Sea-level Reconstructions Research Articles

How can past sea level be evaluated from traces of anthropogenic layers in ancient saltpans?

Footprints of human activities identified in the sedimentary sequence of submerged historical saltpans can reveal the history of the site and can indicate the relative sea level during its operational period. Saltpans are man-made constructions used continuously for salt production in the Mediterranean at least for the last 2000 years. The east Adriatic coast contains many such submerged remains, preserved and well-dated by historical archives. Sedimentological, microfossil and geochemical analyses of the sediments from cores drilled in the saltwork area at Brbinj, Dugi Otok, Croatia, enable the reconstruction of various past environmental conditions. The current study aims to: a) identify the anthropogenic unit in the sedimentary sequence deposited over time, b) determine its age, and c) use it as past sea-level limiting points. Basal units made of terra rossa soil materials were identified in the sedimentary records. These layers are located -120 ±7 cm below mean sea level next to the separation wall and -125 ±7 cm and -135 ±7 cm, respectively, in the inner pools, most likely representing a man-made pavement. The terra rossa layer is overlaid by a unit rich in faunal remains dominated by euryhaline foraminifera and ostracod species such as Ammonia veneta and Cyprideis torosa, representing the saltworks unit. The flooding of the saltpans by the rising sea is manifested by the deposition of an upper sedimentary unit dominated by remains of marine species. The base and the top of the saltwork unit are dated by Optically Stimulated Luminescence to 1040±50 CE and to 1390±30 CE, respectively. The study presents a new approach for obtaining footprints of human activities in ancient, submerged saltpans, by identifying and dating the indicative anthropogenic layers and using these for the reconstruction of paleo sea-level. The described method can be applied all around the Mediterranean.

Mid- to late Holocene sea-level rise recorded in Hells Bells 234U/238U ratio and geochemical composition

Hells Bells are underwater secondary carbonates discovered in sinkholes (cenotes) southeast of Cancun on the north-eastern Yucatán peninsula, Mexico. These authigenic calcite precipitates, reaching a length of up to 4 m, most likely grow in the pelagic redoxcline. Here we report on detailed 230Th/U-dating and in-depth geochemical and stable isotope analyses of specimens from cenotes El Zapote, Maravilla and Tortugas. Hells Bells developed since at least ~ 8000 years ago, with active growth until present day. Initial (234U/238U) activity ratios (δ234U0) in Hells Bells calcite decreas from 55 to 15‰ as sea level converges toward its present state. The temporal evolution of the geochemistry and isotope composition of Hells Bells calcites thus appears to be closely linked to sea-level rise and reflects changing hydrological conditions (desalinization) of the aquifer. We suggest that decelerated leaching of excess 234U from the previously unsaturated bedrock traces Holocene relative sea-level rise. Considering this proxy, the resulting mean sea-level reconstruction contains half as much scatter, i.e. improves by a factor of two, when compared to previously published work for the period between 8 and 4 ky BP.

Reconstruction of the palaeo‐sea level of Britain and Ireland arising from empirical constraints of ice extent: implications for regional sea level forecasts and North American ice sheet volume

ABSTRACTReconstructions of palaeo‐sea level are vital for predicting future sea level change and constraining palaeo‐ice sheet reconstructions, as well as being useful for a wide array of applications across Quaternary Science. Previous reconstructions of the palaeo‐sea level of Britain and Ireland relied on a circular tuning of glacio‐isostatic models: input ice sheet thicknesses and extents were iteratively altered to fit relative sea level data. Here we break that circularity by utilizing new data from the BRITICE‐CHRONO project, which constrains the position of the British–Irish ice sheet margin through time, and we compare derived glacio‐isostatic modelling to the rich relative sea level record. We test a combination of plausible ice thickness scenarios which account for the uncertainty of ice margin position over the North Sea, demonstrating the region where regional sea level data could distinguish between different glaciation scenarios. Our optimal reconstruction is then combined with several global‐scale reconstructions. As the signal of the British–Irish Ice Sheet is constrained, we demonstrate how the relative sea level record of Britain and Ireland can be used to test reconstructions of far‐field ice sheets (e.g. Antarctica, Eurasia and the Laurentide). The derived palaeo‐topography data are likely to be useful for multiple disciplines. Finally, our improved method of sea level reconstruction impacts predictions of contemporary vertical land motion.

