Global Sea Level Variations Research Articles

Significant Increase in Global Steric Sea Level Variations over the Past 40 Years

Understanding global sea level variations and exploring their causes hold significant importance for future climate change predictions and the sustainable development of mankind, with the steric sea level (SSL) variations being one of the primary contributors to these changes. Here, we utilize four types of temperature and salinity products (i.e., EN4, IAP, SODA, and GDCSM) to investigate the spatiotemporal characteristics of global SSL changes from 1980 to 2020. We also explore the relationship between SSL changes and the El Niño-Southern Oscillation (ENSO) phenomenon. The findings reveal a rising trend of 0.64–0.97 mm/a in global SSL over the past 40 years, and the annual amplitudes of SSL time series are within the range of 0–50 mm. The SSL trend at a depth of 0–100 m exerts the greatest influence on the overall trend. The ENSO phenomenon has an obvious influence on sea level changes both in the equatorial Pacific region and global scale. The changes in the global sea level (GSL) associated with ENSO are primarily caused by changes in SSL. This study benefits the understanding of SSL changes and their connection to climate change, serves as a reference for comprehensively assessing sea level change mechanisms using diverse datasets and remote sensing technology, and further provides a scientific basis for the sustainable development of mankind in coastal areas.

Quantifying the contribution of temperature, salinity, and climate change to sea level rise in the Pacific Ocean: 2005-2019

In recent decades, Pacific Ocean’s steric sea level anomaly (SSLA) has shown prominent patterns among global sea level variations. With ongoing global warming, the frequency and intensity of climate and sea level changes have increased, particularly in the tropical Pacific region. Therefore, it is crucial to comprehend the overall trends and mechanisms governing volumetric sea level changes in the Pacific. To accurately quantify the spatiotemporal evolution characteristics of density-driven sea level change in the Pacific Ocean (PO) from 2005 to 2019, we decomposed temperature and salinity into linear trends, interannual variations, seasonal variations, and residual terms using the STL (seasonal-trend decomposition based on loess) method. To evaluate the influence of ocean temperature, salinity, and climate change on density-driven sea level change and its underlying mechanisms, we decompose temperature as well as salinity changes through into the Heaving (vertical displacements of isopycnal surfaces) and Spicing (density-compensated temperature and salinity change) modes. The findings reveal an average steric sea level rise rate of 0.34 ± 0.16 mm/yr in the PO from 2005 to 2019. Thermosteric sea-level accounts for 82% of this rise, primarily due to seawater temperature rise at depths of 0-700 m caused by Heaving mode changes. Accelerated SSLA increase via the thermosteric effect has been connected to interactions between greater Ekman downwelling from surface winds, radiation forcing linked to global greenhouse gases, and changes in the Pacific warm currents triggered by El Niño-Southern Oscillation (ENSO) episodes. Although salinity is affected by the Subantarctic Mode Water (SAMW) and the Antarctic Intermediate Water (AAIW) in the southern Indian Ocean, however the significance of salinity in sea level change is little compared to the role played by thermocline shift. This study offers a substantial contribution to the field, providing robust data and technical support, and facilitating a deeper understanding of the mechanisms underlying the effects of temperature and salinity on sea level changes during periods of rapid climate change, thus enhancing the accuracy of future predictions regarding sea level rise.

Late Quaternary palaeoenvironmental evolution and sea level oscillation of Santa Catarina Island (southern Brazil)

Abstract. Sea level oscillation during the Quaternary played a major role in the geomorphology and vegetation dynamics of coastal areas in southern Brazil, encompassing ecosystems that often have a unique biodiversity. Understanding the natural evolution of these areas is essential for decision making regarding land use regulations towards sustainable development, as well as to preserve the uniqueness of the coastal ecosystems. The southern Brazil coastal plain is formed by marine, transitional and continental Quaternary deposits controlled by past variations of the sea level. These variations shaped the coastal landscape and influenced the development of different Atlantic rainforest formations, such as mangroves and restingas. In particular, the restinga formation corresponds to a specific ecosystem that covers sandy soils of marine and fluvial–marine origin formed during the Quaternary on the Brazilian coastal plain. In this contribution, we present high-resolution palynological and stable isotope data from a Holocene core retrieved from the coastal plain of Santa Catarina Island (southern Brazil). We were able to identify four different environmental zones for the last 6520 yr BP. The first zone (6520–2920 cal yr BP) is characterized by a lagoon with large marine-water influence. Notably, the observed dinoflagellate cyst association suggests that marine waters entering the region had their origin in the relatively warm and saline Brazil Current waters. During the second zone (2920–1520 cal yr BP), marine-water contribution to the lagoon decreased until it became disconnected from the sea. The third zone (1520–550 cal yr BP) was marked by the decrease of the water level until it dried out and led to the colonization of herbaceous vegetation over the palaeo-lagoon. The last zone (550 cal yr BP–recent) is characterized by the consolidation of the coastal-plain Atlantic rainforest (restinga vegetation). Our results form an example of the strong sensitivity of southern Brazilian ecosystem change caused by relative sea level variations. As such, this study might contribute to the debate about the potential effects of current climate change induced by global sea level variations.

