Major Sea-level Changes Research Articles

Decomposition of Deglacial Pacific Radiocarbon Age Controls Using an Isotope‐Enabled Ocean Model

AbstractDuring the last deglaciation Earth’s climate experienced strong and abrupt variations, resulting in major changes in global temperature, sea level, and ocean circulation. Although proxy records have significantly improved our understanding of climate during this period, questions remain regarding the connection between ocean circulation evolution and resulting geotracer distributions, including those of deep waters in the Pacific. Here we use the C‐iTRACE simulation, a transient ocean‐only, isotope‐enabled version of the Community Earth System Model, to better understand deglacial deep Pacific radiocarbon evolution in the context of circulation and reservoir age changes. Throughout the deglaciation, the Pacific Ocean circulation in C‐iTRACE responds strongly to glacial meltwater forcing, leading to large changes in deep Pacific Δ14C age. A multi‐millennial weakening of the overturning circulation from 20 to 15 ka BP leads to increases in deep Pacific Δ14C ages, but from 20 to 18 ka BP, nearly half (40%–60%) of this aging is controlled by changing surface reservoir age, corroborating previous studies showing that Δ14C is not solely a circulation age tracer. As the deglaciation proceeds, circulation change controls progressively more of the Δ14C age, accounting for more than 75% of it across the deep Pacific from 15 to 8 ka BP.

Holocene sea-level changes of the Persian Gulf

High-resolution shallow seismic information supported by sedimentological and geochemical data from sediment cores has been used for reconstruction of Holocene sea-level changes along the northern coast of the Persian Gulf. These investigations in the area of the North Qeshm Island Waterway (NQIW) near the Strait of Hormuz reveal a series of five prominent sea-level fluctuations starting with the second phase of regional postglacial sea-level rise recorded between 15.0 ka and 9.5 ka (ka = thousands of years before present). Based on our study, sea-level had reached −22 m before 9.1 ka. Postglacial sea level high-stand in the study area has been dated to have occurred around 6.0 ka, which was followed by a relative sea-level fall coeval with a regional climate change from humid to arid conditions at about 5.4 ka. After a renewed, but relatively short period of sea level rise associated with climate changes around 4.35 ka, sea-level fell again at about 3.3 ka and remained relatively stable from 3.2 ka until the present. We conclude that major sea-level changes in the Persian Gulf coincided with sea-level fluctuations in the Indian Ocean and observed changes in North Atlantic Ocean and atmospheric circulation and Greenland Ice Sheet melting. Thus, despite of regional tectonic instability, regional sea-level stand in the Persian Gulf has been controlled mainly by eustatic sea-level changes of the global ocean.

The challenges of constraining shelf sea tidal models using seabed sediment grain size as a proxy for tidal currents

Past major changes in sea level have had a significant influence on global- and shelf sea tidal dynamics. Some of these changes are preserved in sedimentary records from the shelf seas, and so appropriate proxy data have the potential to constrain tidal model outputs over the recent geological past. Tidal models which simulate the evolution of tide-dependent parameters over geological timescales are fundamental to understanding the response of the tides to sea-level rise and climate change. This study explores a potential new sedimentary proxy for validating past shelf sea tidal dynamics, interrogating the relationship between tidally-modulated bed shear stress and seabed sediment grain size at discrete sediment core locations over the northwest European shelf seas. Radiocarbon-dated sediment grain size profiles were generated for four British Geological Survey UK shelf sediment vibrocores, spanning a range of physical environments. Changes in observed sediment grain size through time were compared with simulated changes in tidal-induced bed shear through time, using temporal and spatial outputs from the most recently developed palaeotidal model of the Northwest European shelf seas. Although a positive correlation between observed grain size and simulated bed shear stress was observed at three of the four sediment cores sites, no robust relationship could be quantified. The palaeotidal model output failed to resolve the details of the actual sediment dynamics, since only tidal-induced bed shear stresses were considered. Wave processes were neglected, and the model was not sensitive enough to constrain simulated past tidal conditions at point locations; rather it is suitable for examining general trends. There remains a need to develop new proxies for past shelf sea hydrodynamic conditions which can be used to constrain numerical model output of tidal currents at regional scales.

