Interglacial Sea-level Highstand Research Articles

Last Interglacial subsurface warming on the Antarctic shelf triggered by reduced deep-ocean convection

The Antarctic ice-sheet could have contributed 3 to 5 m sea-level equivalent to the Last Interglacial sea-level highstand. Such an Antarctic ice-mass loss compared to pre-industrial requires a subsurface warming on the Antarctic shelf of ~ 3 °C according to ice-sheet modelling studies. Here we show that a substantial subsurface warming is simulated south of 60 °S in an equilibrium experiment of the Last Interglacial. It averages +1.2 °C at ~ 500 m depth from 70 °W to 160 °E, and it reaches +2.4 °C near the Lazarev Sea. Weaker deep-ocean convection due to reduced sea-ice formation is the primary driver of this warming. The associated changes in meridional density gradients and surface winds lead to a weakened Antarctic Circumpolar Current and strengthened Antarctic Slope Current, which further impact subsurface temperatures. A subsurface warming on the Antarctic shelf that could trigger ice-mass loss from the Antarctic ice-sheet can thus be obtained during warm periods from reduced sea-ice formation.

Record of the last interglacial sea level highstand based on new coastal deposits in the Cantabrian margin (Northern Iberian Peninsula)

Quaternary sea level fluctuations have been responsible for significant remodelling of coastal landscapes, generating distinctive sea level indicators at different positions and altitudes that are still visible today. Raised coastal deposits are among the characteristic features of significant value in reconstructing the tectonic and sea-level history of the areas in which they are preserved. To date, few remnants of Quaternary coastal deposits have been described and directly dated along the coast of the Cantabrian margin (on the north of the Iberian Peninsula). In this work, two new deposits located in the towns of Castro-Urdiales and Mendexa were analysed and dated using U/Th geochronology and Optically Stimulated Luminescence. Considering the ages and altitudes of these deposits, it seems most likely that they correlate to the Marine Isotope Stage (MIS) 5e. Furthermore, the data acquired suggest that most of the Cantabrian margin underwent a similar tectonic evolution, with no significant tectonic uplift, since at least MIS 5e.

Neanderthal bones collected by hyena at Grotta Guattari, central Italy, 66–65 ka: U/Th chronology and paleoenvironmental setting

After eight decades since its discovery in 1939, new investigations have been undertaken at Grotta Guattari (Latium, central Italy), a coastal cave by the Tyrrhenian Sea coast and one of the iconic sites of the Italian prehistory, as it yielded an almost complete skull and other remains of Neanderthals. The new excavations of the innermost and untouched cave deposits resulted in an outstanding amount of mammal bones, 40 out of which attributable to Neanderthal, including new large portions of cranial remains. Preliminary taphonomic hints and the collected stratigraphic evidence strongly indicate that the impressive accumulation of the large mammal bones was the work of spotted hyena, in a period in which human frequentation was really sporadic or even completely absent. The new acquired U/Th chronology of Grotta Guattari speleothems provided new constraints for reconstructing the sedimentary and paleoenvironmental history of the archaeological successions and human remains. The accumulation of terrestrial sediments started at ∼112 ka, immediately after the end of the Last Interglacial sea-level highstand (∼116 ka). However, the hyena frequentation, and thus the bone accumulations, occurred several thousands of years after and lasted for a very short time interval, precisely between slightly before 66 ka and 65 ka. The cave became abandoned by hyena after ∼65 ka and before ∼59 ka, because of the cave obstruction and/or the altered environmental conditions related to the Heinrich Event 6 (∼64–60 ka). The regional paleoclimatic records indicate severe conditions during the short interval of the hyena frequentation. In contrast, though providing evidence of open and arid environments, the faunal assemblage and the pollen from Grotta Guattari reveal that the local conditions were less severe, likely because of the mitigating effect of the Tyrrhenian Sea. In the framework of the Italian findings, Grotta Guattari results the richest and better chronologically constrained site of Neanderthal remains, posing it as one of the rare sites of the European prehistory that allows putting into the context an extraordinary large sample of the population of Neanderthals, and of other large mammals, in a very narrow and precisely dated temporal interval of the early Pleniglacial.