Reconstruction of Lake-Level Changes by Sedimentary Noise Modeling (Dongying Depression, Late Eocene, East China)

The late Eocene succession of the Dongying Depression forms a highly productive hydrocarbon source. However, due to lack of an unambiguous fine chronostratigraphic framework for the late Eocene stratigraphy, it is challenging to understand the paleolake’s evolution and the driven mechanism of lake-level variation, a limitation which hinders hydrocarbon exploration. In this work, high-resolution gamma-ray logging data were analyzed to carry out the cyclostratigraphic analysis of the third member (Es3) of the Shahejie Formation in the Dongying Depression. Significant 405-kyr eccentricity cycles were recognized based on time series analysis and statistical modeling of estimated sedimentation rates. We abstracted ~57 m cycles of the GR data in the Es3 member, which were comparable with the long eccentricity cycles (~405-kyr) of the La2004 astronomical solution, yielding a 6.43 Myr long astronomical time scale (ATS) for the whole Es3 member. The calibrated astronomical age of the third/fourth member of the Shahejie Formation boundary (41.21 Ma) was adopted as an anchor point for tuning our astrochronology, which provided an absolute ATS ranging from 34.78 ± 0.42 Ma to 41.21 ± 0.42 Ma in Es3. According to the ATS, sedimentary noise modeling for the reconstruction of lake-level changes was performed through the late Eocene Es3. The lake-level changes obtained based on sedimentary noise modeling and spectrum analysis reveal significant ~1.2 Myr cycles consistent with global sea level variations which were related to astronomical forcing. Potential driven mechanisms of marine incursion and/or groundwater table modulation were linked to explain the co-variation of global sea level changes and regional lake level changes. Our results suggest global sea level fluctuations may have played an important role in driving the hydroclimate and paleolake evolution of the late Eocene Dongying Depression.

East Asian lake hydrology modulated by global sea-level variations in the Eocene warmhouse

Lake hydrological change and its driving mechanisms in the Eocene warmhouse climate are important to evaluating environmental and ecological changes induced by global warming. Our limited understanding of lake hydrological changes during the Eocene is in part due to the scarcity of well-dated and continuous terrestrial records. Here we refine the middle-late Eocene cyclostratigraphy and decipher high-resolution lake hydrology changes using sedimentary noise modeling and proxy data at four terrestrial basins (Dongying Depression, Nanxiang Basin, Jianghan Basin, and Fushun Basin) in mid-latitudes of East Asia. Our new astrochronology provides a robust timescale for the period of 48.5-38.5 Ma, and our tuned magnetostratigraphy is at least comparable with other timescales between Chrons C18n.1r and C20n. The new astrochronology reveals synchronous lake-level changes in the four basins in East Asia following 1.2 Myr obliquity and 2.4 Myr eccentricity cycles, indicating astronomical forcing on lake-level changes. The 2.4 Myr cycles of lake-levels are generally in phase with 2.4 Myr cycles of global sea-level changes, suggesting that East Asian lake hydrology was modulated by global sea-level variations in the Eocene warmhouse. Therefore, global sea-level variations may be an important driver of East Asia hydroclimate and freshwater resources.

