Relative Sea-level Reconstructions Research Articles

Evaluating ancient coastal wells as sea‐level indicators from the coast of Israel

AbstractThis paper re‐evaluates previous records and adds new data to the relative sea level (RSL; namely the sea level related to the level of the national terrestrial datum) reconstruction in Israel from the last 2400 years based on ancient coastal water wells. Different methods for obtaining the most accurate modern offset between coastal groundwater level and mean sea level are also tested, since this is a crucial component for using wells as reliable sea‐level indicators in the Mediterranean and beyond. Very few, if any, sea‐level studies outside of Israel use coastal wells as RSL indicators; refinements to the methods associated with this indicator type will facilitate their use in other regions. Coastal wells in the study are located within 200 m of the present‐day coastline. The functional age range for the wells is based on site stratigraphy, artifacts, historical records, and the coastal water well typology. These archaeological indicators rely on finding the vertical offset between present‐day coastal groundwater and sea level, and applying it to ancient times. The current study explores two methods to calculate accurate vertical offsets: a) modern averages of the upper aquifer groundwater elevation in the vicinity of ancient wells, which are calculated from 60 years of measurements by the Israeli Hydrological Service; and b) a hydrological model where groundwater elevation is a function of distance from the modern coastline. The model uses a simulation of the coastal aquifer with a two‐dimensional, vertical cross‐section. The modeled approach is found to be preferable. The results show that sea level in Roman times (1st century CE) was similar to present‐day, slightly higher in the Byzantine period (4th to 7th centuries CE), and lower in part of the Early Islamic period (9th to 10th centuries CE). For the later Fatimid/Ayyubid and Crusader periods (11th to 13th centuries CE), RSL reaches its lowest elevations (of about −0.5 m) as previously found. Following these low levels, sea level rose going into the 18th and 19th centuries CE.

On the application of contemporary bulk sediment organic carbon isotope and geochemical datasets for Holocene sea-level reconstruction in NW Europe

Bulk organic stable carbon isotope (δ13C) and element geochemistry (total organic carbon (TOC) and organic carbon to total nitrogen (C/N)) analysis is a developing technique in Holocene relative sea-level (RSL) research. The uptake of this technique in Northern Europe is limited compared to North America, where the common existence of coastal marshes with isotopically distinctive C3 and C4 vegetation associated with well-defined inundation tolerance permits the reconstruction of RSL in the sediment record. In Northern Europe, the reduced range in δ13C values between organic matter sources in C3 estuaries can make the identification of elevation-dependent environments in the Holocene sediment record challenging and this is compounded by the potential for post-depositional alteration in bulk δ13C values. The use of contemporary regional δ13C, C/N and TOC datasets representing the range of physiographic conditions commonly encountered in coastal wetland sediment sequences opens up the potential of using absolute values of sediment geochemistry to infer depositional environments and associated reference water levels. In this paper, the application of contemporary bulk organic δ13C, C/N and TOC to reconstruct Holocene RSL is further explored. An extended contemporary regional geochemical dataset of published δ13C, C/N and TOC observations (n=142) from tidal-dominated C3 wetland deposits (representing tidal flat, saltmarsh, reedswamp and fen carr environments) in temperate NW Europe is compiled, and procedures implemented to correct for the 13C Suess effect on contemporary δ13C are detailed. Partitioning around medoids analysis identifies two distinctive geochemical groups in the NW European dataset, with tidal flat/saltmarsh and reedswamp/fen carr environments exhibiting characteristically different sediment δ13C, C/N and TOC values. A logistic regression model is developed from the NW European dataset in order to objectively identify in the sediment record geochemical groups and, more importantly, group transitions, thus allowing the altitude of reference water levels to be determined. The application of this method in RSL research is demonstrated using the Holocene sediments of the Mersey Estuary (UK), in which δ13C, C/N and TOC variability is typical of that encountered in Holocene sediments from C3 coastal wetlands in NW Europe. Group membership was predicted with high probability in the depositional contexts studied and the accuracy of group prediction is verified by microfossil evidence. The method presented facilitates the application of δ13C, C/N and TOC analysis in RSL reconstruction studies in C3 vegetated wetlands throughout temperate NW Europe.