Did eustatic sea-level control deep-water systems at Milankovitch and timescales?: An answer from Quaternary Pearl River margin

As one of the most important forcing factors, sea level fluctuations exert a major influence on deep-water depositional processes, however, it is still not well understood how they control the evolution of the specific deep-water system over different timescales. For relatively longer (1 My) and shorter (100 Ky) timescales, we characterize deep-water sedimentary records on the Pearl River margin using seismic and borehole data, and then compare them with the contemporaneous sea-level curve to exam the varied roles sea levels have played in impacting the development of the deep-marine system. Results indicate that over both the 1 My-scale and the 100 Ky-scale, the studied deep-water system shows systematic variations that are strongly suggested to arise from the modulation of sea levels. Specifically, over the 1 My-scale, facies and depocenters of the deep-water system all show significant changes. Large fan lobes (consisting of both turbidites and mass-transport deposits) with more distal depocenters likely correspond with low-amplitude and high-frequency sea level fluctuations and slightly rising shelf edge trajectories, whereas smaller turbidite fan lobes with more proximal depocenters likely reflect high-amplitude and low-frequency sea level fluctuations and steeply rising shelf-edge trajectories. In contrast, within the 100 Ky-scale of a glacial-interglacial cycle, the composition of deep-water deposits also shows significant variations. Coarser-grained deposits with higher organic carbon (TOC) content and lower calcium carbonate (CaCO3) content are interpreted to reflect periods of glacial sea-level lowstand, whereas finer-grained sediments having lower TOC and higher CaCO3 content reflect interglacial sea-level highstand. Moreover, the spectrum characteristics of these constituent curves are very similar to those of contemporaneous sea levels, reflecting Milankovitch climatic forcings and further validating the tight coupling between sea-level stands and sediment compositions. These correlations between sedimentary records and sea level behaviors suggest that it was mainly a long-term change in the amplitude and frequency of eustatic cycles that controlled the overall architecture of the deep-water system, whereas it was shorter term, changing sea-level stands that played a role and impacted the deep-water sediment composition.

Compilation of Last Interglacial (Marine Isotope Stage 5e) sea-level indicators in the Bahamas, Turks and Caicos, and the east coast of Florida, USA

Abstract. This paper provides a summary of published sea-level archives representing the past position of sea level during the Last Interglacial sea-level highstand in the Bahamas, Turks and Caicos, and the eastern (Atlantic) coast of Florida, USA. These data were assembled as part of a community effort to build the World Atlas of Last Interglacial Shorelines (WALIS) database. Shallow marine deposits from this sea-level highstand are widespread across the region and are dominated by carbonate sedimentary features. In addition to depositional (constructional) sedimentary indicators of past sea-level position, there is also evidence of erosion, dissolution, and/or subaerial exposure in places that can place an upper limit on the position of sea level. The sea-level indicators that have been observed within this region and attributed to Marine Isotope Stage (MIS) 5e include corals, oolites, and other coastal sedimentary features. Here we compile a total of 50 relative sea-level indicators including 36 in the Bahamas, three in West Caicos, and a remaining 10 for the eastern seaboard of Florida. We have also compiled U-Th age data for 24 fossil corals and 56 oolite samples. While some of these archives have been dated using U-Th disequilibrium methods, amino acid racemization, or optically stimulated luminescence, other features have more uncertain ages that have been deduced in the context of regional mapping and stratigraphy. Sedimentary archives in this region that constrain the elevation of the past position of sea level are associated with uncertainties that range from a couple of decimeters to several meters. Across the Bahamas and on West Caicos, one of the observations that emerges from this compilation is that estimation of sea-level position in this region during Marine Isotope Stage 5e is complicated by widespread stratigraphic evidence for at least one sea-level oscillation. This evidence is defined by submarine features separated by erosion and subaerial exposure, meaning that there were likely multiple distinct peaks in sea level rather than just one. To this end, the timing of these individual sea-level indicators becomes important when compiling and comparing data across the region given that different archives may have formed during different sub-orbital peaks in sea level. The database can be found at https://doi.org/10.5281/zenodo.5596898 (Dutton et al., 2021).