Validation of Recent Altimeter Missions at Non-Dedicated Tide Gauge Stations in the Southeastern North Sea

Consistent calibration and monitoring is a basic prerequisite for providing a reliable time series of global and regional sea-level variations from altimetry. The precisions of sea-level measurements and regional biases for six altimeter missions (Jason-1/2/3, Envisat, Saral, Sentinel-3A) are assessed in this study at 11 GNSS-controlled tide gauge stations in the German Bight (SE North Sea) for the period 2002 to 2019. The gauges are partly located at the open water, and partly at the coast close to mudflats. The altimetry is extracted at virtual stations with distances from 2 to 24 km from the gauges. The processing is optimized for the region and adjusted for the comparison with instantaneous tide gauge readings. An empirical correction is developed to account for mean height gradients and slight differences of the tidal dynamics between the gauge and altimetry, which improves the agreement between the two data sets by 15–75%. The precision of the altimeters depends on the location and mission and ranges from 1.8 to 3.7 cm if the precision of the gauges is 2 cm. The accuracy of the regional mission biases is strongly dependent on the mean sea surface heights near the stations. The most consistent biases are obtained based on the CLS2011 model with mission-dependent accuracies from 1.3 to 3.4 cm. Hence, the GNSS-controlled tide gauges operated operationally by the German Waterway and Shipping Administration (WSV) might complement the calibration and monitoring activities at dedicated CalVal stations.

Terrestrial Water Storage Anomalies Emphasize Interannual Variations in Global Mean Sea Level During 1997–1998 and 2015–2016 El Niño Events

AbstractInterannual variations in global mean sea level (GMSL) closely correlate with the evolution of El Niño‐Southern Oscillation. However, GMSL differences occur in extreme El Niños; for example, in the 2015–2016 and 1997–1998 El Niños, the peak GMSL during the mature stage of the former (9.00 mm) is almost 2.5 times higher than the latter (3.72 mm). Analyses from satellite and reanalysis data sets show that the disparity in GMSL is primarily due to barystatic (ocean mass) changes. We find that the 2015–2016 event developed not purely as an Eastern Pacific El Niño event but with Central Pacific (CP) El Niño forcing. CP El Niños contribute to a stronger negative anomaly of global terrestrial water storage and subsequent higher barystatic heights. Our results suggest that the mechanism of hydrology‐related interannual variations of GMSL should be further emphasized, as more CP El Niño events are projected to occur.

Submerged Speleothems and Sea Level Reconstructions: A Global Overview and New Results from the Mediterranean Sea

This study presents a global overview of the submerged speleothems used to reconstruct paleo sea levels and reports new results from two stalactites collected in the Mediterranean Sea. Coastal cave deposits significantly contributed to the understanding of global and regional sea-level variations during the Middle and Late Quaternary. The studied speleothems cover the last 1.4 Myr and focused mainly on Marine Isotope Stages (MIS) 1, 2, 3, 5.1, 5.3, 5.5, 7.1, 7.2, 7.3 and 7.5. The results indicate that submerged speleothems represent extraordinary archives that can provide detailed information on former sea-level changes. The two stalactites collected in the central Mediterranean Sea, at Favignana and Ustica islands (Sicily, Italy), are both characterized by continental, phreatic or marine layers. The U-Th and 14C ages of the new speleothems provide results of great interest for relative sea-level changes over the last 1000 years.

Towards disentangling climatic and tectonic changes of southernmost Africa using strontium isotope stratigraphy and clumped isotope thermometry

Abstract Upper Cretaceous-Cenozoic marine sequences preserved between 30 and 350 masl across southern South Africa record a complex history of climatic and tectonic changes. In this study, we measure the strontium (Sr) isotope composition of fossil shark teeth, echinoderms, corals and oyster shells to chronostratigraphically constrain the ages of these sequences. The method requires careful petrographic screening and micro-drilling of the samples to avoid possible alteration by diagenesis. To assess palaeoenvironmental effects in the shells we measured the Mg/Ca elemental ratios and O isotope values using electron microprobe analysis (EMPA) and secondary ion mass spectrometry (SIMS). In addition, we employed carbonate clumped isotope thermometry to test palaeotemperatures reconstruction. The analysis of recent to modern stromatolites by clumped isotopes yields an average temperature of 20.2°C, in agreement with present day observations. The fossil oyster shells suggest a warmer (23.0°C) seawater palaeotemperature, possibly due to major deglaciation and sea-level rise during the Neogene. We also find that transgressions occurred above 200 to 350 m elevation during: 1) the Campanian-Maastrichian (~75 Ma); 2) the mid-Oligocene (32 to 26 Ma); and 3) the Messinian-Zanclean (6 to 5 Ma). These three episodes are linked to well-known variations in global sea level and regional tectonic processes that could have affected the continental margin differently. The most recent transgression coincides with a maximum global sea-level rise of ~50 m at ca. 5.3 Ma, and a worldwide plate kinematic change around 6 Ma, which in Eurasia led to the closure of the Mediterranean Sea. In the Eastern Cape of South Africa, the new dates of analyzed oyster shells constrain a minimum uplift rate of ca. 150 m/Myr during this tectonic activity. The results have important implications for robust calibration of relative sea level in southern Africa.