Carbon stable isotope (δ13C) and elemental (TOC, TN) geochemistry in saltmarsh surface sediments (Western Brittany, France): a useful tool for reconstructing Holocene relative sea‐level

ABSTRACTProviding a dense network of Holocene relative sea‐level (RSL) records still faces some challenges, such as the paucity of microfossil assemblages within some Holocene sedimentary sequences. Recently, the isotopic (δ13C) and elemental geochemistry [total organic carbon (TOC), total nitrogen (TN) and C/N ratios] of saltmarsh sediments has showed potential for RSL reconstruction. This study presents the results of new δ13C, TOC and TN measurements obtained on surface sediments of two marshes of the Brittany region (France). Overall, the distribution of δ13C, TOC, TN and C/N values shows clear dependencies with elevation. Sediments from above and below the high tide level show contrasting TN contents. Clustering analyses identified three elevation‐dependent groups, characterized by specific values of δ13C, TOC and TN. The use of this geochemical reference for RSL reconstruction is tested on a core retrieved from one of the marshes. The sea‐level index points obtained plot in good accordance with the microfauna‐based RSL positions previously obtained in the region. Some diagenetic losses in TN are observed between modern and Holocene sediments. This does not appear to be harmful to the accuracy of the RSL reconstructions but suggests that C/N ratios may be unreliable palaeoenvironmental proxies in this specific context.

Applicability of transfer functions for relative sea-level reconstructions in the southern North Sea coastal region based on salt-marsh foraminifera

Abstract We studied the foraminiferal distribution in two naturally grown salt marshes from the southern North Sea with respect to the tidal frame, salinity, grain size and pH. The salt marshes are situated on the landward sides of the islands of Sylt (Rantum, Germany) and Fano (Sonderho, Denmark). In both study areas, foraminifera have a vertical distribution with respect to water level, but also show inter-site variability, which can be related to environmental differences (e.g., in salinity and pH) and different flooding dynamics of the coastal salt marsh (Rantum) and the tidal-creek salt marsh (Sonderho). We developed different transfer functions, based on the widely applied standardized water level index (SWLI) approach and on three flooding parameters (duration of submergence (DoS), mean submergence time (MST), and flooding frequency (FF)), in order to assess their predictive ability for relative sea-level estimates for the southern North Sea coastal region. The water-level data used for these approaches are determined based on local water-level conditions, corrected for tidal distortions using water-level measurements for the Sonderho salt marsh and the Rantum tide gauge. The SWLI approach shows a precision (root mean squared error of prediction (RMSEP) of 0.23 m), which is around 15% of the tidal range. All three flooding approaches show comparable results at around mean tide level to mean high water, while at higher elevations, foraminiferal distribution becomes non-linearly correlated to flooding parameters resulting in lower precision of > 1.0 m. Our results suggest that the SWLI approach performs well and that the flooding approaches offer a suitable addition for assessing relative sea-level estimates in the North Sea region. We enhanced the knowledge on changing precision for tide level reconstructions along the elevational gradient in a storm dominated tidal area where elevation and flooding parameters are non-linear correlated.

Exploring mechanisms of compaction in salt-marsh sediments using Common Era relative sea-level reconstructions

Salt-marsh sediments provide precise and near-continuous reconstructions of Common Era relative sea level (RSL). However, organic and low-density salt-marsh sediments are prone to compaction processes that cause post-depositional distortion of the stratigraphic column used to reconstruct RSL. We compared two RSL reconstructions from East River Marsh (Connecticut, USA) to assess the contribution of mechanical compression and biodegradation to compaction of salt-marsh sediments and their subsequent influence on RSL reconstructions. The first, existing reconstruction (‘trench’) was produced from a continuous sequence of basal salt-marsh sediment and is unaffected by compaction. The second, new reconstruction is from a compaction-susceptible core taken at the same location. We highlight that sediment compaction is the only feasible mechanism for explaining the observed differences in RSL reconstructed from the trench and core. Both reconstructions display long-term RSL rise of ∼1 mm/yr, followed by a ∼19th Century acceleration to ∼3 mm/yr. A statistically-significant difference between the records at ∼1100 to 1800 CE could not be explained by a compression-only geotechnical model. We suggest that the warmer and drier conditions of the Medieval Climate Anomaly (MCA) resulted in an increase in sediment compressibility during this time period. We adapted the geotechnical model by reducing the compressive strength of MCA sediments to simulate this softening of sediments. ‘Decompaction’ of the core reconstruction with this modified model accounted for the difference between the two RSL reconstructions. Our results demonstrate that compression-only geotechnical models may be inadequate for estimating compaction and post-depositional lowering of susceptible organic salt-marsh sediments in some settings. This has important implications for our understanding of the drivers of sea-level change. Further, our results suggest that future climate changes may make salt marshes more susceptible to the impacts of RSL rise by enhancing sediment compressibility. We stress, however, that the cause of the softening remains enigmatic. Until this is better constrained, it is premature to widely extrapolate our findings to existing core-based reconstructions of Holocene RSL.