Late Quaternary dynamics of the Lambert Glacier-Amery Ice Shelf system, East Antarctica

The Lambert Glacier-Amery Ice Shelf system (LGAISS) is the largest outlet glacier system in East Antarctica but its response to past climate variability is poorly constrained. In this study, we explore its dynamics over the last ∼520 thousand years using new high-resolution sedimentary records retrieved off Prydz Bay. Episodic occurrences of iceberg rafted debris indicate a dynamic ice sheet throughout this interval, while changes in clay mineral compositions provide detailed evidence on the waxing and waning of the LGAISS. Our data indicate that advance and retreat of the LGAISS was sensitive to oceanic forcing, but also responded to local summer insolation when insolation peaks combined with interglacial sea level high stands. Subglacial bed topography may have played an additional role in modulating the LGAISS behaviour, which could explain sectoral differences in the response of the East Antarctic Ice Sheet to climate change. Overall, our records indicate that the LGAISS advanced more extensively during previous late Quaternary glacial periods than during Marine Isotope Stages 2 and 4. Furthermore, the LGAISS retreated more significantly than present during Marine Isotope Stage 13, and only moderately during Marine Isotope Stage 7, which suggests that the duration of warm climatic states could be a key factor affecting the dynamics of the East Antarctic Ice Sheet.

Last Interglacial sea-level highstand deduced from notches and inner margins of marine terraces at Puerto Deseado, Santa Cruz Province, Argentina

Fil: Bini, Monica. Istituto Nazionale di Geofisica e Vulcanologia; Italia. Universita degli Studi di Pisa; Italia

Flank Margin Caves and the Position of Mid- to Late Pleistocene Sea Level in the Bahamas

Mylroie, J.; Lace, M.; Albury, N., and Mylroie, J., 2020. Flank margin caves and the position of mid- to late Pleistocene sea level in the Bahamas. Journal of Coastal Research, 36(2), 249–260. Coconut Creek (Florida), ISSN 0749-0208.The Bahamian Archipelago has abundant fossil coral reefs and related subtidal deposits as evidence of the last interglacial (marine isotope stage [MIS] 5e) sea-level highstand; evidence of earlier highstands from the mid-Pleistocene is limited and controversial (excepting Mayaguana Island). Data from flank margin cave elevations, used as a sea-level proxy, were initially interpreted to demonstrate an MIS 5e origin derived from ∼60 mapped caves across the archipelago. A new analysis of cave morphologies and distribution has now produced 363 maps of flank margin caves, demonstrating for the first time that 26 caves with associated elevations between +8 and +24 m above modern sea level are found spanning the northwest to the southeast boundaries of the archipelago. Flank margin caves are the primary remaining evidence of past sea-level position, because almost all mid-Pleistocene subtidal deposits, and related sea-level indicators such as subtidal facies, sea caves, and bioerosion notches, have been removed by karst denudation. Cave elevations up to 24 m (above sea level) indicate that prior assumptions as to the rate of subsidence of the Bahama Banks at 1–2 m per 100 ka may not be correct. The activity of MIS 5e was recent enough, and its subtidal deposits voluminous enough, to survive to the present, along with the majority of flank margin caves formed at that time. Karst denudation may have created sufficient mass loss that isostatic subsidence stopped, or was possibly reversed, as has been demonstrated for the Florida peninsula. If true, then mid-Pleistocene flank margin caves may not represent eustatic sea-level position at the time of speleogenesis.

Biological Indicators Reveal Small-Scale Sea-Level Variability During MIS 5e (Sur, Sultanate of Oman)

In this study we aim to document various coastal notches in Sur Lagoon (Oman) and interpret them regarding their use as sea-level indicator. We also unravel any short-term sea-level fluctuations, which are potentially preserved within the trace fossil assemblages of some of the notches. The oldest paleo notches stem from the last interglacial sea-level highstand of MIS 5e. This is concluded from cosmogenic nuclide dating of the fanglomerate bedrock in Sur Lagoon as presented in this study. All outcrops of paleo notches around Sur Lagoon were investigated in regards to the faunal distribution and notch shape. Furthermore, the absolute elevation of the notches and biological markers relative to msl were measured with a differential GPS. The bioerosional notch occurs at the same height around Sur Lagoon indicating that the area remained tectonically stable over the last 125 kyr. According to the elevation of the notch-apex, msl was 3.93 ± 0.12 m higher than today during the last interglacial. The distribution of boring and bioconstructing organisms relative to the notch shape displays at least one phase of short-term sea-level rise subsequent to the notch formation. The beachrocks that are associated with the bioerosion notches in Sur Lagoon show a larger grainsize than any sediment that is deposited in the lagoon nowadays. This, in combination with the occurrence of exceptionally high and deep abrasion notches, indicates that the coastline was more openly exposed and thus experienced a higher wave energy during the notch formation.

