Glacio-eustatic Sea Level Research Articles

Analysis of the late Hirnantian and early Rhuddanian unconformities of southern Ontario: evidence for far-field glacioeustatic effects

Several unconformities have been previously recognized in the Late Ordovician and early Silurian strata of southern Ontario. We examined the Georgian Bay, Queenston, Whirlpool, Manitoulin, Power Glen, and Cabot Head formations and associated unconformities. Detailed sequence stratigraphic and chemostratigraphic analysis of Late Ordovician and early Silurian outcrops between Niagara, New York, and Manitoulin Island, Ontario, reveals new insights on the timing of the erosional unconformities and the Ordovician–Silurian boundary. We recognize three significant lowstand unconformities in this interval, which are referred to as the Cherokee, S1B, and S2 unconformities. Additional small-scale surfaces are present but do not reflect any major change in sea level or pause in sedimentation. Using δ13Ccarb chemostratigraphy, biostratigraphy, and sequence stratigraphy, we tentatively correlate units and the mentioned unconformities from southern Ontario to other eastern North American sections of comparable age, showing how glacioeustasy had a widespread effect on the deposition and removal of strata in far-field, subtropical basins. The Cherokee unconformity appears to be a composite erosion surface found across eastern North America that formed during the two or more episodes of glacioeustatic sea level fall in the early to middle Hirnantian. The overlying S1B and S2 unconformities can also be found across eastern North America and appear to be the result of glacioeustatic sea level falls occurring during the early Silurian. These new insights on the timing of erosional unconformities help better constrain the placement of the Ordovician/Silurian boundary in Ontario. It appears that the boundary does not correspond to a lowstand-related disconformity as previously suggested but can be tentatively placed within the Power Glen or lower Cabot Head formations.

Milankovitch Record From Middle Jurassic Platform Supports Moderate Coolhouse Glaciation

AbstractThere are few documentations of far‐field effects of coolhouse glaciation and its signature preserved within Middle Jurassic carbonate platforms. Through outcrop logging and time series analysis we document the cyclostratigraphy of the upper Bajocian‐Bathonian interval of the Adriatic Platform, Croatia. The 200 m thick, ∼3.3 Myr duration cyclic platform record consists of peritidal carbonate parasequences (n = 109) with high‐energy grainstone‐rudstone bases fining up into micritic carbonate caps. There is little evidence of long‐term exposure near purported sequence boundaries, indicating the succession is conformable above the parasequence scale. Time series were constructed using Dunham rank and stratigraphic position, and parasequence thickness and stratigraphic position. A statistical estimation of accumulation rates using average spectral misfit (ASM) and correlation coefficient (COCO) methods that compare spectral peaks with an astronomical model at various sedimentation rates suggests that these rates ranged from 4.5 to 6 cm/kyr, which is compatible with long‐term rates based on thickness divided by duration. The time series analysis of rank series and cycle thickness series indicates long‐term obliquity modulation (∼1 Myr), short obliquity modulation (173 kyr), strong obliquity, and weak precession. The 173 kyr cycle is particularly evident in the cycle thickness series. The similarity of the spectra to the obliquity‐dominated Cenozoic coolhouse supports moderate southern hemisphere glaciation driving glacioeustatic sea level changes during the Middle Jurassic.

Quaternary Lowstand Prograding Wedges of the Salento Continental Shelf (Southern Adriatic Sea, Italy): Architectural Stacking Patterns and the Control of Glacio-Eustatic Sea Level Fluctuations and Foreland Tectonic Uplift