Recommendations for the design of in situ sampling strategies to reconstruct fine-scale ocean currents in the context of SWOT satellite mission

The new Surface Water and Ocean Topography (SWOT) satellite mission aims to provide sea surface height (SSH) measurements in two dimensions along a wide-swath altimeter track with an expected effective resolution down to 15–30 km. In this context our goal is to optimize the design of in situ experiments aimed to reconstruct fine-scale ocean currents (~20 km), such as those that will be conducted to validate the first available tranche of SWOT data. A set of Observing System Simulation Experiments are developed to evaluate different sampling strategies and their impact on the reconstruction of fine-scale sea level and surface ocean velocities. The analysis focuses (i) within a swath of SWOT on the western Mediterranean Sea and (ii) within a SWOT crossover on the subpolar northwest Atlantic. From this evaluation we provide recommendations for the design of in situ experiments that share the same objective. In both regions of study distinct strategies provide reconstructions similar to the ocean truth, especially those consisting of rosette Conductivity Temperature Depth (CTD) casts down to 1000 m and separated by a range of distances between 5 and 15 km. A good compromise considering the advantages of each configuration is the reference design, consisting of CTD casts down to 1000 m and 10 km apart. Faster alternative strategies in the Mediterranean comprise: (i) CTD casts down to 500 m and separated by 10 km and (ii) an underway CTD with a horizontal spacing between profiles of 6 km and a vertical extension of 500 m. In the Atlantic, the geostrophic velocities reconstructed from strategies that only sample the upper 500 m depth have a maximum magnitude ~50% smaller than the ocean truth. A configuration not appropriate for our objective in both regions is the strategy consisting of an underway CTD sampling one profile every 2.5 km and down to 200 m. This suggests that the thermocline and halocline need to be sampled to reconstruct the geostrophic flow at the upper layer. Concerning seasonality, the reference configuration is a design that provides reconstructions similar to the ocean truth in both regions for the period evaluated in summer and also in winter in the Mediterranean.

The Fish Tanks of the Mediterranean Sea

Roman fish tanks are found in various coastal regions of the Mediterranean, although the vast majority is found on the Tyrrhenian coast of Italy. In this work, a database was developed with information on 62 fish tanks along the Mediterranean coasts to document and compare their features and characteristics. The analysis of the developed database from the Mediterranean fish tanks has shown that, among the 62 fish tanks, ~56% were cut into the rock, indicating that this type of construction was the most popular at that time and probably had advantages over the others. Fish tanks as sea level indicators can provide accurate data on the sea level 2000 years ago. Well-preserved installations with prominent architectural features have a crucial role in determining the paleo sea level. The architectural elements that are mostly used in fish tanks for paleo sea level reconstructions are the crepido, cataractae and channels. Besides the scientific importance of the fish tanks as sea level markers, they also have great cultural and historical significance. Fish tanks can be promoted as heritage monuments and scholarly models to strengthen awareness about climate change, sea level rise and its consequences.

Postglacial relative sea-level changes in the Gulf of Maine, USA: Database compilation, assessment and modelling

Relative sea level (RSL) reconstructions from paleo records are often the most valuable data set for testing and constraining glacial isostatic adjustment (GIA) models. In some regions, the amplitude and rate of the reconstructed RSL changes are large making the data difficult to fit. Arguably, the reconstructed RSL curve from Maine, USA, is the classic example of such a data set with peak values at about 120 m dropping to a low stand of around −40 m within a few thousand years. To our knowledge, no GIA model has captured these extreme variations and the record has been somewhat neglected by the GIA modelling community. Here we critically assess and present a revised pre-10 ka RSL data base for this region and combine it with two recent Holocene compilations. Based on our assessment, we conclude that the large and rapid changes, inferred in previous studies, are robust. To determine if a successful model fit can be found, we consider a GIA parameter set comprising five ice models and 440 spherically-symmetric, Earth viscosity models assuming a Maxwell rheology. Results show that none of the model parameter sets produce a good fit to the data set. The rapid RSL fall and lowstand can be simulated by Earth models with low values of upper mantle viscosity (∼5 × 1019 Pas). However, such low viscosity values result in a large mid-Holocene highstand (order 10 m) while the observations indicate a monotonic RSL rise throughout this interval. Our results indicate that Earth models able to simulate time-dependent viscosity (i.e., those that include transient and/or non-Newtonian deformation) might be required to fit the Maine RSL data set.