Unmixing deep-sea paleoclimate records: A study on bioturbation effects through convolution and deconvolution

Marine sediments are typically subject to bioturbation from benthic macrofauna, which disrupt sediments and can have a substantial impact on the amplitude and timing of climate proxy signals recorded in marine sediment cores. This study explores a computational approach to quantify and remove these post-depositional effects from marine climate proxy records. The bioturbation process is modeled as a linear time-invariant filter building upon prior work (Guinasso and Schinck, 1975). Using forward modeling we find that, given modeled mixing layer depths ≥ 5 cm, simulated centennial scale climate variability is generally not preserved even when the sedimentation rate in our model is above 15 cm/kyr. On the scale of ice ages (104-105 yr), the observed effects of bioturbation have the smallest impact, considering the event scale and typical sedimentation rates and mixed layer depth in the deep sea. For millennial scale events, the signal attenuation strongly depends on the event scale and specific bioturbation and sedimentation parameters. To account for the bioturbation effect on a given climate proxy series, a deconvolution method is proposed. We apply the approach to three individual benthic foraminifera oxygen isotope records to reconstruct δ18O across the last interglacial (Marine Isotope Stage 5e; MIS 5e), and to assess the magnitude of the deglacial shift between MIS 6 and MIS 5e. In the highest resolution records (Site MD952042, Site 1012), the deconvolution results reveal an increase in the amplitude of the recovered deglacial δ18O shift by approximately 0.1‰, with implications for reconstructed global average sea level variations and temperature estimates from benthic foraminifera δ18O. The recovered signal at Site 677 suggests high amplitude δ18O variability during MIS 5e, but exhibits larger uncertainty due to a lower sampling frequency, lower sedimentation rate, and an inferred higher noise level. While uncertainties on the deconvolution results can be large, the results provide a new view of the potential impact of bioturbation on paleoclimate reconstruction. Future work can reduce the uncertainty in the deconvolution estimates, through quantitative integration of additional constraints, provided by our knowledge of the climate and depositional systems.

The Use of Submerged Speleothems for Sea Level Studies in the Mediterranean Sea: A New Perspective Using Glacial Isostatic Adjustment (GIA)

The investigation of submerged speleothems for sea level studies has made significant contributions to the understanding of the global and regional sea level variations during the Middle and Late Quaternary. This has especially been the case for the Mediterranean Sea, where more than 300 submerged speleothems sampled in 32 caves have been analysed so far. Here, we present a comprehensive review of the results obtained from the study of submerged speleothems since 1978. The studied speleothems cover the last 1.4 Myr and are mainly focused on Marine Isotope Stages (MIS) 1, 2, 3, 5.1, 5.3, 5.5, 7.1, 7.2, 7.3, and 7.5. The results reveal that submerged speleothems represent extraordinary archives providing accurate information on former sea level changes. New results from a stalagmite collected at Palinuro (Campania, Italy) and characterized by marine overgrowth are also reported. The measured elevations of speleothems are affected by the local response to glacial and hydro-isostatic adjustment (GIA), and thus might significantly deviate from the global eustatic signal. A comparison of the ages and altitude values of the Mediterranean speleothems and flowstone from the Bahamas with local GIA provides a new scenario for MIS 5 and 7 sea level reconstructions.