The performance of both the tectonic uplift and of the 4th-order glacial eustatic sea level fluctuations in controlling the stratigraphic architecture of Quaternary lowstand prograding wedges of the Salento continental shelf (Southern Adriatic sea, Italy) during a time interval spanning from the Middle Pleistocene to the Holocene has been pointed out through the interpretation of high-resolution seismic reflection profiles and their correlation to the curves of the isotopic stratigraphy. Three main transgressive surfaces of erosion (RS1, RS2 and RS3) punctuate the stratigraphic architecture of the Salento continental shelf, separating Quaternary lowstand prograding wedges between them. All along the Middle Pleistocene, increasing the tectonic uplift of the Puglia offshore, combining with 4th-order glacio-eustatic variations, have dealt with the pattern of a broad forced regression prograding wedge, favoring a platform progradation of approximately 15 km. The architectural stacking patterns of the overlying Late Pleistocene and Holocene prograding wedges are controlled by 4th-order glacio-eustatic sea level changes, allowing for the formation of incomplete depositional sequences. In this period, the eustatic signature overcomes the tectonic mark, implying a decline in the uplift of the Apulian foreland in the course of the final 250 ky.

A Relative Paleointensity (RPI)-Calibrated Age Model for the Corinth Syn-rift Sequence at IODP Hole M0079A (Gulf of Corinth, Greece)

The Corinth basin (Greece) is a young continental rift that recorded cyclic basin paleoenvironment variations (i.e., marine to lacustrine) caused by glacio-eustatic sea level fluctuations during its initial connection to the global oceans. The Corinth syn-rift sequence offers therefore a unique opportunity to determine the timing and modality of connection of young rifts to the global oceans, and investigate how sediment supply change during this extremely dynamic stage of their evolution. Here we use magnetostratigraphic and relative paleointensity (RPI) constraints from 885 discrete samples from International Ocean Discovery Program (IODP) Hole M0079A to generate an unprecedented high-resolution (∼15kyr) age model for the youngest part of the Corinth’s offshore syn-rift sequence. Our RPI-calibrated age model spans the last ∼850 kyr and reveals that initial connection of the Corinth basin to the Mediterranean Sea occurred at ∼530 ky, more recently than previously thought and after a short-lived marine incursion at ∼740 kyr. Accumulation rates calculated from our age model indicate two significant changes in sediment supply at 530 (from 74 to 156 cm/kyr on average) and 70 ka (from 156 to 258 cm/kyr on average), interpreted as due to increased local fault activity in the southern margin of the Corinth basin. Sedimentation rates also display a short-term variation indicating a lower sediment supply (98 cm/kyr on average) during interglacial periods and a higher input (156 cm/kyr on average) during glacial periods. We conclude that long-term variations of sediment supply in young rifts connecting to the global oceans are predominantly controlled by local tectonics (i.e., new depocenters formation), while short-term variations may be driven by global climate (i.e., increased erosion aided by fewer vegetation during glacial periods). Grain size of the Corinth syn-rift sequence shows a weak correlation with glacial cycles too (i.e., coarser sediments during interglacials), but we suggest this to be controlled by the hydrodynamics of the basin (i.e., stronger bottom currents when the basin was connected to the Mediterranean Sea during interglacial periods).

Geochemical Records Reveal Protracted and Differential Marine Redox Change Associated With Late Ordovician Climate and Mass Extinctions

AbstractThe Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove the Late Ordovician Mass Extinction (LOME). Here, we present combined iodine and sulfur isotope geochemical data from three globally distributed carbonate successions to constrain changes in local and global marine redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow carbonate shelves, while sulfur isotopes suggest a reduction in global euxinic (anoxic and sulfidic) conditions. Late Katian sulfate‐sulfur isotope data show a large negative excursion that initiated during elevated sea level and continued through peak Hirnantian glaciation. Geochemical box modeling suggests a combination of decreasing pyrite burial and increasing weathering are required to drive the observed negative excursion suggesting a ∼3% decrease of global seafloor euxinia during the Late Ordovician. The sulfur datasets provide further evidence that this trend was followed by increases in euxinia which coincided with eustatic sea‐level rise during subsequent deglaciation in the late Hirnantian. A persistence of shelf anoxia against a backdrop of waning then waxing global euxinia was linked to the two LOME pulses. These results place important constraints on local and global marine redox conditions throughout the Late Ordovician and suggest that non‐sulfidic shelfal anoxia—along with glacioeustatic sea level and climatic cooling—were important environmental stressors that worsened conditions for marine fauna, resulting in the second‐largest mass extinction in Earth history and the only example during an icehouse climate.