Revised Badenian (middle Miocene) depositional systems of the Austrian Vienna Basin based on a new sequence stratigraphic framework

Abstract This paper presents a revised sequence stratigraphy for the lower, middle and upper Badenian depositional systems of the Austrian Vienna Basin based on the integration of 3D seismic surveys and well data. The study area in the central and northern part of the Austrian Vienna Basin is covered with 3D seismic data. According to a new sequence stratigraphic framework established in the southern part of the Vienna Basin, the Badenian is subdivided into three 3rd order depositional sequences. For each sequence, paleogeographic maps are created, representing coeval depositional systems within a chronostrati-graphic interval. Lower Badenian sediments of the 1st sequence (Ba1) represent fillings of the pre-Badenian sub-basins with a major change of sediment transport direction. The early stage of the 1st sequence is dominated by subaerial braided river deposits which use two pronounced canyon systems (Mistelbach Canyon and Reinthal Canyon) on the northwestern margin of the Vienna Basin as a bypass zone towards the marine depositional system of the North Alpine-Carpathian Foredeep. The late stage of the 1st sequence reflects the change from subaerial to marine depositional environments with main sediment influx from the west, creating two major eastwards prograding delta systems (Zistersdorf Delta and Mühlberg Delta). Depositional systems of the middle Badenian 2nd sequence (Ba2) reflect the interplay between ongoing extensional fault tectonics and major sea-level changes. Lower Badenian paleo-highs in the northern part are drowned during the 3rd sequence (Ba3), thus the Mühlberg Delta and the Zistersdorf Delta merge into one delta system. During the Ba3 the drowning of the Spannberg Ridge initiates a clockwise rotation of the Zistersdorf Delta. Thus, the former Zistersdorf Delta transforms into the Matzen Delta covering the Spannberg Ridge. Together with the Mühlberg Delta, they represent the last full marine depositional system of the eastward prograding paleo-Danube Delta in the Austrian Vienna Basin.

Fluvial response to rising levels of the Black Sea and to climate changes during the Holocene: Luminescence geochronology of the Sakarya terraces

The Sakarya River is among the largest fluvial systems of the southern Black Sea basin, draining most of NW Anatolia. The river crosses the high relief of the Pontide mountain range through successive narrow gorges and strike-slip basins formed by the North Anatolian Fault (NAF) System. We have investigated this fluvial record along the course of the main river channel at its lower reaches. The study site is located south of the Adapazarı Basin, ~50 km inland from the Black Sea, where remnant floodplains are preserved as a three-step terrace staircase resulting from continuous uplift to the south of the NAF. The combination of high resolution mapping with a detailed luminescence (OSL and p-IR-IRSL) and radiocarbon geochronology has shed light on changes in the level of the Black Sea and in the hydrological system during the late Pleistocene to recent. The last glacial period is represented by the highest terrace (T3) indicating deposition during marine isotope stage (MIS) 3 in between two low stands of the Black Sea. Following a long-term erosional period initiated prior to last glacial maximum (LGM), the initiation of the deposition (T2) was synchronous with the proposed catastrophic sea level rise of the Black Sea (cal. C14 9.3 ka BP) which continued throughout the Holocene until the Roman warm period (1.8 ka BP). The late-Holocene to recent morphological evolution of the region is marked with two sequential erosional and depositional (T1 and T0) periods, which can be correlated with the well-documented historical climate shifts affecting the hydrological system. These results reveal that the erosional and depositional periods on the Sakarya River floodplain are controlled by major sea level changes and climatically induced fluctuations in discharge and sediment supply.