Source organic matter analysis of saltmarsh sediments using SIAR and its application in relative sea‐level studies in regions of C4 plant invasion

The introduction of invasive Spartina grasses (C4 plant) into previously C3‐dominated saltmarshes of western Europe complicates the reconstruction of past relative sea‐level (RSL) using sediment geochemistry‐based approaches. We apply a Bayesian mixing model (SIAR – Stable Isotope Analysis in R) to identify organic carbon (Corg) source contributions to saltmarsh sediment and correct δ13C values for the contaminating influence of C4 vegetation. We first assess the performance of SIAR by simulation, producing synthetic saltmarsh sediment samples with variable contributions from three organic matter sources (C3 plants, C4 plants and particulate organic matter (POM)) and then comparing these known contributions to those determined by SIAR. For each source type, SIAR 68% credible intervals include true source contribution in 70% of calculations (n = 60) and all three sources were incorporated within credible intervals for 45% of sediment samples (n = 20). SIAR improves the mean absolute error of source contribution predictions by up to 26% compared to a conventional ternary mixing model approach. We then apply SIAR to real sediment samples recovered along transects spanning the intertidal zones at two contrasting saltmarshes in the inner and outer Shannon estuary, western Ireland. SIAR identified decreasing Corg contributions from POM with increasing elevation in the intertidal zone, reflecting differences in tidal inundation with height. Sediment δ13C values across the intertidal zones varied by 4.4‰ in the inner estuary and 9.4‰ in the outer estuary. Removal of the invasive C4 signatures reduced these differences to 0.5‰ and to 2.4‰ in the inner and outer estuary, respectively, with the larger range due to increased deposition of marine POM. Although subtle, when used in combination with additional proxies, these geochemical gradients can assist in RSL reconstruction. This will be most effective when POM from marine sources is a significant carbon source.

Fossil bog soils (‘dwog horizons’) and their relation to Holocene coastal changes in the Jade Weser region, southern North Sea, Germany

After the deceleration of the postglacial marine transgression, the German North Sea coast was subject to deposition of a complex pattern of subtidal to terrestrial facies. This study aims at reconstructing these facies changes in the ‘Land Wursten’ region (Lower Saxony) by focussing on so-called dwog horizons (incipient soil horizons of the pre-engineered salt marshes). We explore their implications for relative sea-level reconstructions and their indication for early settlement activities. Archive drilling data (1960, provided by the ‘Landesamt fur Bergbau, Energie und Geologie’) were analysed to create five high-resolution stratigraphic cross sections. Three new drilling records were subjected to sedimentological and microfaunal investigations and interpreted to verify and calibrate the archive data. Two dwog horizons were found and 14C-AMS dated. We found basal salt marsh deposits inundated by a high-energy event and covered by tidal flat sediments. The thick tidal flat unit is again overlain by salt marsh deposits indicating the transition from a shallow marine to a terrestrial environment where dwogs were developed and covered by episodic marine incursions. The ages of the dwogs (1128–969 cal BC; cal AD 1426–1467) do not correlate with known layers of adjacent settlement sites and are critically discussed. However, we show that they correlate with phases of stagnant regional relative sea level (RSL) and can be used as RSL indicators. The combined archive and modern data provide valuable information for the RSL reconstruction and palaeoenvironmental changes. However, further research is recommended to accomplish more detailed information about coastal response during the Holocene sea-level changes and implications for settlement dynamics.

Reconstructing the accumulation history of a saltmarsh sediment core: Which age-depth model is best?