Late Glacial Marine Transgression and Ecosystem Response in the Landlocked Elefsis Bay (Northern Saronikos Gulf, Greece)

Coastal landscapes are sensitive to changes due to the interplay between surface and submarine geological processes, climate variability, and relative sea level fluctuations. The sedimentary archives of such marginal areas record in detail the complex evolution of the paleoenvironment and the diachronic biota response. The Elefsis Bay is nowadays a landlocked shallow marine basin with restricted communication to the open Saronikos Gulf. A multi-proxy investigation of a high-resolution sediment core recovered from the deepest part of the basin offered a unique opportunity to record the paleoenvironmental and aquatic ecosystem response to climate and glacioeustatic sea level changes since the Late Glacial marine transgression. The retrieved sedimentary deposits, subjected to thorough palynological (pollen, non-pollen palynomorphs, dinoflagellates), micropaleontological (benthic foraminifera, calcareous nannoplankton, ostracods), and mollusc analyses, indicates isolation of the Elefsis Bay from the Saronikos Gulf and the occurrence of a shallow freshwater paleolake since at least 13,500 cal BP, while after 11,350 cal BP the transition towards lagoon conditions is evidenced. The marine transgression in the Elefsis Bay is dated at 7500 cal BP, marking the establishment of the modern marine realm.

Miocene Atlantic transgressive - regressive events in northeastern and offshore Patagonia: A palynological perspective

Key information for regional biostratigraphic, climatic and environmental reconstructions for the Miocene of the southwestern Atlantic margin can be obtained by qualitative and quantitative palynological analysis at the onshore YPF-CH-PV.es-1 borehole (PV borehole) in the Valdés Basin, combined with previously documented organic-walled dinoflagellate cyst data from the Colorado Basin and well-dated outcropping sections on the east coast of Patagonia. A sequence of eleven significant dinocyst bioevents (highest occurrence, HO; highest common occurrence, HCOs) is recognized across the Valdés and Colorado basins. The bioevents occur in the same stratigraphic order and seem to be synchronous across both basins. From the oldest to the youngest, these are: HO of Emmetrocysta urnaformis, HO of Cannosphaeropsis quattrocchiae, HCO and HO of Hystrichokolpoma rigaudiae, HO of Cousteaudinium auybriae, HCO and HO of Dapsilidinium pseudocolligerum, HO of Cleistosphaeridium ancyreum, HO of Labyrinthodinium truncatum, HO of Operculodinium piaseckii and HO of Reticulatosphaera actinocoronata. The presumed climatically-driven extinctions of Dapsilidinum pseudocolligerum and Hystrichokolpoma rigaudiae around the Burdigalian to earliest Langhian, may be linked to global cooling and the re-establishments of the Antarctic ice-sheets since ~14 Ma. Two maximum flooding episodes, characterized by warm, outer (distal) neritic environmental conditions were identified at the PV borehole, presumably related to glacio-eustatic sea level rise. The older occurred in the Burdigalian – earliest Langhian and the younger, in the Tortonian. The latter maximum flooding is followed by environmental and/or climatically-driven change, implying abrupt shifting from neritic to nearshore conditions, the extinction of the warm-water taxa, and dinocysts being largely replaced by acritarchs.