The evolution of the Great Barrier Reef during the Last Interglacial Period

Reef response to Last Interglacial (LIG) sea level and palaeoenvironmental change has been well documented at a limited number of far-field sites remote from former ice sheets. However, the age and development of LIG reefs in the Great Barrier Reef (GBR) remain poorly understood due to their location beneath modern living reefs. Here we report thirty-nine new mass spectrometry U-Th ages from seven LIG platform reefs across the northern, central and southern GBR. Two distinct geochemical populations of corals were observed, displaying activity ratios consistent with either closed or open system evolution. Our closed-system ages (~129–126ka) provide the first reliable LIG ages for the entire GBR. Combined with our open-system model ages, we are able to constrain the interval of significant LIG reef growth in the southern GBR to between ~129–121ka. Using age-elevation data in conjunction with newly defined coralgal assemblages and sedimentary facies analysis we have defined three distinct phases of LIG reef development in response to major sea level and oceanographic changes. These phases include: Phase 1 (>129ka), a shallow-water coralgal colonisation phase following initial flooding of the older, likely Marine Isotope Stage 7 (MIS7) antecedent platform; Phase 2 (~129ka), a near drowning event in response to rapid sea level rise and greater nutrient-rich upwelling and; Phase 3 (~128–121ka), establishment of significant reef framework through catch-up reef growth, initially characterised by deeper, more turbid coralgal assemblages (Phase 3a) that transition to shallow-water assemblages following sea level stabilisation (Phase 3b). Coralgal assemblage analysis indicates that the palaeoenvironments during initial reef growth phases (1 and 2) of the LIG were significantly different than the initial reef growth phases in the Holocene. However, the similar composition of ultimate shallow-water coralgal assemblages and slow reef accretion rates following stabilisation of sea level (phase 3b) suggest that reefs of both ages developed in a similar way during the main phase of relatively stable sea level.

The Black Sea Dating Game and Holocene Marine Transgression

Dating of major sea-level changes using shells or calcareous microfossils is prone to errors in semi-enclosed marine environments where inputs of seawater and river water vary over time and space. The need to refine mollusc-based age estimates for the rate of the Holocene marine transgression in the Black Sea is the focus of multiple palaeoceanographic and archaeological studies. This ongoing “dating game” seeks to clarify conflicting evidence for a hypothetical catastrophic marine flood that forced the emigration of Neolithic farmers from the shores of a Holocene freshwater lake in the Black Sea. The potential importance of confirming or rejecting this megaflood hypothesis has led to multiple attempts at refining the chronology of the marine transgression and quantifying the palaeosalinity of the Black Sea surface water during the Holocene. Here we report that six new AMS radiocarbon ages of 8890 ± 50 to 8450 ± 40 yr BP were obtained for wood, grass and sedge leaves from peat layers in Core 342 at 33.16 - 32.71 m below present sea level on the Ukrainian Shelf. These plant materials provide critical new ages for quantifying Black Sea carbon reservoir issues. The accuracy of our new AMS wood/peatages is independently supported by palynochronological correlation. The ages of our plant materials have ~100 years precisionandare ~420 - 520 years younger than those previously reported for unsorted detrital peat in Core 342. Paired mollusc—wood ages for brackish—freshwater Dreissena polymorpha shell from detrital peat also shows that an inaccuracy of >1120 yr can arise for shells during times when carbon reservoir values in the semi-isolated, brackish-water Black Sea could depart significantly from global average. Our revised sea level curve shows a gradual early Holocene transgression from water depths of -45.9 to -32.8 m, with initial Mediterranean inflow by 8.9 ka BP.