Asynchronous Antarctic and Greenland ice-volume contributions to the last interglacial sea-level highstand

The last interglacial (LIG; ~130 to ~118 thousand years ago, ka) was the last time global sea level rose well above the present level. Greenland Ice Sheet (GrIS) contributions were insufficient to explain the highstand, so that substantial Antarctic Ice Sheet (AIS) reduction is implied. However, the nature and drivers of GrIS and AIS reductions remain enigmatic, even though they may be critical for understanding future sea-level rise. Here we complement existing records with new data, and reveal that the LIG contained an AIS-derived highstand from ~129.5 to ~125 ka, a lowstand centred on 125–124 ka, and joint AIS + GrIS contributions from ~123.5 to ~118 ka. Moreover, a dual substructure within the first highstand suggests temporal variability in the AIS contributions. Implied rates of sea-level rise are high (up to several meters per century; m c−1), and lend credibility to high rates inferred by ice modelling under certain ice-shelf instability parameterisations.

Nonlinear response of the Antarctic Ice Sheet to late Quaternary sea level and climate forcing

Abstract. Antarctic ice volume has varied substantially during the late Quaternary, with reconstructions suggesting a glacial ice sheet extending to the continental shelf break and interglacial sea level highstands of several meters. Throughout this period, changes in the Antarctic Ice Sheet were driven by changes in atmospheric and oceanic conditions and global sea level; yet, so far modeling studies have not addressed which of these environmental forcings dominate and how they interact in the dynamical ice sheet response. Here, we force an Antarctic Ice Sheet model with global sea level reconstructions and transient, spatially explicit boundary conditions from a 408 ka climate model simulation, not only in concert with each other but, for the first time, also separately. We find that together these forcings drive glacial–interglacial ice volume changes of 12–14 ms.l.e., in line with reconstructions and previous modeling studies. None of the individual drivers – atmospheric temperature and precipitation, ocean temperatures, or sea level – single-handedly explains the full ice sheet response. In fact, the sum of the individual ice volume changes amounts to less than half of the full ice volume response, indicating the existence of strong nonlinearities and forcing synergy. Both sea level and atmospheric forcing are necessary to create full glacial ice sheet growth, whereas the contribution of ocean melt changes is found to be more a function of ice sheet geometry than climatic change. Our results highlight the importance of accurately representing the relative timing of forcings of past ice sheet simulations and underscore the need for developing coupled climate–ice sheet modeling frameworks that properly capture key feedbacks.

First evidence for onshore marine isotope stage 3 aeolianite formation on the southern Cape coastline of South Africa

The southern Cape coast of South Africa boasts an impressive suite of Plio-Pleistocene aeolian dune deposits (aeolianite). Previous research has shown that in this region onshore dune accumulation was generally focused around interglacial sea level highstands, with the locus of coastal dune accumulation shifting onto the adjacent continental shelf during glacial sea level lowstands. Here, using new luminescence dating results, we present the first evidence for preserved onshore glacial age dunes. Specifically, on the Robberg Peninsula, a rocky headland 28 km east of Knysna, two phases of aeolianite formation are identified, corresponding to early (45–60 ka) and late (35–30 ka) marine isotope stage (MIS) 3. Subsequently, during the Holocene, all substantive dune accumulation occurred between 10.2 and 7.0 ka, forming cliff-fronting dunes and filling the limited accommodation space on the headland, including an archaeological rock-shelter. Combining these ages with bathymetric data, we infer that this distinct onshore glacial age aeolianite record reflects: 1) restricted accommodation space during sea level highstands; 2) a regional narrowing of the continental shelf, and 3) liberation of sediments lying on a prominent −45 to −60 m offshore terrace, which would have been exposed during MIS 3. This demonstrates that despite broad regional-scale trends in the timing of coastal aeolian activity - driven by commonalities in relative sea level trends and climate - distinct local variations in late Quaternary coastal evolution can be identified. This is ascribed to local controls on preservation (accommodation space) and sediment supply (shoreline position and antecedent offshore sediment supplies). Such findings may have wider implications for interpretations of site context/resource availability at several notable coastal archaeological sites, and more broadly suggest that local offshore or onshore geologic contexts can at times assume greater influence on a preserved coastal aeolianite record than the regional-scale trends in sea level and climate.

