Effects of sea-level variation and sedimentary noise variation on the development of biogenic reefs since the Pliocene among the Xisha Islands, South China Sea

Carbonate strata are a vital and favorable reservoir for global oil and gas exploration, and carbonate sedimentary systems record ancient oceanic and paleoclimatic conditions, including paleoenvironmental variations throughout geologic periods. Carbonate platforms are widely distributed among the Xisha Islands in the South China Sea and contain large amounts of oil and gas resources. Biogenic reefs are the dominant parts of the carbonate platforms in the Xisha Islands; however, research on the factors that control and affect biogenic reef development is lacking. In this study, a core from well XK-1, which is located on Yongxing Island in the Xisha Islands, a sedimentary noise model, and time-series analyses were used to determine the effects of sea-level fluctuations from 5.3 Ma to present. The results show that coral reefs in the Xisha Islands are sensitive to eustatic fluctuations and that a decrease in sea level essentially corresponds to an increase in sedimentation rate. Indexes of the East Asian monsoon and other environmental indexes show that the Pleistocene and Holocene were suitable for coral growth; however, the trends shown by these indexes and the sea-level variation indicate that the future growth of coral reefs will be at a disadvantage. Research on the controlling factors of biogenic reefs is of significance for understanding reef growth, performing global reef comparisons, and encouraging the future protection of coral reefs.

No demonstrated link between sea-level and eruption history at Santorini

Previous studies have suggested a link between rates of sea-level variation and eruptions globally [McGuire et al., 1997], with Satow and coauthors [2021] presenting the first detailed comparison between sea-level change and eruptive history for a single island-volcano. They use robust, high-resolution ages for volcanic deposits at Santorini, combined with a 2D numerical model to correlate sea-level reduction with volcanism. Lowering sea level reduces overburden pressure and is predicted to increase tensile stress in the magma chamber roof, leading to diking and eventually eruption. Having independently reproduced their results, we disagree with the numerical model for three main reasons: (1) predictions of stress distribution and magnitudes caused by sea level change are solely dependent on the size and boundary conditions of the 2D model; (2) minor changes to the model dimensions, dimensionality (2D to 3D), and/or addition of a mantle analogue, removes correlation between sea level and eruptions; and (3) crustal loading conditions at the volcano absent from the model are more significant than sea level change.

Photogrammetry of sequence and bioherm morphology in the Paradox Formation, Utah, U.S.A.: A test of the coherence of Pennsylvanian (Moscovian) glacio-eustatic sea-level change

ABSTRACT Despite long-standing recognition of high-amplitude, high-frequency sea-level variation resulting from repeated glaciations of Gondwanaland, recorded as “cyclothems” in late Paleozoic strata worldwide, major questions remain concerning their precise timing and expression in the stratigraphic record. A deep meandering canyon in southern Utah cut by the San Juan River exposes cyclic Pennsylvanian (Moscovian) carbonate strata of the Paradox Formation of the Hermosa Group in three dimensions. These peritidally to subtidally deposited strata archive a long record of Pennsylvanian sea-level change that was important to the early development of sequence stratigraphy. Their interpretation with respect to sea level is complicated by lateral variations in sequence thickness and a lack, until recently, of precise geochronologic control. In this study, we use Structure from Motion photogrammetry (SfM) to quantify lateral stratigraphic variation in three dimensions (3D) in the inner gorge of the canyon, then integrate these data with published U-Pb geochronology and conodont biostratigraphy to assess magnitude and temporal significance of depositional cyclicity. 3D SfM models were used to precisely measure the thickness of sixteen depositional sequences in the Barker Creek and Akah intervals, which constitute the lower Paradox Formation. Bioherms ranging from 2–18 m in relief occur within five sequences, and are typically constructed by either microbialites, the sponge Chaetetes, or a mixture of both. On average, bioherms containing Chaetetes are steeper and thicker than those without them. Bioherm-generated bathymetric highs can influence overlying strata in several characteristic ways: 1) by serving as a nucleation site for bioherms in overlying sequences, 2) sequences often onlap bioherms, leading to local stratal truncations, and in some cases, 3) progradational geometries extend laterally outward from preexisting highs in underlying units. Sequence thickness can vary laterally by up to four times in areas with no obvious bioherms; sequences tend to be thickest in the vicinity of bioherms. To better understand the potential orbital significance and correlation of Paradox Formation cyclicity to an increasingly precise global record, we projected photogrammetry-generated average sequence thicknesses onto an age framework using conodont and fusulinid biostratigraphy and CA-TIMS U-Pb zircon ages for correlative strata in Eastern Europe. To mitigate the influence of local variability of accumulation rate, SfM-based average sequence thicknesses in the study area were used to construct a relative-sea-level history that can be compared to global records. While regional averaging cannot correct for the unknown durations of sequence-bounding unconformities, underfilled accommodation, or autocyclicity due to shoal migration, it does provide a more quantitative means to consider timescales of deposition and mechanisms responsible for cyclicity than is possible with a one-dimensional section or core. Paradox Formation cyclicity shares a broad similarity with the global record, suggesting coherent glacio-eustatic sea-level variation across several different tectonic regimes. If one explores a thickness to duration relationship in these sequences, it is plausible that eight thin (2–4 m) sequences in the investigated strata (S2.6–11; S3.5–6) have apparent durations ranging from 100 to 350 kyr, within the plausible range of short and long eccentricity. In contrast, four thicker (6–12 m) sequences in the middle Akah interval (S3.1–S3.4) contain deeper-water lithofacies and have apparent durations ranging from 500 to 1000 kyr, implying that they may be down-dip composites of multiple documented cyclothems in the North American Midcontinent, Urals–Donets, and South China Block successions.

