Glacio-eustatic Sea-level Fluctuations Research Articles

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.

Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition

The Ordovician–Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician–early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48–73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30–64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician–early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δ13Corg values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δ13Corg isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

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).

The earliest occurrence of the ichnogenus Psilonichnus: a new record from the Mississippian of the West of Ireland

The ichnogenus Psilonichnus Fürsich, 1981 is recorded for the first time from the Mississippian (Brigantian regional substage) limestones of the Slievenaglasha Formation from the west coast of County Clare, Ireland. This record extends the known range of Psilonichnus which has previously been recorded from rocks of Jurassic or younger age. The trace maker of the burrows described herein has not been preserved and remains unknown; however, the ichnogenus Psilonichnus is generally ascribed to the burrowing activities of decapod crustaceans such as crabs or shrimp. Decapods from the Paleozoic are poorly known and assigning the traces to a maker must be treated with caution. However, this record indicates that burrowing organisms using a similar life strategy to modern decapods were established by Brigantian times. These examples are assigned to the Glossifungites ichnofacies, as they occur on an omission surface in a shallow marine carbonate firmground. The burrows were subsequently infilled with deposits of possible storm origin. Glacioeustatic sea-level fluctuation related to the onset of the Late Paleozoic Ice Age is a common feature of Carboniferous sedimentary successions at this level, and the Irish (Mississippian) Psilonichnus horizon occurs just below a significant regional sequence boundary at the top of the Slievenaglasha Formation.

Late Pennsylvanian carbonate platform facies and coral reef: new insights from southern China (Guizhou Province)

The Pennsylvanian is characterized by intense paleoenvironmental changes related to glacio-eustatic sea-level fluctuations and major tectonic events, which affected the evolution of biocommunities. Most known Pennsylvanian tropical reefs and mounds are predominantly composed of calcareous algae (e.g. phylloid algae, Archaeolithophyllum), calcareous sponges, fenestrate bryozoans, Tubiphytes, and microbialites. However, in Houchang (southern China), the Late Pennsylvanian carbonate platform records a large coral reef lacking any analogs in age (Gzhelian), size (80–100 m thick) and composition (high biodiversity). The large coral reef developed at the border of the Luodian intraplatform basin. The intraplatform basin is characterized by the deposition of green algal grainstone, coated grain grainstone and bioclastic packstone, grainstone, floatstone and rudstone in shallow-waters. In the deep-water shelf, lithofacies are composed of burrowed bioclastic wackestone, microbioclastic peloidal packstone, grainstone, and fine-grained burrowed wackestone and packstone. In this context, the coral reef developed on a deep-shelf margin, in a moderate to low energy depositional environment, below the FWWB. The scarcity of Pennsylvanian coral reefs suggests global unfavorable conditions, which can be attributed to a complex pattern of several environmental factors, including seawater chemistry (aragonite seas), paleoclimatic cooling related to continental glaciation, and the biological competition with the more opportunistic and adaptive phylloid algal community that occupied similar platform margin paleoenvironments. The existence of the large Bianping coral reef in southern China, as well as a few additional examples of Pennsylvanian coralliferous bioconstructions, provides evidence that coral communities were able to endure the Late Paleozoic fluctuating paleoenvironmental conditions in specific settings. One of such settings appears to have been the deep shelf margin, where low light levels decreased competition with the phylloid algal community.

The onset of the major glaciation of the LPIA: record from South China

The beginning of the Late Paleozoic Ice Age was characterized by localized events with small ice depocenters occurring in South America and a series of cyclic shallow-water limestones in far-field regions, in late Visean to Serpukhovian. This paper documents two topmost Visean–Serpukhovian sections in Guangxi, South China: a stromatolite-bearing deep-water succession in Helv village and a shallow-water succession in Dujie village. Seventeen microfacies types have been recognized and grouped into six microfacies associations. The vertical evolution of the microfacies associations from deep-water to shallow-water depositional environments in both sections indicate a major relative sea-level fall in the latest Visean. The repetitive successions comprising different morphological stromatolites in Helv section and the cyclic alternation of subtidal and peritidal facies in Dujie section imply high-frequency sea-level fluctuations during latest Visean to Serpukhovian, most likely eustatic in origin. The carbon isotope data of Dujie limestone exhibit a pronounced positive δ13C excursion with ± 4.3‰ in the peritidal deposits. This shift is interpreted as the result of enhanced organic carbon burial in other regions and depletion of the ocean in light carbon. The widespread coeval cyclicity occurring in settings in South China, Western Europe, and North America is most likely the results of glacio-eustatic sea-level fluctuations, possibly with local influences of tectonic movements. This pattern is interpreted to represent the expression of the onset of the major glaciation during the LPIA in low-latitudinal successions.