Terrestrial source to deep-sea sink sediment budgets at high and low sea levels: Insights from tectonically active Southern California

AbstractSediment routing from terrestrial source areas to the deep sea influences landscapes and seascapes and supply and filling of sedimentary basins. However, a comprehensive assessment of land-to-deep-sea sediment budgets over millennia with significant climate change is lacking. We provide source to sink sediment budgets using cosmogenic radionuclide–derived terrestrial denudation rates and submarine-fan deposition rates through sea-level fluctuations since oxygen isotope stage 3 (younger than 40 ka) in tectonically active, spatially restricted sediment-routing systems of Southern California. We show that source-area denudation and deep-sea deposition are balanced during a period of generally falling and low sea level (40–13 ka), but that deep-sea deposition exceeds terrestrial denudation during the subsequent period of rising and high sea level (younger than 13 ka). This additional supply of sediment is likely owed to enhanced dispersal of sediment across the shelf caused by seacliff erosion during postglacial shoreline transgression and initiation of submarine mass wasting. During periods of both low and high sea level, land and deep-sea sediment fluxes do not show orders of magnitude imbalances that might be expected in the wake of major sea-level changes. Thus, sediment-routing processes in a globally significant class of small, tectonically active systems might be fundamentally different from those of larger systems that drain entire orogens, in which sediment storage in coastal plains and wide continental shelves can exceed millions of years. Furthermore, in such small systems, depositional changes offshore can reflect onshore changes when viewed over time scales of several thousand years to more than 10 k.y.

The Late Holocene evolution of the Black Sea – a critical view on the so-called Phanagorian regression

Throughout its geologic history, the Black Sea experienced major sea level changes accompanied by severe environmental modifications, including geomorphologic reshaping. The most spectacular changes were driven by the Quaternary glaciations and deglaciations that reflect responses to Milankovitch cycles of 100 and 20 ky periodicity. Major sea level changes were also considered for a shorter and more recent cyclicity. The concept of the Phanagorian re- and transgression cycle, supposedly with a minimum sea level stand of 5–6 m below its present position in the middle of the 1st millennium BC, was established in 1963 by Fedorov for the Black Sea region. It was based on archaeological and palaeogeographical research conducted around the ancient Greek colonies of the Cimmerian Bosporus, in particular at the name giving site of Phanagoria, where underwater prospection had revealed the presence of a large number of submerged relics of the Classical Greek era.Analyses of sediment cores as well as 14C-dated fossil coastal bars in the western and southern parts of Taman Peninsula show that contemporary coastal bars are related to different sea levels. The dissymmetry can reach up to 6 m around 500 BC. This and more evidence from drill cores confirms that on Taman Peninsula many of the apparent sea level changes are tectonically induced. The subsidence may have been initiated by the release of gas from mud volcanoes inherited along anticline axes. Other observations around the Black Sea confirm that submerged archaeological sites correspond to areas where subsidence has taken places, while the so-called Holocene highstand – said to have been located above the present-day sea level – is associated with uplift areas (triggered by the ongoing Caucasus orogeny). Recent oceanographic research carried out in the Black Sea area shows that since the Black Sea was reconnected with the Mediterranean Sea (i.e., 7500 14C BP at the latest), both marine water bodies have been in equilibrium. This fact and arguments from archaeology, history, hydrodynamics etc. lead us to question the existence of the Phanagorian regression. It is important to note that none of the sea level curves established for the (eastern) Mediterranean shows a comparable regression/transgression cycle of several metres during the 1st millennium BC.

Sequence surfaces and paleobathymetric trends in Albian to Maastrichtian sediments of Ariyalur area, Cauvery Basin, India

Exposed Late Cretaceous (Albian–Maastrichtian) marine rocks of the Ariyalur area in the Cauvery Basin have been extensively studied based on biostratigraphy and paleobathymetry with paleobathymetric interpretation carried out using vertical and lateral relationships of rock facies, macro- and microfossil assemblages, textural characteristics and diagenetic changes of the lithologic units. The integration of these data reveals four Transgressive–Regressive (T–R) cycles, viz. Dalmiapuram, Garudamangalam, Sillakkudi and Kallankurichchi (in stratigraphic order). These T–R cycles have been compared with global published relative sea level curves of the study area. The major sea level changes during the Late Turonian and Late Maastrichtian in the study area correlate well with global sea level changes of Vail et al. (1977), Haq et al. (1987) and Miller et al. (2005). Based on biostratigraphy, stratal patterns and their relationship, the Late Cretaceous succession of the Ariyalur area is thus subdivided into four 2nd/3rd order sequences.