Internal Variability Role on Estimating Sea Level Acceleration in Fremantle Tide Gauge Station

Low frequency internal signals bring challenges to signify the role of anthropogenic factors in sea level rise and to attain a certain accuracy in trend and acceleration estimations. Due to both spatially and temporally poor coverage of the relevant data sets, identification of internal variability patterns is not straightforward. In this study, the identification and the role of low frequency internal variability (decadal and multidecadal) in sea level change of Fremantle tide gauge station is analyzed using two climate indices, Pacific Decadal Oscillation (PDO) and Tripole Interdecadal Pacific Oscillation (TPI). It is shown that the multidecadal sea level variability is anticorrelated with corresponding components of climate indices in the Pacific Ocean, with correlation coefficients of −0.9 and −0.76 for TPI and PDO, respectively. The correlations are comparatively low on decadal time scale, −0.5 for both indices. This shows that internal variability on decadal and multidecadal scales affects the sea level variation in Fremantle unequally and thus, separate terms are required in trajectory models. To estimate trend and acceleration in Fremantle, three trajectory models are tested. The first model is a simple second-degree polynomial comprising trend and acceleration terms. Low passed PDO, representing decadal and interdecadal variabilities in Pacific Ocean, added to the first model to form the second model. For the third model, decomposed signals of decadal and multidecadal variability of TPI are added to the first model. In overall, TPI represents the low frequency internal variability slightly better than PDO for sea level variation in Fremantle. Although the estimated trends do not change significantly, the estimated accelerations varies for the three models. The accelerations estimated from the first and second models are statistically insignificant, 0.006 ± 0.012 mm yr−2 and 0.01 ± 0.01 mm yr−2, respectively, while this figure for the third model is 0.018 ± 0.011 mm yr−2. The outcome exemplifies the importance of modelling low frequency internal variability in acceleration estimations for sea level rise in regional scale.

Probabilistic Tsunami Hazard Assessment in Meso and Macro Tidal Areas. Application to the Cádiz Bay, Spain

Tsunami hazard can be analyzed from both deterministic and probabilistic points of view. The deterministic approach is based on a “credible” worst case tsunami, which is often selected from historical events in the region of study. Within the probabilistic approach (PTHA, Probabilistic Tsunami Hazard Analysis), statistical analysis can be carried out in particular regions where historical records of tsunami heights and runup are available. In areas where these historical records are scarce, synthetic series of events are usually generated using Monte Carlo approaches. Commonly, the sea level variation and the currents forced by the tidal motion are either disregarded or considered and treated as aleatory uncertainties in the numerical models. However, in zones with a macro and meso tidal regime, the effect of the tides on the probability distribution of tsunami hazard can be highly important. In this work, we present a PTHA methodology based on the generation of synthetic seismic catalogs and the incorporation of the sea level variation into a Monte Carlo simulation. We applied this methodology to the Bay of Cádiz area in Spain, a zone that was greatly damaged by the 1755 earthquake and tsunami. We build a database of tsunami numerical simulations for different variables: faults, earthquake magnitudes, epicenter locations and sea levels. From this database we generate a set of scenarios from the synthetic seismic catalogs and tidal conditions based on the probabilistic distribution of the involved variables. These scenarios cover the entire range of possible tsunami events in the synthetic catalog (earthquakes and sea levels). Each tsunami scenario is propagated using the tsunami numerical model C3, from the source region to the target coast (Cádiz Bay). Finally, we map the maximum values for a given probability of the selected variables (tsunami intensity measures) producing a set of thematic hazard maps. 1000 different time series of combined tsunamigenic earthquakes and tidal levels were synthetically generated using the Monte Carlo technique. Each time series had a 10000-year duration. The tsunami characteristics were statistically analyzed to derive different thematic maps for the return periods of 500, 1000, 5000, and 10000 years, including the maximum wave elevation, the maximum current speed, the maximum Froude number, and the maximum total forces.