Marine terrace development on reefless volcanic islands: New insights from high-resolution marine geophysical data offshore Santa Maria Island (Azores Archipelago)

Submerged marine terraces are relict coastal erosional landforms now underwater due to rising sea level and/or land subsidence. Using as case study the shelf around Santa Maria Island (North Atlantic Ocean), we intend to advance our knowledge on the formation and preservation of these features on reefless volcanic islands. Santa Maria is an ideal place to study their combined generation, since it displays a sequence of subaerial and submerged marine terraces (the latter not studied before), distributed between 7/230 m in elevation, and −40/−140 m in depth, respectively. Based on some geological constraints, we investigated a possible correlation between the formation of the different terraces with known sea-level changes. Our results suggest that the spatial distribution of marine terraces at Santa Maria depends on the complex interplay between glacio-eustatic sea-level fluctuations, the island's vertical motion trends, the morphology of the shelf, and the intensity of marine erosion. Subaerial terraces probably developed from ~3.5 Ma to ~1 Ma following a fortuitous conjugation of optimal exposure to energetic waves and a suitable arrangement/lithology of the stratigraphic units promoting easier erosion. Their preservation was likely promoted by the uplift trend the island experienced in the last 3.5 Ma, which was rapid enough to prevent their destruction by subsequent highstands. The submerged terraces, presumably all younger than ~1 Ma, were largely influenced by shelf gradient, leading to more developed and preserved terraces in wider and low-gradient sectors. Displacement by active faults also conditioned the formation and further development of both subaerial and submerged terraces, with tectonic activity documented for the 0.693 Ma–2.7 Ma period.

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.

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.

Sedimentary architecture of the Bohai Sea China over the last 1 Ma and implications for sea-level changes

Sedimentary architecture dominated by transgression–regression cycles in the shallow Bohai shelf region contains information about global sea-level, climate and local tectonics. However, previous studies of transgression–regression cycles in this region at orbital timescales that extend back to the early Pleistocene are sparse, mainly because of the shortage of well-dated long cores. Although transgression–regression sedimentary cycles in the region have been interpreted in terms of local tectonics, sea-level, and climate change, the detailed structure of marine transgressions and their significance for Quaternary global sea-level variations remains to be examined. In this study, we present an integrated sedimentological, geochemical and paleontological study of a 212.4 m (∼1 Ma) core (BH08) recovered from the Bohai Sea, China, for which an astronomically-based age model is available. Correspondence between marine-terrestrial sedimentary cycles and global sea-level fluctuations suggests that stacking of marine and terrestrial sediments was driven mainly by glacio-eustatic sea-level fluctuations in a context in which tectonic subsidence was largely balanced by sediment supply over the last ∼1 Ma. We report a dominant 100-kyr cycle beginning at ∼650 ka, which reflects the worldwide influence of the mid-Pleistocene transition (MPT) in sea-level records. We find that neritic deposits after the MPT were relatively thicker than before the MPT, which indicates an important control of the MPT on sedimentary architecture through lengthening of the duration of sea-level highstands.

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.

Mixed Carbonate–Siliciclastic Sedimentation Along the Great Barrier Reef Upper Slope: A Challenge To the Reciprocal Sedimentation Model