Saltmarsh-based reconstructions of relative sea-level (RSL) change play a central role in current efforts seeking to quantify the relationship between climate and sea-level rise. The development of an accurate chronology is pivotal, since errors in age–depth relationships will propagate to the final record as alterations in both the timing and magnitude of reconstructed change. A range of age-depth modelling packages are available but differences in their theoretical basis and practical operation mean contrasting accumulation histories can be produced from the same dataset. We compare the performance of five age-depth modelling programs (Bacon, Bchron, Bpeat, Clam and OxCal) when applied to the kinds of data used in high resolution, saltmarsh-based RSL reconstructions. We investigate their relative performance by comparing modelled accumulation curves against known age–depth relationships generated from simulated stratigraphic sequences. Bpeat is particularly sensitive to non-linearities which, whilst maximising the detection of small rate changes, has the potential to generate spurious variations, particularly in the last 400 years. Bacon generally replicates the pattern and magnitude of change but with notable offsets in timing. Bchron and OxCal successfully constrain the known accumulation history within their error envelopes although the best-fit solutions tend to underestimate the magnitude of change. The best-fit solutions of Clam generally replicate the timing and magnitude of changes well, but are sensitive to the underlying shape of the calibration curve, performing poorly where plateaus in atmospheric 14C concentration exist. We employ an ensemble of age-depth models to reconstruct a 1500 year accumulation history for a saltmarsh core recovered from Connecticut, USA based on a composite chronology comprising 26 AMS radiocarbon dates, 210Pb, 137Cs radionuclides and an historical pollen chronohorizon. The resulting record reveals non-linear accumulation during the late Holocene with a marked increase in rate around AD1800. With the exception of the interval between AD1500 and AD1800, all models produce accumulation curves that agree to within ∼10 cm at the century-scale. The accumulation rate increase around AD1800 is associated with the transition from a radiocarbon-based to a 210Pb-dominated chronology. Whilst repeat analysis excluding the 210Pb data alters the precise timing and magnitude of this acceleration, a shift to faster accumulation compared to the long-term rate is a robust feature of the record and not simply an artefact of the switch in dating methods. Simulation indicates that a rise of similar magnitude to the post-AD1800 increase (detrended increase of ∼16 cm) is theoretically constrained and detectable within the radiocarbon-dated portion of the record. The absence of such a signal suggests that the recent rate of accumulation is unprecedented in the last 1500 years. Our results indicate that reliable (sub)century-scale age-depth models can be developed from saltmarsh sequences, and that the vertical uncertainties associated with them translate to RSL reconstruction errors that are typically smaller than those associated with the most precise microfossil-based estimates of palaeomarsh-surface elevation.

Postglacial relative sea-level history of the Prince Rupert area, British Columbia, Canada

This paper presents a history of relative sea level (RSL) change for the last 15,000 years in the Prince Rupert region on the northern coast of British Columbia, Canada. One hundred twenty-three radiocarbon ages of organic material from isolation basin cores, sediment sequence exposures, and archaeological sites having a recognized relation to past sea levels constrain postglacial RSL. The large number of new measurements relating to past sea-level provides a well constrained RSL curve that differs in significant ways from previously published results. After deglaciation following the Last Glacial Maximum, the region experienced an isostatically-induced rapid RSL drop from as much 50 m asl to as low as −6.3 m asl in as little as a few centuries between 14,500 BP and 13,500 BP. After a lowstand below current sea level for about 2000 years during the terminal Pleistocene, RSL rose again to a highstand at least 6 m asl after the end of the Younger Dryas. RSL slowly dropped through the Holocene to close to its current position by 2000-1500 BP, with some potential fluctuations between 3500 and 1500 BP. This study highlights variation in RSL histories across relatively short distances, which must be accounted for by local RSL reconstructions such as this one. This RSL curve aided in the identification of an 8000–9000 year old archaeological site on a 10–12 m asl terrace, which is currently the earliest dated archaeological site in the area, and it provides guidance for searching for even older archaeological remains. We highlight the utility and potential of this refined RSL history for developing surveys for other archaeological sites associated with paleoshorelines.

Quantifying organic carbon storage and sources in sediments of Dong Rui mangrove forests, Tien Yen district, Quang Ninh province using carbon stable isotope

Quantifying organic carbon storage and sources in sediments of Dong Rui mangrove forests, Tien Yen district, Quang Ninh province using carbon stable isotope