Hourly Sea Level Prediction‐Based GNSS‐IR Inversions by Combining the Least Squares Learning Cross‐Checking Method With the Gaussian Kernel Model L2 Constraint and LSTM

AbstractMultisatellite systems and multi‐signal‐to‐noise ratio types provide a more prosperous data basis for the inversion of sea level by GNSS‐IR technology. However, there are few studies on data reconstruction of the retrieved sea level height. This study takes the SPBY station as an example and introduces the least squares learning cross‐checking method with the Gaussian kernel model (GKM) L2 constraint. Furthermore, research on hourly sea level reconstruction based on GNSS‐IR is carried out. Compared with the measured sea level, the sea level height in 2021 obtained after the fusion of multisource data has an R of 0.905 and an RMSE of 0.144 m. And the result obtained after data reconstruction has an R of 0.958 and an RMSE of 0.090 m. Compared with the multisource data fusion results before reconstruction, R is increased by 5.9%, and RMSE is decreased by 37.5%. Finally, using the reconstructed sea level data based on Long Short‐Term Memory (LSTM) artificial neural network to carry out the research on sea level prediction, which verifies the conclusion that more reliable forecast values can be obtained based on 5 months of training data. Among them, the R from 1 to 24 hr is 0.905, and the RMSE is 0.145 m. Compared with the inversion accuracy of GNSS‐IR, the R is increased by 2.0%, and the RMSE is decreased by 21.4%. This study demonstrates the feasibility of GNSS‐IR technology and L2‐constrained least squares learning cross‐checking method based on the GKM to reconstruct sea level data with high temporal resolution and high accuracy. The reliability of the sea level prediction based on GKM reconstruction and LSTM is verified in the sea level forecast of the next 24 epochs, which has essential applications in sea level data recovery and forecasting.

Thorium isotope evidence for glacial–interglacial dust storminess and productivity in the North Pacific gyre

The flux of dust deposition to the ocean and its impact on marine biological productivity are outstanding issues of the Earth’s climate system. The North Pacific gyre (NPG) is the Earth’s largest ecosystem and is important for marine nitrogen fixation, which is sensitive to Asian dust-regulated surface nutrient stoichiometry. Meanwhile, it is one of the least studied regions in the global ocean during the late Pleistocene, because of the extremely slow sediment accumulation and lack of biological material preservation. 232Th and 230Th concentrations recorded by hydrogenetic ferromanganese crusts in the deep NPG provide a novel and quantitative proxy of dust deposition and export productivity in this region. For this purpose, we have established an in situ U–Th isotope analytical technique on hydrogenetic ferromanganese crusts collected from the deep NPG. Iron, Mn, and Co of our samples have also been analyzed to constrain the impact of dust dissolution on the Fe geochemical cycle. Remarkably, variability of the 232Th-based dust flux over the last ∼900 ky exhibits a striking resemblance to grain-size records of the Chinese loess deposits and sea-level reconstructions but clearly decouples from the global temperature evolution. On orbital time scales, dust storms over the NPG are apparently sensitive to effects related to continental ice sheet sizes but decoupled from major global cooling during the early glaciation. While our 232Th data indicate a 2- to 3-fold variability of glacial–interglacial dust flux, scavenging of Fe, Mn, and Co in the deep NPG is much less variable through the late Pleistocene. We suggest that variable dust fluxes have a limited impact on the inventory of dissolved Fe in the deep waters of NPG, even though the scavenging of Fe has likely been regulated by glacial–interglacial paleoceanography. The paired 230Th-scavenging-based biological productivity tracks gyre sea surface temperature rather than dust flux during the last glacial cycle. We observe no correlation between dust flux and export productivity in the NPG, indicating that dust-derived Fe is not the productivity-limiting factor on orbital time scales in this region. Instead, these results imply that changes in gyre mixing and mode water formation might have led to increased supply of phosphorous and thus enhanced glacial biological carbon sequestration, with potentially stronger nitrogen fixation under higher eolian input.