Simulation of oxygen isotopes and circulation in a late Carboniferous epicontinental sea with implications for proxy records

Reconstructions of ancient ocean chemistry are largely based on geochemical proxies obtained from epicontinental seas. Mounting evidence suggests that these shallow inland seas were chemically distinct from the nearby open ocean, decoupling epicontinental records from broader ocean conditions. Here we use the isotope-enabled Community Earth System Model to evaluate the extent to which the oxygen isotopic composition of the late Carboniferous epicontinental sea, the North American Midcontinent Sea (NAMS), reflects the chemistry of its open-ocean sources and connect epicontinental isotope variability in the sea to large-scale ocean-atmosphere processes. Model results support estuarine-like circulation patterns demonstrated by past empirical studies and suggest that orographic runoff produced decreases in surface seawater δ18O (δ18Ow) of up to ∼3‰ between the NAMS and the bordering ocean. Simulated sea surface temperatures are relatively constant across the sea and broadly reproduced from proxy-based δ18O paleotemperatures for which model-based values of epicontinental δ18Ow are used, indicating that offshore-onshore variability in surface proxy δ18O is primarily influenced by seawater freshening. Simulated bottom water temperatures in the NAMS are also reproduced from biogenic calcite δ18O using model-based values of epicontinental δ18Ow, suggesting that benthic marine fossil δ18O is also influenced by seawater freshening and coastal upwelling. In addition, glacial-interglacial variations in nearshore seawater freshening counteract the effects of temperature on marine biogenic δ18O values, suggesting that salinity effects should be considered in δ18O-based estimates of glacioeustatic sea level change from nearshore regions of the NAMS. Our results emphasize the importance of constraining epicontinental dynamics for interpretations of marine biogenic δ18O as proxies of paleotemperature, salinity, and glacioeustasy.

DEGRADACIÓN DE DUNAS LITORALES DE CHILE CENTRAL: UNA VISIÓN BIOGEOMORFOLÓGICA

Los campos de dunas costeras de Chile central son resultado de las interacciones entre las variaciones glacioeustáticas del nivel del mar, cambios climáticos y el solevantamiento tectónico de la costa ocurridos durante el Cuaternario. La evolución actual es debida a morfodinámicas naturales ligadas al viento, la vegetación y a la ocupación e instalación de actividades e infraestructuras humanas. Se evidencia ladegradación de dos campos de dunas en Chile central debido a la extensión de especies vegetales invasivas y la instalación de especies arbóreas productivas. Se aprecian alteraciones de las dinámicas eólicas y con ello se espera la modificación de las morfologías existentes.
 Palabras clave: dunas litorales, biogeomorfología, Chile central, especies invasivas

Sequence biostratigraphic framework for the Oligocene to Pliocene of Malaysia: High-frequency depositional cycles driven by polar glaciation

This paper reviews quantitative biostratigraphic data from 101 petroleum exploration wells from the Malay, Sarawak and Sabah basins, and places the stratigraphy of each area into a precise chronostratigraphic framework comprising 41 transgressive-regressive depositional packages which are fully defined here. Each package is ‘fingerprinted’ by foraminiferal, nannofossil and palynological index fossils, and then tied to the global pattern of glacio-eustatic sea level and climate changes based on the global δ18O and δ13C datasets used to calibrate the geological timescale. The result is a tightly constrained chronostratigraphic framework that is applicable from deep to shallow water and is of high accuracy. It is concluded that there is a sedimentation ‘pulsebeat’ across Southeast Asia, independent of local and regional tectonics, driven by patterns of global climate change. For the Oligocene and Miocene, Antarctic glaciation is the key driver, whereas the Pliocene cycles are mainly driven by the expansion and contraction of Northern Hemisphere glaciers. The new chronostratigraphic framework allows unconformities across Malaysia to be better characterised, and the different classification schemes of seismic “Groups” in Malay and Penyu basins, “Cycles” in offshore Sarawak and “Stages” in offshore western Sabah to be correlated. Also, with a clear understanding of the effect of fluctuating sea levels on the stratigraphy of the region, it is easier to make firm judgments regarding the effect of tectonics. The study also characterises systems tracts from shelf to basin floor using biostatigraphic assemblages, which should help in ground-truthing the architecture of depositional sequences in the region based on seismic. The new framework is proposed as an alternative to the outdated TB scheme of Haq et al. (1988), widely used to interpret depositional cycles across Sarawak and Sabah.