Relative sea-level change in northeastern Florida (USA) during the last ∼8.0 ka

An existing database of relative sea-level (RSL) reconstructions from the U.S. Atlantic coast lacked valid sea-level index points from Georgia and Florida. This region lies on the edge of the collapsing forebulge of the former Laurentide Ice Sheet making it an important location for understanding glacio-isostatic adjustment and the history of ice-sheet melt. To address the paucity of data, we reconstruct RSL in northeastern Florida (St. Marys) over the last ∼8.0 ka from samples of basal salt-marsh sediment that minimize the influence of compaction. The analogy between modern salt-marsh foraminifera and their fossil counterparts preserved in the sedimentary record was used to estimate paleomarsh surface elevation. Sample ages were determined by radiocarbon dating of identifiable and in-situ plant macrofossils. This approach yielded 25 new sea-level index points that constrain a ∼5.7 m rise in RSL during the last ∼8.0 ka. The record shows that no highstand in sea level occurred in this region over the period of the reconstruction. We compared the new reconstruction to Earth-ice models ICE 6G-C VM5a and ICE 6G-C VM6. There is good fit in the later part of the Holocene with VM5a and for a brief time in the earlier Holocene with VM6. However, there are discrepancies in model-reconstruction fit in the early to mid Holocene in northeastern Florida and elsewhere along the Atlantic coast at locations with early Holocene RSL reconstructions. The most pronounced feature of the new reconstruction is a slow down in the rate of RSL rise from approximately 5.0 to 3.0 ka. This trend may reflect a significant contribution from local-scale processes such as tidal-range change and/or change in base flow of the St. Marys River in response to paleoclimate changes. However, the spatial expression (local vs. regional) of this slow down is undetermined and corroborative records are needed to establish its geographical extent.

Relative sea-level changes during the Holocene in the Río de la Plata, Argentina and Uruguay: A review

This study critically reviews the relative sea-level (RSL) data from the Río de la Plata (RDP) (Argentina and Uruguay) in order to analyze the Holocene RSL changes in this region and thus capture the general trend illustrated by both the data and the high-degree polynomial regression analyses which show good agreement with the ICE-6G_C (VM6) model. The previously inferred Holocene sea-level histories reconstructed from palynological and diatom records from the northern and the southern RDP are also compared to the RSL curves proposed in this work. Analysis of the RSL database revealed that the RSL rose to reach the present level at or before c. 7000 cal yr BP, with the peak of the sea-level highstand c. +4 m between c. 6000 and 5500 cal yr BP (depending on the statistical method used) or at c. 7000 cal yr BP according to the ICE-6G model prediction, gradually falling after this time to the present position. The subaerial data tell a consistent story for the last 6000 years for the RDP but the subsurface data are not in agreement with these data or with the glacial isostatic adjustment (GIA) predictions. Since much of the subaerial data come from shells on beaches, beach ridges and storm beaches, they cannot be strictly interpreted as RSL index points. It seems likely that the predicted curves are somewhat high and could be overestimated. The “smooth” model of the late Holocene sea-level decline is in close agreement with both the GIA model predictions and the reconstruction of RSL from palynological and diatom records. The models provide no evidence to suggest that there has been a significant trend of change in the speed of sea-level fall or oscillations during the falling stage system tract. The non-parametric regression model, the GIA predictions and the trend inferred by palynological and diatom data trace out a slow sea-level fall for the last c. 6000 cal yr BP. We critically assessed published Holocene sea-level data from the RDP to produce a Holocene RSL curve of sufficient quality to provide a location in this area for testing theoretical models for the Atlantic coast of South America.

Temperature-driven global sea-level variability in the Common Era

We assess the relationship between temperature and global sea-level (GSL) variability over the Common Era through a statistical metaanalysis of proxy relative sea-level reconstructions and tide-gauge data. GSL rose at 0.1 ± 0.1 mm/y (2σ) over 0-700 CE. A GSL fall of 0.2 ± 0.2 mm/y over 1000-1400 CE is associated with ∼ 0.2 °C global mean cooling. A significant GSL acceleration began in the 19th century and yielded a 20th century rise that is extremely likely (probability [Formula: see text]) faster than during any of the previous 27 centuries. A semiempirical model calibrated against the GSL reconstruction indicates that, in the absence of anthropogenic climate change, it is extremely likely ([Formula: see text]) that 20th century GSL would have risen by less than 51% of the observed [Formula: see text] cm. The new semiempirical model largely reconciles previous differences between semiempirical 21st century GSL projections and the process model-based projections summarized in the Intergovernmental Panel on Climate Change's Fifth Assessment Report.