Resolving Uncertainties in Foraminifera-Based Relative Sea-Level Reconstruction: a Case Study from Southern New Zealand

Abstract Since the pioneering work of David Scott and others in the 1970s and 1980s, foraminifera have been used to develop precise sea-level reconstructions from salt marshes around the world. In New Zealand, reconstructions feature rapid rates of sea-level rise during the early to mid-20th century. Here, we test whether infaunality, taphonomy, and sediment compaction influence these reconstructions. We find that surface (0–1 cm) and subsurface (3–4 cm) foraminiferal assemblages show a high degree of similarity. A landward shift in assemblage zones is consistent with recent sea-level rise and transgression. Changes associated with infaunality and taphonomy do not affect transfer function-based sea-level reconstructions. Applying a geotechnical modelling approach to the core from which sea-level changes were reconstructed, we demonstrate compaction is also negligible, resulting in maximum post-depositional lowering of 2.5 mm. We conclude that salt-marsh foraminifera are indeed highly accurate and precise indicators of past sea levels.

Spatial and Temporal Distributions of Live Salt-Marsh Foraminifera in Southern New Jersey: Implications for Sea-Level Studies

ABSTRACT Geological reconstructions of relative sea-level change have been greatly enhanced by continuous high-resolution records with the use of salt-marsh foraminifera due to their relationship with tidal level in modern environments and subsequent preservation of tests in sediments. A detailed understanding of how live foraminifera assemblages compare to dead or total (live + dead) assemblages and the influence of environmental variables on foraminiferal distributions is essential for their use as a proxy to reconstruct sea level. Here, we evaluated small-scale spatial and temporal (seasonal and interannual) variability of live foraminifera assemblages from four high marsh monitoring stations along a salinity gradient in southern New Jersey over three years. In addition, we measured porewater and sedimentary variables and stable carbon isotopes during each sampling period every three months. In the 184 samples, we identified 11 live agglutinated foraminifera species and four distinct clusters of live foraminifera that correspond to the stations from which they were sampled and to the dead and total assemblages. We found no clear correlation over time between variability in live assemblages and measured environmental variables; however, elevation was the primary controlling factor influencing foraminiferal distributions, with secondary influences from salinity and substrate. The consistency of foraminiferal assemblages on spatial and temporal scales and among live, dead, and total assemblages further reinforces the value of salt-marsh foraminifera as reliable sea-level indicators.

The World Atlas of Last Interglacial Shorelines (version 1.0)

Abstract. This paper presents version 1.0 of the World Atlas of Last Interglacial Shorelines (WALIS), a global database of sea-level proxies and samples dated to marine isotope stage 5 (∼ 80 to 130 ka). The database includes a series of datasets compiled in the framework of a special issue published in this journal (https://essd.copernicus.org/articles/special_issue1055.html, last access: 15 December 2022). This paper collates the individual contributions (archived in a Zenodo community at https://zenodo.org/communities/walis_database/, last access: 15 December 2022) into an open-access, standalone database (Rovere et al., 2022, https://doi.org/10.5281/zenodo.7348242). The release of WALIS 1.0 includes complete documentation and scripts to download, analyze, and visualize the data (https://alerovere.github.io/WALIS/, last access: 15 December 2022). The database contains 4545 sea-level proxies (e.g., marine terraces or fossil beach deposits), 4110 dated samples (e.g., corals dated with U-series), and 280 other time constraints (e.g., biostratigraphic constraints or tephra layers) interconnected with several tables containing accessory data and metadata. By creating a centralized database of sea-level proxy data for the Last Interglacial, the WALIS database will be a valuable resource to the broader paleoclimate community to facilitate data–model integration and intercomparisons, assessments of sea-level reconstructions between different studies and different regions, as well as comparisons between past sea-level history and other paleoclimate proxy data.