The Brachiopod δ11B Record Across the Carboniferous‐Permian Climate Transition

AbstractWe present the δ11B of well‐preserved brachiopod fossils coupled with geochemical modeling to examine how seawater boron responded to abrupt and dynamic climate changes in the Late Paleozoic. The Late Carboniferous, a time of major coal formation and glacioeustatic sea level changes, is characterized by relatively stable brachiopod δ11B of 15–17‰, similar to values seen in modern brachiopods. Brachiopod δ11B dropped by ~5‰ in the early Permian and then restabilized at a new value of 10‰ within a few million years. Mass balance models of seawater δ11B reproduced the overall trends in our brachiopod data but failed to capture the large drop in δ11B in the early Permian. Published seawater 87Sr/86Sr and δ44/40Ca data based on brachiopod shells also shift to lower values in the early Permian, suggesting a common control on all three seawater isotope systems. The Permian terrestrial record of evaporites and eolian deposits suggests a prolonged reduced delivery of dissolved weathering products to the ocean, accounting for the change in seawater 87Sr/86Sr. This reduced weathering, in turn, led to increased atmospheric CO2 and lowered seawater pH, which may have significantly decreased major removal mechanisms for seawater calcium and boron leading to declines in both isotope systems. We propose that boron removal via coprecipitation in carbonates and adsorption onto clay minerals was significantly diminished due to a reduction in the availability of the borate aqueous species caused by lowered seawater pH.

A Persistent Kuroshio in the Glacial East China Sea and Implications for Coral Paleobiogeography

AbstractThe Kuroshio Current is a major hydrographical feature of the modern East China Sea, but it has been suggested that its flow was diverted to the east of the Ryukyu Arc at the Last Glacial Maximum. Shoaling of the Yonaguni Depression has also been proposed as a cause of Kuroshio Current diversion which, while unlikely to have been significant at the Last Glacial Maximum, may have been an important consideration further back in time. Using an ensemble of high‐resolution ocean simulations with climatic boundary conditions emulating those of the Last Glacial Maximum, we present the first regional state estimates of the glacial East China Sea which are both physically consistent and compatible with sea surface temperature proxy compilations. We find that while the Kuroshio Current transport in the East China Sea is slightly reduced at the Last Glacial Maximum, its path is relatively unchanged, with limited sensitivity to glacioeustatic sea level change, glacial‐interglacial changes in climate, and tectonic shoaling of the Yonaguni Depression. Simulations with the best model‐proxy agreement predict only limited contraction of the reef front at the Last Glacial Maximum, and strong surface currents associated with the glacial Kuroshio may have maintained or even improved long‐distance coral larval dispersal along the Ryukyu Arc, suggesting that conditions may have enabled coral reefs in this region to remain widespread throughout the last glacial. Further field studies investigating whether this is genuinely the case will provide insights into how the coral reef front responds to long‐term environmental change.

High-resolution palaeomagnetic record from Sea of Marmara sediments for the last 70 ka

SUMMARYMagnetostratigraphic and geochemical analyses were performed on two sediment cores recovered from the Sea of Marmara to investigate geomagnetic field variations over the last 70 ka. A chronology for each of the two cores was developed from eight AMS 14C datings, tephrochronology, and tuning of Ca concentrations with stadials and interstadials observed in Greenland ice core oxygen isotope data. Based on the age models, cores MD01–2430 and MRS-CS19 reach back to 70 and 32 ka, respectively. High average sedimentation rates of 43 cm kyr–1 for core MD01–2430 and 68 cm kyr–1 for core MRS-CS19 allow high-resolution reconstruction of geomagnetic field variations for the Sea of Marmara. Mineral magnetic properties are sensitive to glacioeustatic sea level changes and palaeoclimate variations in this region, reflecting the variable palaeoenvironmental conditions of the Sea of Marmara during last 70 ka. Despite the impairment of the palaeomagnetic record in some stratigraphic intervals due to early diagenesis, relative palaeointensity variations in the Sea of Marmara sediments correlate well with similar records derived from other regions, such as the nearby Black Sea and the GLOPIS-75 stack. The directional record derived from the Sea of Marmara cores exhibits typical palaeosecular variation patterns, with directional anomalies at 41 and 18 ka, representing the Laschamps and postulated Hilina Pali excursions, respectively. Both directional anomalies are also associated with palaeointensity minima. A further palaeointensity minimum at 34.5 ka is likely related to the Mono Lake excursion, with no directional deviation documented in the Sea of Marmara palaeomagnetic record so far.