An examination of spatial variability in the timing and magnitude of Holocene relative sea-level changes in the New Zealand archipelago

Holocene relative sea-level (RSL) changes have been reconstructed for four regions within the New Zealand archipelago: the northern North Island (including Northland, Auckland, and the Coromandel Peninsula); the southwest coast of the North Island; the Canterbury coast (South Island); and the Otago coast (South Island). In the North Island the RSL highstand commenced c. 8100–7240 cal yr BP when present mean sea-level (PMSL) was first attained. This is c. 600–1400 years earlier than has been previously indicated for the New Zealand region as a whole, and is consistent with recent Holocene RSL reconstructions from Australia. In North Island locations the early-Holocene sea-level highstand was quite pronounced, with RSL up to 2.75 m higher than present. In the South Island the onset of highstand conditions was later, with the first attainment of PMSL being between 7000–6400 cal yr BP. In the mid-Holocene the northern North Island experienced the largest sea-level highstand, with RSL up to 3.00 m higher than present. This is demonstrably higher than the highstand recorded for the southwest North Island and Otago regions. A number of different drivers operating at a range of scales may be responsible for the spatial and temporal variation in the timing and magnitude of RSL changes within the New Zealand archipelago. One possible mechanism is the north-south gradient in RSL that would arise in the intermediate field around Antarctica in response to the reduced gravitational attraction of the Antarctic Ice Sheet (AIS) as it lost mass during the Holocene. This gradient would be enhanced by the predicted deformation of the lithosphere in the intermediate field of the Southern Ocean around Antarctica due to hydro-isostatic loading and mass loss of the AIS. However, no such long-wavelength signals in sea-surface height or solid Earth deformation are evident in glacial isostatic adjustment (GIA) model predictions for the New Zealand region, while research from Australia has suggested that north-south variations in Holocene RSL changes due to hydro-isostatic influences are limited or non-existent. At the regional-to local-scale, post-glacial meltwater loading on the continental shelf around New Zealand is predicted by GIA modelling to have a significant effect on the timing and magnitude of RSL changes through the phenomenon of continental levering. The spatial variation in continental levering is controlled by the configuration of the coast and the width of the adjacent continental shelf, with continental levering providing a robust explanation for the observed spatial and temporal variations in RSL changes. Further research is required to characterise the regional and local effects of different tectonic regimes, wave climates, and sediment regimes. These are potentially very significant drivers of RSL variability at the regional-to local-scale. However, the magnitude of their potential effects remains equivocal.

Red Sea isolation history suggested by Plio-Pleistocene seismic reflection sequences

High evaporation rates in the desert climate of the Red Sea ensure that, during glacial sea level lowstands when water exchange with the Indian Ocean was more restricted, water salinity and δ18O became unusually extreme. Modeling of the effect on Red Sea sedimentary δ18O has been used previously to reconstruct relative sea level to 500 ka and now poses the question of whether that sea-level model could be extended if continuous core material of older sediment became available. We attempt to address this question here by examining seismic reflection data. The upper Pleistocene hemipelagic sediments in the Red Sea contain intervals of inorganic aragonite precipitated during supersaturated conditions of sea-level lowstands. Seismic impedance changes associated with boundaries to those aragonite-rich layers appear to explain seismic reflection sequences. A segment of Chirp sediment profiler data from the central Red Sea reveals prominent reflections at ∼1, ∼5, ∼23, ∼26 and ∼36 ms two-way travel time (TWT) from the seabed. Based on depths to the glacial marine isotope stages (MIS) in cores, we relate the upper three reflections to the tops of aragonite-rich layers and hence the sea level rises immediately following MIS 2, 6 and 12. The reflection at 26 ms is related to an unusually rapid fall into MIS 12 predicted by one sea level reconstruction, which may have created an abrupt lower boundary to the MIS 12 aragonite-rich layer. With the aid of seismogram modeling, we tentatively associate the ∼36 ms reflection with the top of an aragonite-rich layer formed during MIS 16. Furthermore, some segments of lower frequency (airgun and sparker) seismic data from the central and southern Red Sea show a lower (earlier) Plio-Pleistocene (PP) interval that is less reflective than the upper (late) PP interval. This implies less variability in sediment impedance and that extreme variability in water salinity did not develop; water exchange with the Indian Ocean likely continued throughout this interval. These results suggest that viable relative sea level reconstructions may be recovered from Red Sea sediment δ18O data to at least MIS 16 and perhaps even as far back as the early Pliocene.