Phylogeography of the Indo-Pacific parrotfish Scarus psittacus: isolation generates distinctive peripheral populations in two oceans

Phylogenetic, phylogeographic, population genetic and coalescence analyses were combined to examine the recent evolutionary history of the widespread Indo-Pacific parrotfish, Scarus psittacus, over a geographic range spanning three marine biogeographic realms. We sequenced 164 individuals from 12 locations spanning 17,000 km, from 55oE to 143oW, using 322 base pairs of mitochondrial control region (D-loop). S. psittacus displayed high haplotype (h = 0.83–0.98), but low nucleotide ( 83%) genetic variation was within populations. AMOVA revealed significant partitioning and identified five geographic groups. These included one central population and four populations peripheral to the centre. The central population occupied reefs from Western Australia to Tahiti and represented the central Indo-Pacific biogeographic realm. Cocos Keeling was distinct from central and western Indo-Pacific biogeographic realms occupying a position intermediate to these. Peripheral populations (Hawaii, Marquesas) represented the eastern Indo-Pacific biogeographic realm, while Seychelles represented the western Indo-Pacific biogeographic realm. All but the central population expanded (<163 kya). Whilst all populations experienced major sea level and SST changes associated with Pleistocene glaciation cycles, the genetic structure of the central population was relatively homogenous unlike the remaining genetically distinctive populations.

Adaptive Cycles of Coastal Hunter-Gatherers

Along the southeastern Atlantic coast of Georgia, hunter-gatherer groups substantially altered the landscape for more than three millennia (ca. 4,200-1,000 B.P.) leaving behind a distinct material record in the form of shell rings, middens, and burial mounds. During this time, these groups experienced major changes in sea level and resource distribution. Specifically, we take a resilience theory approach to address these changes and discuss the utility of this theory for archaeology in general. We suggest that despite major destabilizing forces in the form of sea-level lowering and its concomitant effects on resource distribution, cultural systems rebounded to a structural pattern similar to the one expressed prior to environmental disruption. We propose, in part, the ability for people to return to similar patterns was the result of the high visibility of previous behaviors inscribed on the landscape in the form of shell middens and rings from the period preceding environmental disruption. Finally, despite a return to similar cultural formulations, hunter-gatherers experienced some fundamental changes resulting in modifications to existing behaviors (e.g., ringed villages) as well as the addition of new ones in the form of burial-mound construction.

Large perturbations of the carbon and sulfur cycle associated with the Late Ordovician mass extinction in South China

High-resolution δ 13 C data of organic carbon from a continuous section of the Late Ordovician–Early Silurian reveal two positive δ 13 C excursions that are associated with the mass extinction in South China. The fi rst stratigraphic δ 34 S measurements on pyrite tied to well-established biostratigraphy indicate a large perturbation of the sulfur cycle, consistent with major sea-level changes related to the glaciation. The elevated δ 34 S values of pyrites and a large, short-lived negative δ 34 S excursion of ~20‰ associated with the decay of the glaciation suggest deep-water anoxia during the Hirnantian Stage, in contrast to the conventional view that the global oceans were oxygenated. We suggest that deep-water anoxia may have contributed to the Late Ordovician mass extinction in South China and possibly elsewhere.