Links between iron supply from Asian dust and marine productivity in the Japan Sea since four million years ago

AbstractAeolian dust input exerts significant influence on oceanic biogeochemical cycles and further potentially controls atmospheric CO2 concentrations. However, the possible link between long-term aeolian dust supply and primary productivity in the western North Pacific remains poorly understood. Here, we present a comprehensive study of major and trace elements and total organic carbon (TOC) concentrations of sediments from Integrated Ocean Drilling Program (IODP) Site U1430 in the southern Japan Sea, in order to reconstruct oceanic palaeoproductivity evolution and test its possible link to Asian dust input since 4 Ma. Palaeoproductivity proxies indicate remarkable increases in productivity at ∼3–2 Ma followed by high-frequency oscillations in productivity since 1.2 Ma. We suggest that higher dust-derived iron supply from Central Asia at 3–2 Ma, which was likely driven by the growth of the Northern Hemisphere ice sheets, could account for enhanced primary productivity and export production in the Japan Sea. Such increased oceanic palaeoproductivity could enhance organic carbon burial, which might contribute to the decrease in atmospheric CO2 concentrations, and provide a positive feedback to the global cooling. However, the Tsushima Warm Current (TSWC) intrusion via the southern Tsushima Strait, which was controlled by glacioeustatic sea level changes, has been the principal cause of the rapid changes in primary productivity and benthic redox condition since 1.2 Ma, regardless of continuously increased Asian dust input.

Amazon fluid mud impact on tide- and wave-dominated Pliocene lobes of the Orinoco Delta

The modern Orinoco Delta is a major sink for the world's largest alongshore (littoral) mud dispersal system; it receives some 108 tons/yr from the Amazon river delta. The influence of these huge volumes of Amazon mud on the paleo-Orinoco Delta succession is now investigated for the first time. Abundant fluid-mud deposits are preserved in outcrops from the Pliocene Orinoco Delta deposits (Lower Morne L'Enfer, Manzanilla and Mayaro formations on Trinidad) with different styles in tide-dominated, in tide-dominated and wave-influenced, and in storm wave-dominated delta lobes. Fluid-mud deposits in sandy parasequences (10–20 m thick) of the Lower Morne L'Enfer Formation change from thin layers amalgamating to form mud bedsets (up to 4 cm) or draping across ripple laminae in the lower part, to thick layers (up to 10 cm) occasionally with deformed and load structures at their top, in the upper part. The Manzanilla Formation parasequences (30–40 m thick) exhibit coarsening-to-fining-upward (CUFU) facies successions with fluid-mud deposits located mainly in the middle to upper parts of the parasequences, near the transition between the CU and FU parts of the units. Most of the fluid-mud layers are interbedded with bi-directional, current rippled-sandstones, whereas minor mud layers are associated with erosion-based sandstones with small-scale swaley strata and bi-directional symmetrical ripples. The Mayaro Formation parasequences (5–15 m thick) are characterized by alternating fluid-mud intervals and sets of hummocky cross-stratified sandstones (HCS) passing upward into amalgamated swaley cross-stratified (SCS) sandstones. The fluid-mud intervals have flat or irregular tops occasionally overlain by amalgamated SCS sandstones with fluid-mud clasts.Very thick (1–1.5 m) fluid-mud intervals occur in places in both tide- and wave-dominated strata and these are probably large muddy bedforms similar to mud banks that migrate currently along the modern Amazon-Orinoco coast. The outcrop analysis strongly suggests that the fluid-mud banks, as they approached the Orinoco delta-front area, were handled differently depending on whether the delta-front lobes were wave- or tide-dominated. Tide-dominated delta lobes tended to trap the muds near the shoreline and on the subaqueous delta-front platform in water depth < 10 m with only minor volumes escaping offshore. In contrast, storm wave-dominated delta lobes had a more dynamic impact on the approaching mudbanks. Mud accumulated in the shallow water tend to be eroded and re-deposited into deeper water near the storm wave base (15–50 m water depth).The volumes of Pliocene Amazon mud transported along the innermost shelf and arriving on the Orinoco Delta were likely influenced by high-amplitude, Pliocene glacio-eustatic sea level fluctuations. During sea-level fall and lowstand periods, increased volumes of Amazon mud would have bypassed to the deep water and thus decreased mud volumes drifted and became incorporated in the Orinoco delta front. In contrast, sea-level rise and highstand periods would have promoted increased volumes of Amazon mud both on the shelf and onto the Orinoco, similar to the Holocene and modern muddy system.