Relative sea-level change in Connecticut (USA) during the last 2200 yrs

We produced a relative sea-level (RSL) reconstruction from Connecticut (USA) spanning the last ∼2200 yrs that is free from the influence of sediment compaction. The reconstruction used a suite of vertically- and laterally-ordered sediment samples ≤2 cm above bedrock that were collected by excavating a trench along an evenly-sloped bedrock surface. Paleomarsh elevation was reconstructed using a regional-scale transfer function trained on the modern distribution of foraminifera on Long Island Sound salt marshes and supported by bulk-sediment δ13C measurements. The history of sediment accumulation was estimated using an age-elevation model constrained by radiocarbon dates and recognition of pollution horizons of known age. The RSL reconstruction was combined with regional tide-gauge measurements spanning the last ∼150 yrs before being quantitatively analyzed using an error-in-variables integrated Gaussian process model to identify sea-level trends with formal and appropriate treatment of uncertainty and the temporal distribution of data. RSL rise was stable (∼1 mm/yr) from ∼200 BCE to ∼1000 CE, slowed to a minimum rate of rise (0.41 mm/yr) at ∼1400 CE, and then accelerated continuously to reach a current rate of 3.2 mm/yr, which is the fastest, century-scale rate of the last 2200 yrs. Change point analysis identified that modern rates of rise in Connecticut began at 1850–1886 CE. This timing is synchronous with changes recorded at other sites on the U.S. Atlantic coast and is likely the local expression of a global sea-level change. Earlier sea-level trends show coherence north of Cape Hatteras that are contrasted with southern sites. This pattern may represent centennial-scale variability in the position and/or strength of the Gulf Stream. Comparison of the new record to three existing and reanalyzed RSL reconstructions from the same site developed using sediment cores indicates that compaction is unlikely to significantly distort RSL reconstructions produced from shallow (∼2–3 m thick) sequences of salt-marsh peat.

Insight into the late Holocene sea-level changes in the NW Atlantic from a paraglacial beach-ridge plain south of Newfoundland

Constructional sedimentary features can provide insight into past changes in relative sea-level (RSL) in regions where traditional bio-stratigraphic markers are absent. The paraglacial beach-ridge plain at Miquelon-Langlade, located 50km south of Newfoundland, is an example of a well-preserved regressive barrier. Initiation of this plain correlates with a decrease in the rate of RSL rise (from +4.4mm/yr to ~1.3mm/yr) at around 3000years ago. It developed under conditions of normal regression during a period of slow RSL rise (<1.3mm/yr). The barrier is composed of two oppositely prograding mixed sand-and-gravel beach-ridge systems, which evolved contemporaneously along two open coasts. The growth of these features reflects high rates of sediment influx that was sourced from the erosion of proximal glacigenic sediment (moraines) and reworked alongshore and across-shore by wave action. The combination of stratigraphic (ground-penetrating radar and sediment cores), topographic (RTK-GPS) and chronologic (optically stimulated luminescence, OSL) data provide a detailed understanding of the constructional history of the plain. The well-defined contact between coarse-grained, wave-built facies and overlying aeolian deposits is used to demonstrate the dominant influences of RSL change in the development of the barrier system and produce a RSL curve over the period of its formation. A net increase of 2.4m in the surface elevation of wave-built facies is observed across the plain, corresponding to the increase in mean sea-level during its formation. Coupled with OSL dates, trends in elevation of the wave-built facies across the plain are used to reconstruct the relative sea-level history during this period. Acknowledging the uncertainties inherent in the method applied in this study, three distinct periods of sea-level rise can be distinguished: (1) an increase from 2.4 to 1m below modern MSL between 2400 and 1500years (average rate of +1.3mm/yr); (2) relatively stable or slowly rising RSL (<+0.2mm/yr) from 1400 to 700years; and (3) a rise of ca. 0.7m during the past 700years (+1.1mm/yr). This study not only produces the first RSL reconstruction for the Saint-Pierre-et-Miquelon archipelago but also provides: (i) additional details of RSL changes in a region exhibiting great spatial variations in RSL histories (Newfoundland); (ii) field confirmation that wave-built/aeolian stratigraphic contacts in beach ridges can provide a powerful tool for sea-level reconstructions in mixed clastic systems; and (iii) evidence that sediment influxes can outpace the rate of accommodation creation producing a broad, progradational coastal system.