A new Neogene biostratigraphy for Denmark

In Denmark most of the water used in private households, in the industry and for irrigation in agriculture comes from subsurface aquifers. Some of the most important aquifers in Jylland, western Denmark, are sand layers deposited from 23 to 15 Ma ago, in the Early Neogene (Early to Middle Miocene). About 23 Ma ago, in the Early Miocene, the coastline ran NW–SE across present-day Jylland (Rasmussen 2004). Global climatic variations led to major sea-level changes (Zachos et al. 2001), which in combination with increased sediment transport from the north (the present Norway) resulted in deposition of several huge, fluvio-deltaic sand systems intercalated with marine clay (e.g. Rasmussen 1961; Rasmussen 2004; Rasmussen &amp; Dybkjær 2005). The Geological Survey of Denmark and Greenland (GEUS) and the regional Environment Centres (the former counties (amter)) in Jylland are working in close cooperation to study the Early Neogene succession; the main purposes are: (1) to find new aquifers, (2) to map the extent of known aquifers and clarify their mutual relationships, in order to evaluate the size of the water resources and optimise production, and (3) to protect the aquifers from pollution due to leaching from the surface. In order to map the complex sedimentary succession, it has been necessary to combine several geological disciplines, including seismic interpretation, sedimentology, correlation of geophysical logs, and biostratigraphy (e.g. Dybkjær 2004; Rasmussen 2004; Rasmussen et al. 2004; Piasecki 2005; Rasmussen &amp; Dybkjær 2005; Dybkjær &amp; Rasmussen 2007). This article shows some results of a detailed dinoflagellate cyst stratigraphy, which is based on an extensive database (Fig. 1). We present here for the first time a dinoflagellate cyst zonation for the complete Neogene succession in the Danish area.

Middle Pleistocene to Holocene geochronology of the River Aguas terrace sequence (Iberian Peninsula): Fluvial response to Mediterranean environmental change

Our results from the River Aguas basin suggest that fluvial archives, travertine and slope deposits provide sensitive resolution records of environmental changes during the last 170 kyr. From the chronostratigraphic data sets we have established a model of middle and late Pleistocene river response for littoral basins on the southern Iberian Peninsula. U/Th and OSL dating indicate the major periods of travertine formation of the Alfaix travertine platform, which range from 169 to 26 kyr. At least four incision events interrupted this aggradation period: between 167 and 148 kyr, between 148 and 110 kyr, around 95 kyr and at 71 kyr. Aggradation ceased after 26 kyr and incision occurred during OIS 2. Subsequently, the terraces T4a and T4b were deposited. OSL dating of the T4a channel deposit provides maximum ages of 28, 20 and 18 kyr. However, short climatic events, such as the Younger Dryas, produced two more river incision episodes during OIS 2. Nonetheless, for river systems influenced by tectonics, climate and sea-level changes it is difficult to assess the weight of each controlling factor. Regarding the three mechanisms of Pleistocene river dynamics in middle-size catchment areas of the littoral region of southeastern Spain, our results support the hypothesis that large scale tectonics triggered the general downcutting trend, whereas the main aggradation and incision phases occurred during periods of major sea-level changes. Over short-time scales the influence of climate variability, as documented by pollen records, plays a decisive role. Thus, the river responses to the three cyclic mechanisms operate at different time scales although synergetic processes should be considered with respect to the magnitude of abrupt incision/aggradation events.

Evidence for perturbation of the carbon cycle in the Middle Frasnian punctata Zone (Late Devonian)

New carbon isotopic data from the Devonian of Ardennes (Belgium) and partly from the Holy Cross Mountains (Poland) highlight an abrupt and high-amplitude negative excursion in the punctata conodont Zone. Published information from Moravia and China suggests that this Middle Frasnian negative excursion, jointly with the preceding large-scale positive shift, should be used as a global chemostratigraphic marker. Causation scenarios for this negative ‘punctata Event’ are correlated neither with major biota turnover nor major sea-level changes, but may be related to: (1) the Alamo Impact Event, that led to (2) the massive dissociation of methane hydrates and (3) the rapid onset of global warming.