Abstract Results of studies involving numerous cores and ODP holes along the Great Barrier Reef (GBR) margin and adjacent Queensland Trough and Queensland Plateau have challenged the use of a reciprocal sedimentation model to describe the sedimentary response of slope and basin settings to glacioeustatic sea-level fluctuations. Upper-slope sedimentation results from the relationships between sea-level fluctuations, antecedent topography, and regional climate that play an important role in the type and amount of sediment deposited on the upper slope during glacial, deglacial, and interglacial times. During the Last Glacial Maximum (LGM, > 20 ka ago) upper-slope sediments generally lacked siliciclastic material and are characterized by very low accumulation rates, whereas early deglacial-time (Termination I, TI) deposits are dominated by a siliciclastic and neritic carbonate pulse. Siliciclastic sedimentation was significantly reduced in the Holocene, while carbonate sedimentation remains elevated. A new borehole, IODP Expedition 325 Hole M0058A (Hole 58A), recovered 82% of a 40.4 m hole on the upper slope east of Noggin Passage on the central GBR margin near Cairns, Australia. Hole 58A provides a detailed sedimentary record during Termination II (TII), Marine Isotope Stage 6/5e (MIS-6/5e), deglacial transition, and through most of interglacial MIS-5. This hole, along with two others (ODP Leg 133 Holes 820A and 819A from the upper slope east of Grafton Passage), show carbonate–siliciclastic cyclicity as the result of glacioeustatic change with the GBR shelf. Sedimentation at Hole 58A is consistent with that of previous studies along the GBR margin (focusing on the LGM to present), and extends the upper-slope sedimentary record back to TII and interglacial MIS-5. A siliciclastic pulse similar to the one during TI occurred during the penultimate deglaciation, TII; however, the maximum neritic aragonite export to the upper slope occurred not during peak MIS-5e highstand when sea level was a few meters above modern position, but subsequently during a time (MIS-5d to 5a) when lowered sea level fluctuated between 30 and 50 m below present sea level. Siliciclastic sediments were reworked and exported to the upper slope when the lowstand fluvial plain was re-flooded, whereas neritic carbonate export to the slope reached a maximum when sea level fell and much of the mid to outer shelf re-entered the photic zone, subsequent to a drowning interval. Thus, this analysis refines the mixed-sedimentation models of upper-slope sedimentation along the central GBR margin during the penultimate deglacial transgression and subsequent interglacial early and late highstand. This study provides further evidence that mixed carbonate–siliciclastic margins do not always behave in a predictable manner and that mixed margins both modern and ancient would benefit from detailed study of sediment transport in the context of sea-level rise and fall.

Methane Gas Hydrate Stability Models on Continental Shelves in Response to Glacio-Eustatic Sea Level Variations: Examples from Canadian Oceanic Margins

We model numerically regions of the Canadian continental shelves during successive glacio-eustatic cycles to illustrate past, current and future marine gas hydrate (GH) stability and instability. These models indicated that the marine GH resource has dynamic features and the formation age and resource volumes depend on the dynamics of the ocean-atmosphere system as it responds to both natural (glacial-interglacial) and anthropogenic (climate change) forcing. Our models focus on the interval beginning three million years ago (i.e., Late Pliocene-Holocene). They continue through the current interglacial and they are projected to its anticipated natural end. During the current interglacial the gas hydrate stability zone (GHSZ) thickness in each region responded uniquely as a function of changes in water depth and sea bottom temperature influenced by ocean currents. In general, the GHSZ in the deeper parts of the Pacific and Atlantic margins (≥1316 m) thinned primarily due to increased water bottom temperatures. The GHSZ is highly variable in the shallower settings on the same margins (~400–500 m). On the Pacific Margin shallow GH dissociated completely prior to nine thousand years ago but the effects of subsequent sea level rise reestablished a persistent, thin GHSZ. On the Atlantic Margin Scotian Shelf the warm Gulf Stream caused GHSZ to disappear completely, whereas in shallow water depths offshore Labrador the combination of the cool Labrador Current and sea level rise increased the GHSZ. If future ocean bottom temperatures remain constant, these general characteristics will persist until the current interglacial ends. If the sea bottom warms, possibly in response to global climate change, there could be a significant reduction to complete loss of GH stability, especially on the shallow parts of the continental shelf. The interglacial GH thinning rates constrain rates at which carbon can be transferred between the GH reservoir and the atmosphere-ocean system. Marine GH can destabilize much more quickly than sub-permafrost terrestrial GHs and this combined with the immense marine GH reservoir suggests that GH have the potential to affect the climate-ocean system. Our models show that GH stability reacts quickly to water column pressure effects but slowly to sea bottom temperature changes. Therefore it is likely that marine GH destabilization was rapid and progressive in response to the pressure effects of glacial eustatic sea level fall. This suggests against a catastrophic GH auto-cyclic control on glacial-interglacial climate intervals. It is computationally possible but, unfortunately in no way verifiably, to analyze the interactions and impacts that marine GHs had prior to the current interglacial because of uncertainties in temperature and pressure history constraints. Thus we have the capability, but no confidence that we can contribute currently to questions regarding the relationships among climate, glacio-eustatic sea level fluctuations and marine GH stability without improved local temperature and water column histories. We infer that the possibility for a GH control on climate or oceanic cycles is speculative, but qualitatively contrary to our model results.

Late pleistocene sedimentation history of the Shirshov Ridge, Bering Sea

The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was the Yukon-Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and glacial cycles in Alaska.