Reappraisal of sea-level lowstand during the Last Glacial Maximum observed in the Bonaparte Gulf sediments, northwestern Australia

During the Last Glacial Maximum (LGM), ice volume reached a relative maximum and global temperature was lower than today. Understanding continental ice volume change requires accurate reconstruction of relative sea level, but tectonic uplift and isostatic adjustment effects complicate many records. Using marine sediment cores from the Bonaparte Gulf in northwestern Australia, a so-called “far field” tectonically stable site, Yokoyama et al. (2000) reported that the LGM terminated abruptly at 19 ka with a rapid sea-level rise (19 ka event). Their sea-level reconstruction determined the age of the LGM termination, but the timing of its inception remained less well constrained, partly because the number of radiocarbon analyses was insufficient to clarify LGM duration. Here we return to the Bonaparte Gulf and present a new relative sea level that better constrains LGM duration with high-resolution radiocarbon dating of a recently recovered marine sediment core (water depth: 120 m, length: 583 cm). Radiocarbon dating of 23 molluscs and 26 bulk organic-matter samples, together with total organic carbon, total nitrogen, and stable carbon isotopes, provide a record of paleoenvironmental variability in response to sea level change. Our results indicate that the LGM sea-level minimum occurred at 20.8 cal ka BP and the LGM duration was shorter than previously reported.

Radiocarbon dating of mangrove sediments to constrain Holocene relative sea-level change on Zanzibar in the southwest Indian Ocean

Mangrove sedimentary deposits are sensitive to changes in sea level and can be used to reconstruct mid- to late Holocene sea-level fluctuations in intermediate and far-field locations, distant to the former polar ice sheets. However, they can be difficult to date using 14C because mangrove sediment can contain mixtures of carbon of different ages. The two main potential causes of error are younger mangrove roots penetrating down through the sediment column and bioturbation by burrowing animals which moves carbon up and down the sediment column. Both processes may introduce carbon not representative of the age of deposition of the layer being dated. This study reports new 14C dates on organic concentrates (10–63 µm) from mangrove sediments from Makoba Bay on Zanzibar (Unguja) where previous bulk sediment 14C age–depth profiles contained inversions and were therefore less useful for relative sea-level (RSL) reconstruction. Dates on organic concentrates provide a more coherent sequence of 14C ages compared with those from bulk sediments. These new data provide an improved environmental history and mid- to late Holocene RSL record for this site. Our reconstructions show that RSL rose during the mid-Holocene and reached within −3.5 m of present by c. 7900 cal. yr BP. RSL slowed as it reached present at or shortly after c. 7000 cal. yr BP, with falling and/or stable RSL from c. 4400 cal. yr BP to present. We are not able to determine whether there was a RSL highstand above present on Zanzibar during the mid- to late Holocene. The RSL reconstruction agrees broadly with changes predicted by the ICE-5G geophysical model, which includes 4 m of ice equivalent sea-level rise between 7000 and 4000 cal. yr BP. Our new dating approach has the potential to provide improved chronologies with which to interpret sea level data from this and other mangrove environments.

Beachrock: A tool for reconstructing relative sea level in the far-field

Abstract Today's understanding of sea-level change developed through a combination of process-based physical modelling and observational data. Observational data of sea-level change derives from coral reefs in the far-field of the former ice sheets where a geographically variable relative sea-level signal is expected as a response of the earth to ocean loading. Given this variability and the limited geographical distribution of coral reefs, there is a need to explore other, non-coral based sea-level markers to further understand sea-level change and, for example, to ‘fingerprint’ melt-water. Here, we present beachrock as a coastal deposit suitable for relative sea-level (RSL) observations in the far-field. Beachrock is an intertidal deposit forming in the zone where carbonate saturated meteoric and marine water mix and p CO 2 decreases. We provide the conceptual framework for beachrock analysis and describe techniques suitable for analysing and dating the deposit. The approach is standardised by outlining the sediment characteristics in terms of RSL indicative meaning and indicative range, and is tested against published data. A study conducted on coasts of the Mediterranean Sea exemplifies the utility of beachrock for RSL reconstruction. It is shown that the precision of the reconstruction is derived from the combined uncertainty of age and tidal amplitude or tidal range. The uncertainty can be reduced to half the tidal amplitude when a deposit can be ascribed to the upper (or lower) intertidal zone. Beachrock-based data benefit from the lack of non-quantifiable error terms such as post-depositional compaction due to the instantaneous formation and high preservation potential of the deposit. This underlines the high precision of beachrock-based RSL reconstruction, which is a prime requirement for testing and extending coral-based records.