Identification of a penultimate interglacial (marine isotope stage 7) alluvium in South Australia and its climatic and sea-level implications

ABSTRACTAlluvial sequences proximal to coastlines offer opportunities to establish associations between terrestrial, sea-level and climatic events. South Australia hosts a globally significant Pleistocene interglacial sea-level record and numerous terrestrial sediment sources. However, only fragmentary evidence of pre-Last Interglacial alluvium has been identified. This paper presents the first definitive recognition of MIS 7 alluvium in South Australia, which occurs beneath the surface of extensive river terraces flanking Currency Creek and the Finniss River, between the Mount Lofty Ranges and the River Murray Lakes. A thermoluminescence age, 227 ± 24 ka, correlates with the penultimate interglacial global sea-level highstand. Nearby, last interglacial fossils of the estuarine bivalve Spisula trigonella at 2.53 ± 0.25 m APSL occupy a hollow eroded into the MIS 7 alluvium. Increasing aridity and decreased fluvial activity in the late Quaternary have preserved the MIS 7 alluvium. The fragmentary record of alluvium pre-dating the Last Interglacial is attributed to three principal causes: (1) tectonic subsidence of the Murray Estuary, which increased the potential for burial or coastal erosion of sediments; (2) erosion and reworking of previously existing alluviums, especially during low sea levels of glacial times; (3) the absence of reliable dating controls on the potentially older alluvial sediments.

Episodic reef growth in the granitic Seychelles during the Last Interglacial: Implications for polar ice sheet dynamics

We present a detailed taxonomic and lithologic analysis of fossil reefs in the Granitic Seychelles to improve constraints on global mean sea level behavior during the last interglacial sea-level highstand, ~129,000–116,000 years ago. In particular, we have assessed the coralgal reef outcrops for any sedimentalogic evidence of sub-orbital sea level oscillations that may have occurred during this warm, interglacial period. At the outcrop scale, we observe at least three distinct episodes of reef growth punctuated by two discontinuities that typically manifest as coral rubble layers or extensive lateral encrustations of the hydrozoan coral Millepora exaesa. Hiatuses in reef growth at other sites around the world have been interpreted as relative sea level falls and/or environmental disturbances. We have tested the hypothesis that these discontinuities, that we refer to as disturbance horizons, are related to ephemeral drops in sea level that have been inferred at last interglacial reef sites from other sites around the globe. At two sites, there is sedimentological evidence in the form of extensive dissolution and freshwater cements associated with the disturbance horizons that would require subaerial exposure followed by marine inundation. At the remaining sites, evidence is consistent with, but not necessarily indicative of, an ephemeral drop in sea level. To establish if these disturbance horizons are coeval between the outcrops and if they are local or global features, requires high-precision dating of the disturbance horizons along with a critical analysis of sedimentary and chronological evidence from other sites around the globe. If the disturbance horizons observed in the Seychelles represent transient sea-level falls or still stands during the last interglacial, this would imply a dynamic behavior of the polar ice sheets during this past warm period.

The last interglacial (MIS 5e) sea level highstand from a tectonically stable far-field setting, Yorke Peninsula, southern Australia

Yorke Peninsula in southern Australia is an important region for reconstructing relative sea-level histories due to its location on the eastern margin of the tectonically stable Gawler Craton and in one of the world's geographically most remote far-field locations from the Pleistocene ice sheets. Richly fossiliferous, skeletal carbonate sands of the last interglacial (125 ka) Glanville Formation crop out in the coastal cliffs along large sectors of southern Yorke Peninsula. Sedimentary facies include deepening-upward intertidal to shallow subtidal facies, relict storm beach facies and cobble and boulder beach deposits in more exposed, higher energy locations. During deposition of the Glanville Formation, southern Yorke Peninsula had a different coastal geography with two prominent marine corridors extending across the southern-most portion of the entire peninsula. In a 3 km long coastal cliff section in southern Hardwicke Bay, the Glanville Formation crops out as an upward-deepening intertidal-subtidal succession capped by supratidal and subaerially-exposed sediments with pervasive calcrete development. The sedimentary succession passes upwards from a basal unit of intertidal sand flat facies with abundant gastropods (Batillaria diemenensis) near the upper bounding (disconformity) surface, upwards into richly fossiliferous shelly sands (coquina) representing a shallow subtidal facies formed by sediment aggradation in response to a relative sea-level rise. The subtidal facies is dominated by the bivalve molluscs Katelysia sp. and Amesodesma angusta, signifying a water deepening event. The subtidal facies is in turn overlain by pedogenically modified skeletal carbonate sands with pervasive calcrete development signifying a relative sea level fall at the end of the Last Interglacial Maximum. The upper-bounding surface of the shallow-water subtidal facies ranges from 2.4 to 3.0 m Australian Height Datum (AHD) and by analogy with modern sedimentary environments suggests a maximum palaeo-sea level of 4.8 ± 1.0 m during the Last Interglacial Maximum. Uranium-series ages of 127.3 ± 2.1 to 115.0 ± 5.4 ka on specimens of the solitary coral Plesiastrea versipora from the subtidal facies confirm that the succession was deposited during the Last Interglacial Maximum, and are consistent with the independent stratigraphical evidence that the highstand event was represented by a single phase of relative sea-level rise. Correlation with other occurrences of the Glanville Formation in southern Australia has also been confirmed by aminostratigraphy.