東南極における最終間氷期以降の氷床・海水準変動の復元：現状と今後の課題

地球温暖化が危惧される現在，将来の気候変動に対する南極氷床の振る舞いを理解することは学術的にも社会的にも喫緊の課題である．南極氷床は多様な時間スケールで変動するため，人工衛星や現地測地観測など比較的短期の変動を捉える観測手法に加え，地質記録やモデルシミュレーションを用いた数十年から数十万の長期的な時間スケールの南極氷床変動の復元が重要である．海水準記録は地質記録とglacial isostatic adjustment（GIA）モデルから復元が可能であり，全球的な氷床量すなわち海水量を反映する．そのため，長期的な時間スケールの南極氷床変動の復元において重要な指標である．本論文では，東南極における海水準変動の先行研究とGIAモデルの現状をまとめ，東南極海水準変動復元の研究における今後の展望を述べる．

Reconstruction of Mediterranean coastal sea level at different timescales based on tide gauge records

Abstract. A coastal sea level reconstruction based on tide gauge observations is developed and applied to the western basin of the Mediterranean sea. The reconstructions are carried out in four frequency bands and are based on an optimal interpolation method in which the correlation between tide gauge data and all coastal points has been determined from the outputs of a numerical model. The reconstructions for frequencies lower than 1 month use monthly observations from the Permanent Service for Mean Sea Level (PSMSL) database and cover the period from 1884 to 2019. For the reconstruction of higher frequencies, hourly observations from the Global Extreme Sea Level Analysis (GESLA-2) dataset are used and cover from 1980 to 2015. Total sea level is retrieved with high accuracy from the merging of the different frequency bands. Results of a cross-validation test show that independent tide gauge series are highly correlated with the reconstructions. Moreover, they correlate significantly better with the reconstructions than with altimetry data in all frequency bands, and therefore the reconstruction represents a valuable contribution to the attempts of recovering coastal sea level. The obtained reconstructions allow us to characterize the coastal sea level variability, estimate coastal sea level trends along the entire coastline, and examine the correlation between western Mediterranean coastal sea level and the main North Atlantic climate indices. The limitations and applicability of the method to other regions are also discussed.

Data rescue process in the context of sea level reconstructions: An overview of the methodology, lessons learned, up‐to‐date best practices and recommendations

AbstractCoastal water level measurements represent one of the earliest geophysical measurements and allow an assessment of historical sea level rise and trends in tides, river flow and storm surge. However, recovery and digitization of archival tidal records have been much less widespread and systematic than, for example meteorological records. In this contribution, we discuss data rescue efforts and lessons learned in France, the United States and the United Kingdom, countries with early and extensive tide gauge networks by the mid‐19th century. We highlight the importance of (a) cataloguing the historical gauge records, as a first step towards locating them; (b) locating data in archives, and then recovering and saving data by any means necessary, including photographs and scanning; (c) obtaining metadata, including both quantitative survey records, gauge checks and clock data, but also qualitative records such as gauge notes, letters and reports; and (d) quantitative statistical analysis of data and datum quality, using both standard data‐entry checks but also tools that leverage the unique predictability of tide measurements. Methods for digitizing original analogue records are also discussed, including semi‐automatic, computer‐based methods of digitizing tidal charts (marigrams). Although the current best practice is described, future improvements are desirable and needed to make the more than estimated 10,000 station years of unused, undigitized records available to the scientific community.

Toward New and Independent Constraints on Global Mean Sea‐Level Highstands During the Last Glaciation (Marine Isotope Stage 3, 5a, and 5c)

AbstractEstimates of ice volume over the last 120 ka, from marine isotope Stage (MIS) 5d (∼110 ka) through MIS 3 (60–26 ka) are uncertain. Weiss et al. (2022, https://doi.org/10.1029/2021PA004361) offer an innovative new constraint on past sea level using the oxygen isotopes (δ18O) of planktic (surface and thermocline dwelling) foraminifers to infer the salinity of the Sulu Sea in the Indo‐Pacific Ocean and assess flow through the Karimata Strait (Indonesia) over the last glaciation. Based on the timing of Karimata Strait flooding, the study concludes that local relative sea level in the Karimata Strait was >−8 6 m during MIS 5c (∼100 ka) and >−12 6 m during MIS 5a (∼80 ka), relative to present. For MIS 3, a maximum possible relative sea level of −16 6 m is determined. Here, these results are placed into the context of current knowledge of last glacial sea‐level change and the implications for climate forcings and feedbacks (e.g., global average surface temperature and greenhouse gases) and ice sheet growth are discussed. By tracing past ocean circulation patterns that are modulated by the depth of shallow straits such as the Karimata Strait, Weiss et al. (2022, https://doi.org/10.1029/2021PA004361) provide independent constraints on local sea level, which are essential for improving global mean sea level reconstructions on late Pleistocene glacial‐interglacial cycles.