Global Mean Sea Level Nexus Climate Change: A Dynamic Cointegration and Causality Analyses

Nowadays, the impact of climate change on the global mean sea level variations is becoming an important issue to the researchers and policy makers as a general concern about environmental degradation is making their way into global policy agenda. That is why, in this paper the principal purpose has been made to investigate the dynamic cointegration and causality relationships between global mean sea level (GMSL) and its determinants. Also another purpose has been made to examine empirically the short-run and long-run effects of climate change, carbon dioxide emissions, energy consumption, trade openness and urbanization on global mean sea level based on the time series data from 1970-2019. From the Augmented Dickey-Fuller (ADF), Phillips-Perron (PP) and Kwiatkowski-Phillips-Schmidt-Shin (KPSS) tests results it is found that all the variables are integrated of order one i.e. I(1). From the bounds test approach, it is found that there exits only one cointegration relationship when GMSL is the dependent variable. From the Granger causality test results of VEC model, it is found that the short-run unidirectional causalities are running from carbon dioxide emissions and energy consumption to global mean sea level, from energy consumption and trade openness to carbon dioxide emissions and from trade openness to energy consumption. It has also been found that the error correction term is statistically significant at any significance level when global mean sea level is treated as endogenous variable indicating that there exists a long-run relationship among the variables in the form of Equation (1). It has also been found that in the short-run the variables temperature, carbon dioxide emissions, and urbanization have significant positive effects while energy consumption and trade openness have significant negative effects on global mean sea level at any significance level. The coefficient of ECM (-1) is statistically significant at any significance level, indicates that speed of adjustment for short-run to research in the long-run equilibrium is statistically significant and within the first year, when global mean sea level is above or below its equilibrium level, it adjusts 66.52%. It has also been found that in the long-run the variables temperature, carbon dioxide emissions, and urbanization have significant positive effects on global mean sea level at any significance level but the variable energy consumption has significant negative effects at 5% significance level. It is also found that over time higher temperature, and carbon dioxide emissions will increase global mean sea level rapidly. The CUSUM and CUSUMSQ tests results support that the VEC model is stable and can be applied for policy formulation and decision making purposes.

Paleo-water depth variations since the Pliocene as recorded by coralline algae in the South China Sea

Coralline algae are excellent archives for recording paleo-water depth, but to date few studies have used them for long-term paleo-water depth reconstruction. This study focuses on the Plio-Pleistocene section of Well CK2, which was drilled in the northern South China Sea. The section consists of 55.2 m of volcanic basement overlain by 873.55 m of carbonates. The chronological framework of Well CK2 has been well established using magnetostratigraphy and bulk sediment strontium isotope ratios. The Plio-Pleistocene section includes the uppermost 341 m (5.18 Ma). Based on observations of 201 petrologic thin sections, we identified nine coralline algae genera (nongeniculate: Mesophyllum, Spongites, Lithothamnion, Lithoporella, Lithophyllum, and Hydrolithon; articulated: Amphiroa, Jania, and Corallina), and five coralline algal assemblages (Mesophyllum assemblage, Spongites assemblage, Lithothamnion assemblage, Mesophyllum-Lithophyllum assemblage, and Lithoporella-Lithophyllum assemblage). The Mesophyllum assemblage dominates in the interval between 341 and 312 m (5.18–4.36 Ma; early Zanclean) and developed in a water depth between 15 and 25 m. The Spongites assemblage dominates in the interval between 312 and 309 m (4.36–4.28 Ma; middle Zanclean) and is characterized by subspherical rhodoliths (3 to 9 cm in diameter). This assemblage is probably related to the very shallow (0 to 5 m) water of the distal reef flat/reef crest. The Lithothamnion assemblage dominates in the interval between 309 and 200 m (4.28–2.20 Ma) and developed in a reef platform zone at a water depth of more than 25 m. The Mesophyllum-Lithophyllum assemblage dominates in the interval between 200 and 80 m (2.20–1.16 Ma) and developed in a water depth between 15 and 20 m. The Lithoporella-Lithophyllum assemblage dominates in the interval between 80 and 0 m (1.16–0 Ma) and develops in a water depth of 15 m or less. Our paleo-water depths are basically in agreement with global sea-level variations over time, indicating that paleo-water depths in the South China Sea were controlled mainly by eustatic variations. Based on the relationship between coral reef development and water depth, we further divided the Quaternary intervals into four long-term developmental stages. They consist of two catch-up stages: 1) from 5.18 to 4.28 Ma and 2) from 2.2 to 1.16 Ma; a give-up stage from 4.28 to 2.2 Ma; and a keep-up stage from 1.16 Ma to present. Our results confirm that the coralline algae in the coral reefs of the South China Sea can accurately record past water depth variations.