Extreme sea levels at the Finnish coast due to large-scale wind storms

<p>In the Baltic Sea, the short-term sea level variation might be several meters, even if the tides in the Baltic Sea are negligible. The short-term sea level fluctuations are caused by passing wind storms, inducing sea level variation through wind-induced currents, inverse barometric effect and seiches. Due to the shape of the Baltic Sea with several bays, the highest sea levels are found in the ends of bays like the Gulf of Finland and the Bothnian Bay. The sea level extremes caused by the large-scale windstorms depend strongly on the storm tracks. Within the natural climatic variability during the past centuries, there have most likely been higher sea level extremes than the extreme values found in the tide gauge records.</p><p>To study this variability of sea levels, induced by varying tracks of the passing windstorms, we construct an ensemble of synthetic low-pressure systems. In this ensemble, the parameters of the low-pressure systems (e.g. point of origin, velocity of the center of the system and depth of the pressure anomaly) are varied. The ensemble of low pressure systems is used as an input to a numerical sea level model based on shallow-water hydrodynamic equations. The sea level model is fast to calculate, enabling a study of a large set of varying storm tracks. As a result we have an ensemble of simulated sea levels. From the simulation results we can determine the low-pressure system that induces the highest sea level on a given location on the coast. We concentrate our studies on the Finnish coast, but the method can be applied to the entire Baltic coast. </p>

Collapse of Holocene mangrove ecosystems along the coastline of Oman

Sedimentological, geochemical, and paleontological investigations of the coastline of northeastern Oman have provided the authors with an in-depth insight into Holocene sea levels and climate conditions. The spatial distribution and species assemblage of mangrove ecosystems are analyzed. These ecosystems are sensitive to changes in sea level and precipitation and thus reflect ecological conditions. The close proximity to archaeological sites allows us to draw conclusions regarding human interaction with the mangrove ecosystems. Our interdisciplinary inquiry reveals that the mangrove ecosystems along the east coast of Oman collapsed ~6000 cal yr BP on a decadal scale. There is no sedimentological evidence for a mid-Holocene sea-level highstand. The ecosystem collapse was not caused by sea-level variation or anthropogenic interferences; rather, it was the consequence of reduced precipitation values related to a southward shift of the Intertropical Convergence Zone. This resulted in a decrease of freshwater input and an increase in soil salinity. Further, the aridification of the area caused increased deflation and silting up of the lagoons.

Intraseasonal Variability in the Persian Gulf Revealed by GRACE and Altimetry

<p>The Persian Gulf is a semi-enclosed marginal sea of the Indian Ocean. It connects to the Arabian Sea through the Gulf of Oman and the Strait of Hormuz. The Persian Gulf has a large coastal population, and is relevant economically and geopolitically, and so it is important to understand sea-level changes in the region. We use satellite observations from the Gravity Recovery and Climate Experiment (<strong>GRACE</strong>) and satellite altimetry to study intraseasonal sea level variation over the Persian Gulf during 2002-2015. We interrogate the spatial scales and forcing functions of the variation and its relation to large-scale circulation and climate over the Indian Ocean. Empirical orthogonal function analysis applied to sea level data from satellite altimetry reveals that the intraseasonal sea level variation in the Persian Gulf is dominated by a basin-wide, single-signed mode of fluctuation. Maximum covariance analysis applied to altimetry and GRACE satellite retrievals shows that these basin-wide intraseasonal sea level fluctuations are largely barotropic in nature and coupled to variations in ocean bottom pressure. To interpret the results, we develop a simple linear barotropic theory based on volume and momentum conservation. The theory describes Persian Gulf sea level in terms of freshwater flux over the region, wind stress along the Strait of Hormuz, and sea level in the Gulf of Oman. To test this theory, we perform a complex multiple linear regression using these regional freshwater flux, wind stress, and sea level as inputs, and Persian Gulf sea level as output. The regression model explains ~70% of the intraseasonal Persian Gulf sea level variance. The magnitudes and phases of the coefficients determined from the regression model are consistent with expectations from the simple theory. The Gulf of Oman sea level boundary condition shows significant lagged correlation with intraseasonal sea level upstream along the Indian Subcontinent, Maritime Continent, and equatorial Indian Ocean. This hints at a large-scale circulation and climate influence on intraseasonal sea level variation of the Persian Gulf mediated by waves propagating along equatorial and coastal waveguides. This study highlights the value of GRACE retrievals of ocean bottom pressure for understanding sea level in an understudied semi-enclosed marginal sea.</p>