most significant melt source for global sea-level rise.

Late Quaternary sea-level changes and palaeoseismology of the Bering Glacier region, Alaska

Glacial isostatic adjustment and multiple earthquake deformation cycles produce temporal and spatial variability in the records of relative sea-level change across south-central Alaska. Bering Glacier had retreated inland of the present coast by 16 ka BP and north of its present terminus by ∼14 ka BP. Reconnaissance investigations in remote terrain provide new but limited insights of post-glacial relative sea-level change and the palaeoseismology of the region. Relative sea-level was above present ∼9.2 ka BP to at least 5 ka BP before falling to below present. It was above present by the early 20th century, before land uplift in the 1964 M 9.2 earthquake. The pattern of relative sea-level change differs what may be expected in comparison with model predictions for other seismic and non-seismic locations. Buried mud–peat couplets show a great earthquake ∼900 cal BP, including evidence of a tsunami. Correlation with other sites suggest simultaneous rupture of adjacent segments of the Aleutian megathrust and the Yakutat microplate.

An overview of the stratigraphy and facies development of the Jurassic System on the Tabas Block, east-central Iran

Abstract The Tabas Block of east-central Iran shows very thick and well-exposed Upper Triassic–Jurassic sequences, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate. The succession is subdivided into major tectonostratigraphic units based on widespread unconformities related to the Cimmerian tectonic events. As elsewhere in Iran, there is a dramatic change from Middle Triassic platform carbonates (Shotori Formation) to the siliciclastic rocks of the Shemshak Group (Norian–Bajocian), reflecting the onset of Eo-Cimmerian deformation in northern Iran. Following the marine sedimentation of the Norian–Rhaetian Nayband Formation, the change to non-marine, coal-bearing siliciclastic rocks (Ab-e-Haji Formation) around the Triassic–Jurassic boundary is related to the main uplift phase of the Cimmerian orogeny. Condensed limestones of the Toarcian–Aalenian Badamu Formation indicate widespread transgression, followed by rapid lateral facies and thickness variations in the succeeding Lower Bajocian Hojedk Formation. This tectonic instability culminated in the middle Bajocian compressional–extensional Mid-Cimmerian event. The resulting Mid-Cimmerian unconformity separates the Shemshak Group from the Upper Bajocian–Upper Jurassic Magu (or Bidou) Group. The succeeding Late Bajocian–Bathonian onlap of the Parvadeh and Baghamshah formations (Baghamshah Subgroup) was caused by increased subsidence of the Tabas Block rather than a eustatic sea-level rise, followed by the development of a large-scale platform–basin carbonate system (Callovian–Kimmeridgian Esfandiar Subgroup). Block faulting starting in the Kimmeridgian (Late Cimmerian event) resulted in the destruction of the carbonate system, which was covered by Kimmeridgian–Tithonian limestone conglomerates, red beds and evaporites (Garedu Subgroup or Ravar Formation). Virtually the same pattern of relative sea-level change, facies development and succession of geodynamic events is recorded from the Late Triassic–Jurassic of northern Iran (Alborz Mountains), suggesting that the Iran Plate behaved as a single structural unit at that time.

Foraminifera and tidal notches: Dating neotectonic events at Korphos, Greece

Fossil assemblages of foraminifera and thecamoebians from three salt-marsh cores recovered at Korphos, Greece, provided evidence for five transgression events since the mid Holocene. Marsh accretion rates based on radiocarbon-dated peat and geomorphic evidence from a series of discrete, v-shaped, submerged tidal notches indicated that these transgression events were rapid and episodic. Correlation of the tidal notches with the transgression horizons in the salt-marsh stratigraphy revealed a stepwise pattern of relative sea-level change at Korphos, which is best explained by coseismic subsidence related to fault displacement (earthquakes) associated with the Hellenic subduction zone. A comparison between the Korphos data and a model of Holocene sea-level change for the Peloponnesus reinforces this interpretation as sea-level rose in a series of jumps by amounts greater than accounted for by eustatic and glacio-hydro-isostatic factors (up to ~ 2.0 m). This study illustrates that by combining microfossil, sedimentary and geomorphic records of past sea-level change, problems frequently encountered with each record individually (e.g. dating submerged notches and autocompaction of marsh sediments) may be overcome.

Paleoshoreline record of relative Holocene sea levels on Pacific islands

Understanding the history of relative Holocene sea levels on Pacific islands is important for constraining fundamental geodynamic theories, interpreting the environments of early human occupation sites, and forecasting future environmental conditions on the islands. An observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels, hence standing at higher elevations than modern counterparts. Emergent paleoshoreline notches in limestone seacliffs record paleo-high-tide levels and emergent paleoreef flats record paleo-low-tide levels, whereas emergent paleobeachrock locally records paleo-intertidal levels. Both paleonotches and paleoreefs occur along the coasts of high-standing islands exposing volcanic bedrock and uplifted reef complexes, but low-lying coralline atolls lack sufficient relief to preserve paleonotches. Controls on relative Holocene sea level include global eustatic and regional hydro-isostatic changes in ambient sea level relative to island landmasses, and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism, by flexure of the lithosphere under the load of growing volcanic edifices (Hawaii, Samoa, Society Islands), by arching of the lithosphere over trench forebulges (Loyalty Islands, Niue, Bellona–Rennell), and by tectonism within forearc belts between active volcanic chains and trenches (Mariana Islands, Tonga, Vanuatu). The dominant pattern of relative sea-level change, where not overprinted by local tectonism or lithospheric flexure, was a uniform early Holocene rise in eustatic sea level followed by a regionally variable late Holocene hydro-isostatic drawdown in sea level. The resultant was a mid-Holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical Pacific Ocean. The earliest human migrations into intra-oceanic island groups of both the northwestern and southwestern Pacific Ocean followed close on the heels of the mid-Holocene sea-level highstand, and took advantage of newly attractive coastal environments engendered by sea-level drawdown. The effects of the mid-Holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence.

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.