Climate-driven connectivity changes of the Black Sea since 430 ka: Testing a dual palynological and geochemical approach

The Black Sea experienced multiple episodes of connection with both the Mediterranean and Caspian seas during the Quaternary. Global sea-level variation has been proposed as the main driver of changes in Mediterranean-Black Sea connectivity, while positive water budgets drove Caspian overspill. We present a new, two proxy, low-resolution record from the Black Sea that allows reconstruction of connectivity history from 430 to 50 ka: dinoflagellate cysts (dinocysts) provide direct evidence for properties of surface waters and strontium isotopes constrain the source(s) of water entering the basin. Dinocysts and 87Sr/86Sr suggest that the Black Sea was isolated from the Mediterranean during global sea-level lowstands associated with glacials MIS 4, 8 and 10. Both proxies also strongly suggest that marine (Mediterranean) water flowed into the Black Sea during the eustatic highstands associated with peak interglacials during MIS 5 and 9. However, while the contribution of marine waters during MIS 5e was similar to the present day, lower 87Sr/86Sr during MIS 9 suggests lower than present input. Connectivity during MIS 11, MIS 7 and MIS 6 is more enigmatic. Lower 87Sr/86Sr than those of the isolated Black Sea and dinocyst assemblages dominated by species of Paratethyan lineage are compatible with input from the Caspian Sea. Dinocyst taxa recovered in MIS 11 can be found in both the Caspian Sea and the Black Sea today. All four interglacials studied contain different dinocyst assemblages, suggesting that different conditions may have prevailed during each warm period. However, high-resolution studies are needed to confirm this observation. Further work on the same sequence could be valuable in elucidating the connectivity history of the Black Sea over the glacial-interglacial cycles of the late Quaternary.

The certitude of a global sea level acceleration during the satellite altimeter era

Abstract Recent studies reported a uniform global sea level acceleration during the satellite altimetry era (1993–2017) by analyzing globally averaged satellite altimetry measurements. Here, we discuss potential omission errors that were not thoroughly addressed in detecting and estimating the reported global sea level acceleration in these studies. Our analyses results demonstrate that the declared acceleration in recent studies can also be explained equally well by alternative kinematic models based on previously well-established multi-decadal global mean sea level variations of various origins, which suggests prudence before declaring the presence of an accelerating global mean sea level with confidence during the satellite altimetry era.

100,000-year rhythmicity in glacial cycles and global sea level variations

100,000-year rhythmicity of paleoclimate variations, that aroused in the Late Pleistocene, can be linked with corresponded long-term oscillations of insolation and seafloor volcanism, forced by gravitational forces in Solar system. This conclusion is based on wavelet analysis of variations of the earth’s orbit eccentricity, different paleoclimatic characteristics, and their known spectral estimates and seafloor volcanism data.

Test-Bed Performance of an Ice-Coring Drill Used with a Hot Water Drilling System

Ice cores from ice shelves contain abundant paleoclimatic information and provide essential information concerned with the prediction of future climatic change and global sea level variations. Efficient retrieval of ice cores is always an engineering challenge in polar ice and marine research. Here, we present design and other information of a new hot-water ice-coring drill used in combination with a hot-water drilling system that provides a rapid and environmentally friendly ice coring system. The coring system shares the surface equipment and hydraulic hose with the hot-water drilling system. Tests with the drill were carried out at an ice drill testing facility, and theoretical estimations were performed to predict the rate of penetration (ROP) and water flow rates. The results indicate the optimal water temperature for ice-coring to be 50 °C, and the most suitable water flow rate to be from 42 L/min to 55 L/min. With those drilling parameters, the maximum ROP is 27.8 m/h and the ice cores are 55–59 mm in diameter.