Off-shelf sedimentary record of recurring global sea-level changes during the Plio-Pleistocene: evidence from the cyclic fills of exhumed slope systems in central Italy

The markedly cyclic sedimentary successions of four late Pliocene to early Pleistocene slope turbidite systems exposed in eastern central Italy have been resolved into 31 high-frequency sequences. Chronological constraints from biostratigraphy and magnetostratigraphy indicate that these successions form a composite, partially overlapping stratigraphic record and sequence-bounding surfaces can be convincingly correlated with glacial oxygen isotope stages G2–60 ( c . 2.65–1.7 Ma) inclusive. The studied successions, therefore, preserve an extraordinary and legible record of recurring, orbitally dictated glacio-eustatic sea-level fluctuations and provide an unprecedented opportunity to examine the deep-water sedimentary response to such high-frequency changes from an outcrop perspective. Supplementary material: Sedimentological attributes of lithofacies identified in the studied successions are available at www.geolsoc.org.uk/SUP18549 .

The incomplete history of mitochondrial lineages between two rockfishes, Sebastes longispinis and Sebastes hubbsi (Scorpaeniformes: Scorpaenidae)

The genetic divergence between two closely related rockfishes, Sebastes longispinis and Sebastes hubbsi, was inferred from both mitochondrial DNA (mtDNA) sequence variations and amplified fragment length polymorphism (AFLP) markers. The two species were placed into two distinct clades in a neighbour-joining tree based on the AFLP data, clearly indicating that they represented separate species. Although this evidence, together with a previous morphological study, revealed clear differences between the two species, no obvious clustering of haplotypes by species was detected in the minimum spanning network inferred from sequence variations in the mtDNA control region (c. 500 base pairs). In fact, the significant Φ(ST) estimates indicated only a restriction of gene flow between the two species. Uncorrected pairwise sequence differences in mtDNA between two species were small (1·8% at maximum, on the lower end of the range of control region divergence between previously studied sister species pairs), suggesting their speciation event as having been fairly recent. The incongruent results of AFLP and mtDNA phylogenies suggested incomplete lineage sorting and introgression of mtDNA in the course of the evolution of the two species. Differences in their main distributional ranges and the small level of sequence divergence in mtDNA suggests that speciation and dispersal may have been associated with glacio-eustatic sea level fluctuations between the Japanese Archipelago and the Korean Peninsula during the past 0·4 million years.

Glacioeustatic cyclicity of a Pennsylvanian carbonate platform in a foreland basin setting: An example from the Bachende Formation of the Cantabrian Zone (NW Spain)

By analysis of outcrop data, sequence stratigraphy was used to interpret the stratal architecture and evolution of a Moscovian (Pennsylvanian) carbonate platform (Bachende Formation) located in the Variscan foreland basin of the Cantabrian Zone (NW Spain). The Bachende Formation represents a delta-top carbonate platform developed in highly subsiding (average subsidence rate of 254m/My) distal shelf areas of the basin. It reached a thickness of 650m and covered an area of ~170km2 (after conservative palinspastic restorations). The platform interior succession is mostly composed of low-relief mound-shaped micritic boundstones, alternating with bioclastic mudstones/wackestones, peloidal–foraminiferal packstones/grainstones, calcareous algal bafflestones and skeletal and ooidal grainstones. Landwards (to the W), carbonate platform beds alternate and interfinger with deltaic sandstones and shallow marine shales.The platform interior consists of three sequences (S1, S2 and S3), ranging from 180 to 230m in thickness and from 1.3 to 0.5My in age. These sequences are in turn subdivided into several meter-scale (10.6m average thickness), high-frequency (110 to 15ky), subtidal transgressive–regressive cycles. These cycles are bounded by subaerial exposure or marine flooding surfaces and are inferred to be laterally continuous across the platform-top. They vary from mixed carbonate-siliciclastic to carbonate-dominated cycles.Cyclicity on the Bachende platform was strongly influenced by glacioeustatic sea level variations and changes in subsidence and sedimentation rates. Subaerial exposure surfaces capping high-frequency cycles are better developed in the sequence 1 (late Kashirian/early Podolskian), when subsidence was relatively low and glacioeustatic sea level fluctuations could have exposed subtidal deposits during base-level falls. On the basis of the conservative estimated duration of each cycle and spectral analysis of cycle thickness, the precessional (~20ky) and obliquity (~40ky) cycles were detected in all the sequences, the short eccentricity cycle (~100ky) in the sequence 1, and the long eccentricity cycle (~400ky) in the sequences 2 and 3 (Podolskian). Nevertheless, precessional and obliquity frequencies seem to dominate during sequences 2 and 3, which were deposited under significant higher subsidence and sedimentation rates. Additionally, spectral peaks at ~60–80ky are randomly present across the whole succession, and they cannot be explained as a linear response of the climate system to the Milankovitch forcing.