Glacial Cycles Influence Marine Methane Hydrate Formation

AbstractMethane hydrates in fine‐grained continental slope sediments often occupy isolated depth intervals surrounded by hydrate‐free sediments. As they are not connected to deep gas sources, these hydrate deposits have been interpreted as sourced by in situ microbial methane. We investigate here the hypothesis that these isolated hydrate accumulations form preferentially in sediments deposited during Pleistocene glacial lowstands that contain relatively large amounts of labile particulate organic carbon, leading to enhanced microbial methanogenesis. To test this hypothesis, we apply an advection‐diffusion‐reaction model with a time‐dependent organic carbon deposition controlled by glacioeustatic sea level variations. In the model, hydrate forms in sediments with greater organic carbon content deposited during the penultimate glacial cycle (~120–240 ka). The model predictions match hydrate‐bearing intervals detected in three sites drilled on the northern Gulf of Mexico continental slope, supporting the hypothesis of hydrate formation driven by enhanced organic carbon burial during glacial lowstands.

MAGNETIC SUSCEPTIBILITY CHANGES CORRELATED WITH PALEOENVIRONMENTAL AND LITHOLOGICAL CHANGES, WITHIN BOREHOLE CORES ON EITHER SIDES OF THE KALAMAKI-ISTHMIA FAULT IN CORINTH CANAL

Τhe most important active fault that intersects the eastern tip of Corinth Canal,the Kalamaki-Isthmia fault, is studied in detail, involving data analysis from nine boreholes, magnetic susceptibility measurements within boreholes and paleoenvironmental interpretations. Samples taken from boreholes were analysed and paleoenvironmental changes in the sequence are described. We correlate magnetic susceptibility (MS) measurements with paleoenvironmental and lithological alternations within the boreholes. We have ascribed low MS values to marine highstand deposits and high MS values to lowstand terrestrial deposits. Glacioeustatic sea level changes and tectonic movements have led to a very complicated lithosedimentary pattern that involves subaerial exposure, fluvialterrestrial, lagoonal, shallow marine environments and possibly even some lake sediments. Our results indicate that the Kalamaki fault is active and has a low slip rate of 0.05±0.02 mm/yr.

Milankovitch cycles in an equatorial delta from the Miocene of Borneo

The factors controlling sedimentary cyclicity in deltaic systems are a subject of intense debate, and more research, in different deltaic environments and time periods, is needed to better understand the possible mechanisms. Offshore and Pleistocene case studies are more common than proximal and more ancient, greenhouse-climate examples. Furthermore, many studies lack a (statistical) cyclostratigraphic element. The paleo-Mahakam delta of Eastern Kalimantan, Borneo developed during the globally warm middle Miocene, in an equatorial setting, making it of interest to comprehend cyclic sedimentation in a period of warmer yet rapidly changing climate. In this study, statistical analysis of lithological changes shows that regular sandstone/shale alternations occur in a distinct pattern of cycles with thicknesses of ∼90, ∼30, and ∼17 m. Using independent dating, these thicknesses translate into periods of about 100, 40, and 20 kyr, matching the known periods of Earth's orbital eccentricity, obliquity and precession. The obliquity dominance in the middle interval is markedly similar to that observed in the global marine isotope (benthic δ18O) and other cyclic proxy records for this time interval. Despite a mismatch in the number of 40 kyr cycles compared to the global record that can be plausibly linked to the major sea-level drop at ∼13.8 Ma and facies shifts, it appears that the proximal setting of the paleo-Mahakam's sedimentation was dominantly controlled by allogenic orbital forcing, probably as a consequence of glacioeustasy. In particular, the observed obliquity dominance at paleo-equatorial latitudes, as seen in other records, highlights the dominance of orbital forcing, and potentially glacioeustatic sea level change, during this crucial period of warmer climate.