Climate, sea level and tectonic controls on sediment discharge from the Sepik River, Papua New Guinea during the Mid- to Late Pleistocene

Amongst the rivers draining the mountainous islands of the Indonesian Archipelago, the Sepik River of Papua New Guinea is the largest contributor of solute and particulate material to the world ocean. Sites U1484 and U1485, drilled during International Ocean Discovery Program (IODP) Expedition 363 provide a continuous, ~555 kyr long, high-resolution record of mainly siliciclastic slope sedimentation on the northern continental margin of PNG, just offshore the mouth of the Sepik River. Sedimentological analysis, based on a combination of smear slide petrography, particle size analysis, high-resolution physical properties track data and visual core description, offers an unprecedented opportunity to investigate the evolution of this major tropical river throughout changing climate and sea-level conditions during the mid- to late Pleistocene. The Sites U1484 and U1485 sediment records exhibit a dramatic lithologic change at ~ 370 ka: the oldest deposits are dominated by pelagic mud, suggesting that the coarser-grained terrigenous discharge from rivers draining the New Guinea Highlands (including a “proto” Sepik River) was captured before reaching the ocean, when the Sepik River basin was an epicontinental sea. The occurrence of coarser-grained, mass-gravity (mainly hybrid) flows after ~370 ka suggests that the epicontinental sea became a more restricted, shallow-water to nonmarine basin, probably due to both basin infilling and the uplift of local coastal ranges. During the last three glacial-interglacial cycles (~300 kyr), this shallow inland basin was strongly affected by global variations in sea level: during sea level lowstands, the Sepik River cut into older sediments and discharged further offshore onto the shallow continental margins, promoting mass-gravity flows. Pelagic mud deposition on the continental margin during the most extreme highstands following deglaciations suggests a return to shallow marine conditions in the Sepik Basin and repositioning of the river mouth further inland, away from the shelf and from the location of the IODP sites. This also indicates that variations in terrigenous fluxes during these extreme highstands were not solely controlled by the intensity of the hydrological cycle and that global sea level variations also influenced sediment deposition. The sequence is interrupted by several massive grain flow deposits, occurring diachronously at Sites U1484 and U1485 and related to a period of intensified tectonic activity between ~280 ka and ~140 ka. The sedimentologic characteristics of these mass-gravity deposits and their lack of correlation between sites are interpreted as resulting from channelized flows along margin-parallel, fault-controlled channels caused by local and regional failures of the continental margins including the area of the Yalingi Canyon, ~50 km north of the sites, where large tsunamigenic events have also occurred in recent years. Time series analyses of magnetic susceptibility and natural gamma radiation (proxies for %sand and %clay content, respectively) indicate that river discharge fluxes were modulated at orbital frequencies (including obliquity-scale cycles), which suggests that precipitation and river discharge were not only linked to precessionally driven shifts in the mean position of the Intertropical Convergence Zone, but also to high-latitude climate change.

Seasonal variations in global mean sea level and consequences on the excitation of length-of-day changes

Seasonal variations in global mean sea level and consequences on the excitation of length-of-day changes

The interplay of dynamic topography and eustasy on continental flooding in the late Paleozoic

Global sea level change can be inferred from sequence stratigraphic and continental flooding data. These methods reconstruct sea level from peri-cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts, and from spatio-temporal correlations across basins. However, it has been understood that long-wavelength (typically hundreds of km) and low-amplitude (<2 km) vertical displacements of the Earth's surface due to mantle flow, namely dynamic topography, can occur in the absence of crustal deformation. Dynamic topography can drive marine inundation or regional emergence of continents and must be taken into consideration for eustasy estimates. Our analysis indicates that the long-term trend in global-scale maximum flooding over the late Paleozoic generally correlates with global sea level curves. The first-order flooding history of North America correlates with some estimates of eustasy. The Paleozoic inundation of South America does not follow long-term sea level variations. The flooding lows during the Early Carboniferous and high during the Late Carboniferous are at odds with estimates of eustasy and can be explained by dynamic uplift and subsidence, respectively. Our dynamic topography models indicate that the Yangtze Platform of South China experienced significant dynamic subsidence during the transition from Permian to Triassic largely due to proto-Pacific subduction and its northward motion to collide with North China. The reference districts – Western New York, Oklahoma and Kansas, and West Texas in North America – were to some degree affected by dynamic uplift and subsidence associated with long-lived Panthalassa subduction zones, closure of the Rheic Ocean and large-scale upwelling above the African deep-mantle structure during late Paleozoic times. This indicates that some published global sea level curves may include non-eustatic signals such as dynamic uplift or subsidence. The interpretation of stratigraphic data gathered from these reference districts should be treated with caution to estimate global sea level variations.