Quaternary coral, coralline algal and vermetid assemblages as sea-level indicators: a review

Corals, coralline algae and vermetid gastropods are indirect (marine limiting) relative sea-level (RSL) indicators. The precision in sea-level reconstruction based on fossils of those organisms depends on the probable palaeodepth in which they grew. Constraining such palaeodepth depends, in turn, on the available information about the habitats of their living counterparts. Diverse genera, species and species assemblages of corals, coralline algae and vermetid gastropods have historically been proposed as reliable indicators of narrow, shallow depth ranges. However, the increased information on depth distribution of marine benthos in the last two decades has challenged some early assumptions about depth ranges of taxa considered diagnostic of precise palaeodepths. Here, the authors test the reliability of coral, coralline algal and vermetid assemblages that have been extensively used in RSL reconstructions in the light of data from Ocean Biogeographical Information System (OBIS) and other recently published data. In the Indo-Pacific province, these data support the use of the robust-branching and the shallow, high-energy encrusting coral assemblages with a 0 to 10 m uncertainty. In both cases many component species have unimodal distributions and both median and average water depths are shallower than 10 m. The reliability of these coral assemblages as indicative of shallow water depths is strengthened when corals are encrusted by thick plants of the coralline alga Porolithon gr. onkodes. According to OBIS data, coralline algae of this species group in the Indo-Pacific are restricted to very shallow waters (95% probability of occurrence shallower than 0.2 m and in 99.6% of records shallower than 6 m). However, such a narrow depth range and the overall scarce data on coralline algal species in the OBIS database are questionable due to difficulties of coralline algal species identification with the naked eye. A comprehensive survey of the modern distribution of coralline algae at One Tree Reef (southern Great Barrier Reef) indicates that P. gr. onkodes has a log-normal distribution with median depth of less than 5 m and 95% of occurrence probability of thick crusts (> 0.2 mm) shallower than 8.8 m. Data on modern distribution of vermetids are scarce. In the OBIS database, vermetid species are reported from relatively wide depth ranges. However, relatively high densities (> 10 individuals per m2) on coral and coralline algal surfaces only occur from above mean low tide to some 6 m depth. In the Western Atlantic-Caribbean province Acropora palmata is the most precise RSL marker and no additional components of fossil assemblages improve its palaeodepth information. The confident use of coralgal and vermetid assemblages as RSL indicators relies on the identification of fossil corals and coralline algae at the species or species-group level. The scarcity of available data highlights the need for further studies on distribution of coralline algal species and vermetid in modern coral reefs from a variety of oceans and reef settings.

Holocene Paleoenvironmental Evolution of a Semi-Enclosed Shallow Aegean Basin: A Combination of Seismic Stratigraphy and Sediment Core Proxies

The island of Astypalea (Greece), known for its rich and pristine archeological sites, encompasses a semi-enclosed silled basin that has been very susceptible to global sea levels and regional climate changes due to its relatively modern shallow sill of 4.7 m water deep that connects the Vathy bay with the adjacent Aegean Sea. To identify the causal relationship between regional climate, sea-level trajectories, and environmental change and their potential impact on hominine habitats on the island, we investigated a high-resolution seismic profile together with sediment, stable isotope, geochemical, and biotic proxies retrieved from a marine sediment core (ASTC1). Our results show that the basin was once isolated, and a marine inundation occurred at around 7.3 ka BP, which is older than expected, based on global sea level reconstructions. The entire transition from isolation to full marine conditions was accomplished in three major phases: (1) non-marine isolated conditions between 9–7.3 ka BP, (2) semi-isolated hypersaline marsh and lagoonal conditions between 7.3 and 4.1 ka BP, and (3) semi-isolated shallow marine conditions of today (4.1 ka BP to present). High water alkalinity, elevated organic content, and heavier isotopic signals indicate relatively arid conditions in the region that favored Sr-rich carbonate precipitation within the 7.3–6 ka BP interval. On the other hand, freshwater biota, along with a high Corg/N ratio and lighter isotopic signal, showed wetter conditions, at least for the intervals 8–7.3 ka and 6–5.4 ka BP, in contrast to the aridification trend seen as 4.1 ka to present. Finally, the hominine habitat evolution at around 6 ka BP might be attributed to the wetter conditions and the freshwater source provided by the bay at that time.