Temporal Changes of Plio—Pleistocene Ostracoda from the Takanabe Formation, Miyazaki Group, Southwest Japan

A total of 145 ostracode taxa were obtained from the upper part of the Plio—Pleistocene Takanabe Formation, Miyazaki Group, in the northern part of the Miyazaki Plain, Southwest Japan. Most of the assemblages are extant temperate and subtropical water species that have been reported from the recent western North Pacific Ocean around Southwest Japan, the Tsushima Strait, and the shelves off China. Four ostracode bioassociations (UA, MA, LA, and DA) and two biofacies (SF and DF) were discriminated on the basis of Rand Q-mode cluster analyses. Biofacies SF is divided into six sub-biofacies (SF1–6). The results revealed at least two transgressions and one regression during the deposition of the upper part of the Takanabe Formation. The sea-level fluctuation at approximately 2.6 Ma inferred in this study is extremely large compared with the glacio-eustatic sea-level fluctuations during the late Pliocene to early Pleistocene, suggesting that local tectonic movement in the study site during the deposition of the upper part of the studied interval at ∼2.6 Ma may have led to the large sea-level fluctuation. This paper also includes descriptions of two new species, Acanthocythereis abei sp. nov. and Acanthocythereis takanabensis sp. nov.

Plio-Pleistocene Bryozoan faunas of the Wanganui Basin, New Zealand: stratigraphic distribution and diversity

The New Zealand continental shelf has a rich bryozoan fauna, and Bryozoa are relatively common as fossils in local marine-deposited strata. We record 103 species of Bryozoa occurring in the Wanganui Basin sequence spanning the Plio-Pleistocene (3.0–0.3 Ma), 77 of which belong to the order Cheilostomata and the remainder to the order Cyclostomata. The majority are conspecifics of the modern New Zealand shelf fauna. Palaeoecological characteristics of Wanganui Basin bryozoan faunas including diversity, taxonomic and colony growth-form combinations are identified, defining three biofacies groupings corresponding to depositional sequences (systems tracts). Water depth, sedimentation, substratum characters and taphonomic processes have all influenced the distribution and preservation of bryozoans in the local fossil record. Broader stratigraphic variations in Wanganui Basin faunas are explained by ‘habitat tracking’: a lateral shifting of bryozoan palaeocommunity zones in response to changing environments related to Plio-Pleistocene glacio-eustatic sea-level fluctuations.

Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)

Globally documented carbon isotope excursions provide time-varying signals that can be used for high-resolution stratigraphic correlation. We report detailed inorganic and organic carbon isotope curves from carbonate rocks of the Ellis Bay and Becscie Formations spanning the Ordovician-Silurian boundary on Anticosti Island, Quebec, Canada. Strata of the Anticosti Basin record the development of a storm-dominated tropical carbonate ramp. These strata host the well-known Hirnantian positive carbon isotope excursion, which attains maximum values of ~4.5‰ in carbonate carbon of the Laframboise Member or the Fox Point Member of the Becscie Formation. The excursion also occurs in organic carbon, and δ^(13)C_carb and δ^(13)C_org values covary such that no reproducible Δ^(13)C (= δ^(13)C_carb – δ^(13)C_org) excursion is observed. The most complete stratigraphic section, at Laframboise Point in the west, shows the characteristic shape of the Hirnantian Stage excursion at the global stratotype section and point (GSSP) for the Hirnantian Stage in China and the Silurian System in Scotland. We therefore suggest that the entire Hirnantian Stage on Anticosti Island is confined to the Laframboise and lower Fox Point Members. By documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin, we are able to reconstruct glacioeustatic sea-level fluctuations corresponding to maximum glacial conditions associated with the end-Ordovician ice age. The combined litho- and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and Becscie limestones; the former transgresses from west to east, and the latter progrades from east to west. The sea-level curve consistent with our sequence-stratigraphic model indicates that glacioeustatic sea-level changes and the positive carbon isotope excursion were not perfectly coupled. Although the start of the isotope excursion and the initial sea-level drawdown were coincident, the peak of the isotope excursion did not occur until after sea level had begun to rise. Carbon isotope values did not return to baseline until well after the Anticosti ramp was reflooded. The sea-level–δ13Ccarb relationship proposed here is consistent with the “weathering” hypothesis for the origin of the Hirnantian δ^(13)C_carb excursion.