Last Interglacial sea-level highstand deduced from notches and inner margins of marine terraces at Puerto Deseado, Santa Cruz Province, Argentina

Fil: Bini, Monica. Istituto Nazionale di Geofisica e Vulcanologia; Italia. Universita degli Studi di Pisa; Italia

Sensitivity of Last Interglacial sea-level high stands to ice sheet configuration during Marine Isotope Stage 6

Estimates of peak global mean sea level (GMSL) during the Last Interglacial (LIG, ∼129-116 ka) based on geological sea level high-stand markers require a correction for the contaminating influence of glacial isostatic adjustment (GIA). This correction is obtained by calculating the viscoelastic response of the Earth to changes in the ice and ocean load prior to and following the LIG. While ice retreat over the last deglaciation is relatively well constrained, changes in ice cover prior to the LIG are more uncertain. We investigate the sensitivity of numerical predictions of GIA during the LIG to variations in the geometry of pre-LIG ice cover and the timing of the deglaciation into the LIG, with a particular focus on Marine Isotope Stage (MIS) 6 (∼190-130 ka). We demonstrate that reconstructing the pre-LIG ice history by replicating the last glacial cycle back in time, rather than using ice volume approximations based on oxygen isotope records, can introduce errors in LIG high-stand predictions of ∼5 m at sites on the peripheral bulge of major ice complexes, and up to ∼2 m at far-field sites. We also demonstrate that predictions of LIG sea level are more sensitive to the geographic distribution of ice cover during MIS 6 than previously recognized. Adopting simulations which vary the relative size of Late Pleistocene ice cover over North America and Eurasia can yield a change in predicted high-stand elevations of ∼5 m in both the near and far field of northern hemisphere ice sheets. This far-field sensitivity arises, in part, from the reorientation of Earth's rotation axis during MIS 6, which in turn drives sea-level changes with a distinct geographic signature. In future work we will apply the insights gained here to re-evaluate the observed geographic variability in geological high-stand markers of LIG age and estimates of GMSL based upon them.

Low Florida coral calcification rates in the Plio-Pleistocene

Abstract. In geological outcrops and drill cores from reef frameworks, the skeletons of scleractinian corals are usually leached and more or less completely transformed into sparry calcite because the highly porous skeletons formed of metastable aragonite (CaCO3) undergo rapid diagenetic alteration. Upon alteration, ghost structures of the distinct annual growth bands often allow for reconstructions of annual extension ( = growth) rates, but information on skeletal density needed for reconstructions of calcification rates is invariably lost. This report presents the bulk density, extension rates and calcification rates of fossil reef corals which underwent minor diagenetic alteration only. The corals derive from unlithified shallow water carbonates of the Florida platform (south-eastern USA), which formed during four interglacial sea level highstands dated approximately 3.2, 2.9, 1.8, and 1.2 Ma in the mid-Pliocene to early Pleistocene. With regard to the preservation, the coral skeletons display smooth growth surfaces with minor volumes of marine aragonite cement within intra-skeletal porosity. Within the skeletal structures, voids are commonly present along centres of calcification which lack secondary cements. Mean extension rates were 0.44 ± 0.19 cm yr−1 (range 0.16 to 0.86 cm yr−1), mean bulk density was 0.96 ± 0.36 g cm−3 (range 0.55 to 1.83 g cm−3) and calcification rates ranged from 0.18 to 0.82 g cm−2 yr−1 (mean 0.38 ± 0.16 g cm−2 yr−1), values which are 50 % of modern shallow-water reef corals. To understand the possible mechanisms behind these low calcification rates, we compared the fossil calcification rates with those of modern zooxanthellate corals (z corals) from the Western Atlantic (WA) and Indo-Pacific calibrated against sea surface temperature (SST). In the fossil data, we found a widely analogous relationship with SST in z corals from the WA, i.e. density increases and extension rate decreases with increasing SST, but over a significantly larger temperature window during the Plio-Pleistocene. With regard to the environment of coral growth, stable isotope proxy data from the fossil corals and the overall structure of the ancient shallow marine communities are consistent with a well-mixed, open marine environment similar to the present-day Florida Reef Tract, but variably affected by intermittent upwelling. Upwelling along the platform may explain low rates of reef coral calcification and inorganic cementation, but is too localised to account also for low extension rates of Pliocene z corals throughout the tropical WA region. Low aragonite saturation on a more global scale in response to rapid glacial–interglacial CO2 cyclicity is also a potential factor, but Plio-Pleistocene atmospheric pCO2 is generally believed to have been broadly similar to the present day. Heat stress related to globally high interglacial SST only episodically moderated by intermittent upwelling affecting the Florida platform seems to be another likely reason for low calcification rates. From these observations we suggest some present coral reef systems to be endangered from future ocean warming.

OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz

The comparison of optically stimulated luminescence (OSL) ages of different grain-size fractions has given rise to conflicting results over recent years. To test the reliability and validity of OSL dating of fine- and coarse-grained fractions, 25 samples were collected from a 46-m sediment core taken from the southwestern coast of Korea, which contained two typically fine-grained tidal units separated stratigraphically by a coarse-grained sand and gravel layer. A single aliquot regenerative dose (SAR) procedure was applied to chemically separated fine- (4–11 μm, n = 16) and coarse- (90–212 μm, n = 24) grained quartz. The ages of coarse-grained quartz in the upper unit were relatively consistent with those of fine-grained quartz in the same unit of Holocene age (12–0 ka). In contrast, in the lower unit the coarse quartz fraction OSL ages are 30–60% (mean 46%) lower than the fine quartz fraction ages. OSL ages for all fine-grained quartz in the lower unit (139–110 ka) were in good agreement with the last interglacial sea-level highstand [Marine Isotope Stage (MIS) 5e]. However, all OSL ages for coarse-grained quartz in this lower unit (90–45 ka) were younger, corresponding broadly to the last glacial period (MIS 4 and 3). OSL ages obtained for fine-grained quartz were in agreement with indirect age controls provided by lithological and palynological data, implying that the use of the SAR protocol to date fine-grained quartz is appropriate for age control of coastal sediments extending to the last interglacial period. The discrepancies between the ages of different grain-size fractions may have arisen from early saturation of the natural signal and, in some cases, from contamination of the feldspar signal in the coarse quartz. However, more research is required to assess the reliability of OSL ages for fine-grained quartz obtained from the high-dose linear region of the dose response curve.

Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at the last interglacial sea-level highstand: Kaua'i, Hawai'i

Upstream knickpoint propagation is an important mechanism for channel incision, and it communicates changes in climate, sea level, and tectonics throughout a landscape. Few studies have directly measured the long-term rate of knickpoint retreat, however, and the mechanisms for knickpoint initiation are debated. Here, we use cosmogenic ^3He exposure dating to document the retreat rate of a waterfall in Ka’ula’ula Valley, Kaua‘i, Hawai‘i, an often-used site for knickpoint-erosion modeling. Cosmogenic exposure ages of abandoned surfaces are oldest near the coast (120 ka) and systematically decrease with upstream distance toward the waterfall (<10 ka), suggesting that the waterfall migrated nearly 4 km over the past 120 k.y. at an average rate of 33 mm/yr. Upstream of the knickpoint, cosmogenic nuclide concentrations in the channel are approximately uniform and indicate steady-state vertical erosion at a rate of ∼0.03 mm/yr. Field observations and topographic analysis suggest that waterfall retreat is dominated by block toppling, with sediment transport below the waterfall actively occurring by debris flows. Knickpoint initiation was previously attributed to a submarine landslide ca. 4 Ma; however, our dating results, bathymetric analysis, and landscape-evolution modeling support knickpoint generation by wave-induced sea-cliff erosion during the last interglacial sea-level highstand ca. 120–130 ka. We illustrate that knickpoint generation during sea-level highstands, as opposed to the typical case of sea-level fall, is an important relief-generating mechanism on stable or subsiding steep coasts, and likely drives transient pulses of significant sediment flux.