Lithochemical composition of sandstones of the Vendian Asha Group, South Urals

The mineralogical–petrographic and chemical study of sandstones of the Vendian Asha Group in the Bashkir anticlinorium, the western slope of the South Urals, showed that this large stratigraphic unit consists of sedimentary associations formed in different conditions: (1) Pre-Uryuk sediments (Tolparovo, Suirovo, and Bakeevo formations) accumulated during marine regression possibly in the course of significant glacioeustatic sea level fluctuations and formation of the foredeep of Timanides. (2) Sediments of the Uryuk Formation, including alluvial and several related sediments. Analysis of the Qm–F–Lt, Qt–F–L, and ln(Q/L + CE)–ln(Q/F) diagrams showed that they were derived from magmatic/plutonic rocks in the inner parts of the East European Craton. Based on the distribution of data points of psammites in the Qt/(F + R)–Qp/(F + R) diagram, they were accumulated in the semihumid/semiarid conditions. (3) Coastal, shallowmarine, and fluvial/proluvial (?) sediments of the Basa, Kukkarauk, and Zigan formations. They were formed by the erosion of provenances located supposedly east of the present-day Bashkir anticlinorium. The psammites of the Asha Group were analyzed using the sandstone formation model proposed models proposed in (Dickinson et al., 1985; Garzanti et al., 2007). The distribution of data points of psammites from three uppermost formations of the Asha Group in the Qm–F–Lt and Qt–F–L diagrams suggests that they were accumulated by the redeposition of erosion products of the so-called clastic wedges of recycled orogens (clasticwedge provenance) made up of the fluvial and turbidite complexes of the foreland, fore-arc, or residual oceanic basins.

Genetic variability in anadromous fishes, chum salmon Oncorhynchus keta (Walbaum, 1792), and Sakhalin taimen Parahucho perryi (Brevoort, 1856) from the Northwestern Pacific as a reflection of paleoclimate oscilations

The genetic variability distribution of two mtDNA segments of chum salmon (Oncorhynchus keta) (Walbaum, 1792) and Sakhalin taimen (Parahucho perryi) (Brevoort, 1856) was examined in populations of the Sea of Japan and the Sea of Okhotsk. The values of haplotype and nucleotide variability in these species are, in general, of the same level. The dating of the divergence time of species haplotypes revealed four evolutionary periods in Sakhalin taimen and three in chum salmon. In the taimen, the first divergence time occurred approximately 430 thousand years (kyr) ago, the second 220 kyr ago, and the third 70 kyr ago. In the chum salmon, the first divergence time corresponds to 220 kyr; the second is approximately 100 kyr ago. In both species, the main portion of presently revealed haplotypes evolved over the past 50–10 kyr. Certain glacioeustatic sea level fluctuations influenced each stage of evolution history of species, contributing to their geographic isolation. Demographic population history research found that the initial stage of population growth in the taimen occurred at the time period of approximately 12 kyr ago and was apparently associated with the end of the Last Glacial Maximum. In the chum salmon, this period began somewhat earlier, 30–35 kyr ago; it has accelerated in the past 10–15 kyr. The last glaciation to a lesser extent impacted the demographics of chum salmon, probably due to the greater eurythermity and to the larger range of this species.