Relative Contributions of Open‐Ocean Forcing and Local Wind to Sea Level Variability Along the West Coasts of Ocean Basins

AbstractThis study aims to reconstruct sea level at the coast by quantifying relative contributions of local wind forcing and the penetrated open‐ocean signals, and thus to build a dynamical connection that seamlessly links the ocean basin to the coast. A linear, modified arrested topographic wave (ATW) model is used to examine the transmission of interior sea level across the continental shelf and the role of alongshore wind in determining the coastal sea level. The local wind stress substantially influences the coastal sea level but does not affect the transmission of open‐ocean signals in the linear framework. Increased bottom friction or gentler slope of shelf favors the penetration of open‐ocean signals. This modified ATW model is then applied to the shelf seas of North Pacific and North Atlantic, with model parameters being set according to the realistic features of regional oceanography. The offshore boundary condition is determined by the integrated barotropic and baroclinic ocean responses in the ocean basin and the transmission processes across western boundary currents. Based on the proposed “basin‐to‐coast” dynamical linkage framework, one can estimate the relative contributions of open‐ocean forcing and local wind to the total coastal sea level. According to these partitions, the reconstructed coastal sea levels along the west coasts of North Pacific and North Atlantic on seasonal time scale are in close agreement with tide‐gauge observations. The idea and specific calculations of coastal sea level reconstruction could be readily applied to other coasts of the global ocean.

High interstadial sea levels over the past 420ka from the Huon Peninsula, Papua New Guinea

The history of sea level across the Quaternary is essential for assessing past and future climate. Global sea-level reconstructions are typically derived from oxygen isotope curves, but require calibration with geological constraints that are scarce prior to the last glacial cycle (>130 thousand years ago). Here we show that the coral reef terrace sequence at the Huon Peninsula (Papua New Guinea) provides such constraints up to ∼420 thousand years ago, through a geometric analysis of high-resolution topographic data. We derive a northward tectonic tilt as regional deformation pattern, and estimate relative sea level for 31 Quaternary periods, including several periods for which no relative sea level data exists elsewhere. Supported by numerical reef models, these estimates suggest that oxygen isotope-based global mean sea-level curves systematically underestimate interstadial sea-level elevations, by up to ∼20 m. Compared to those curves, our results imply a stronger degree of non-linearity between ice-sheet volumes and global temperatures within Quaternary glacial cycles.

Reconstruction of long-term sea-level data gaps of tide gauge records using a neural network operator

The coastal sea level is an important factor in understanding and clarifying the physical processes in coastal seas. However, missing values and outliers of the sea level that occur for various reasons often disrupt the continuity of its time series. General-purpose time-series analysis and prediction methods are not tolerant of missing values, which is why researchers have attempted to fill these gaps. The disadvantage of conventional time-series reconstruction techniques is the low accuracy when missed sea-level records are longer than the timescales of coastal processes. To solve this problem, we used an artificial neural network, which is a novel tool for creating multivariate and nonlinear regression equations. The trained neural network weight set was designed to enable long-term reconstruction of sea level by acting as a one-step prediction operator. In addition, a data assimilation technique was developed and adapted to ensure seamless continuity between predicted and observed sea-level records. The application of our newly developed method to 3-day gaps of seal level records at 16 tide gauge stations around the Korean peninsula confirms that it can successfully reconstruct missing values with root-mean-squared errors of 0.5–1.1 cm on average.