Events around the Triassic–Jurassic boundary in northern and eastern Spain: A review

More than 20 successions containing the Triassic–Jurassic (T–J) boundary were studied in five of the major geological units of Spain. The data are from outcrop, cored boreholes, and interpreted well-logs. A consistent − 3.4‰ δ 13C org isotope excursion, starting in the Rhaetian and continuing in the Hettangian, was recorded in Asturias. Climate changes in the T–J transition show a warming episode in the early Hettangian. Other climate changes are indicated by an increase in hygrophytic miospores above the T–J boundary; this reflects a more humid episode at the beginning of the Hettangian. The Messejana Dyke in southeastern Iberia is part of the Central Atlantic Magmatic Province (CAMP). CO 2 outgassing related to extensive magmatic activity in the CAMP is thought to be one of the possible factors responsible for the end-Triassic mass extinction. No major sea-level changes or unconformities were recorded at the T–J boundary. Only in the Asturias area was there a well-defined shallow carbonate platform during the T–J transition. The other areas of eastern and northern Spain were occupied by coastal playas and sabkhas that developed in arid climatic conditions. The biotic crisis around the T–J boundary is reflected in the palynomorph record. In Asturias, seven species do not persist beyond the late Rhaetian, and only six appeared in the Triassic–Jurassic transition. However, 22 taxa appeared in the early Hettangian.

Biodiversity curves for the Ordovician of Baltoscandia

Biodiversity curves for the Ordovician of Baltoscandia show a substantial increase in taxonomical diversity through the period, as seen also in global data sets. A database of 10,340 records of first and last appearances of species at different localities in the region has been analysed using simple species counts, and partly validated with resampling methods. While the biodiversity curve for all fossil groups combined is probably reasonably accurate except for an unknown scaling constant, taxonomical or geographical subsets may not be sufficiently well sampled to allow precise measurement of their species counts through time. The analysis shows a major diversification event commencing in the middle Arenig (Ibex-Whiterock boundary), and more limited diversification events in the Llanvirn, Caradoc and Ashgill. The Scandinavian (Norwegian and Swedish) biodiversity curves are broadly correlated with major changes in sea level, with low biodiversity at highstands and high biodiversity at lowstands, although this pattern is not clear for all fossil groups. In the Arenig, graptolites and trilobites appear to have higher diversities at high sea levels, while the brachiopods and ostracodes show higher diversities at low sea level. As a consequence, the Arenig diversification is delayed for the latter two groups until the upper end of the interval.

Sulfate profiles and barium fronts in sediment on the Blake Ridge: present and past methane fluxes through a large gas hydrate reservoir

Ocean Drilling Program (ODP) Sites 994, 995, and 997 were drilled into a large gas hydrate deposit on the crest of the Blake Ridge (southeast U.S. margin) where upward CH 4 fluxes (F Out) are related to depths of pore water SO 4 2− depletion. High-resolution pore water SO 4 2− and sediment Ba profiles have been constructed at these sites to assess present and past F Out. Pore water SO 4 2− profiles are linear with zero SO 4 2− concentration occurring at 21.4, 21.6, and 22.8 mbsf at holes 994A, 995A and 997A, respectively. Using steady state solutions to diffusion equations with appropriate parameters, the steep SO 4 2− gradients support upward CH 4 fluxes between 7.2 and 8.6 mol/m 2 · ky at present-day, with the range primarily reflecting different approaches for incorporating porosity (φ). Taking into account the generally decreasing φ with depth and the high clay content of the sediment, the best estimates for F Out are 7.9, 7.6 and 7.2 mol CH 4/m 2 · ky at sites 994, 995 and 997, respectively. However, non-steady state solutions to diffusion equations show that the SO 4 2− gradients do not imply steady state conditions. Elevated Ba concentrations (530–1410 ppm) exist in sediment between 18.23 and 20.65, between 17.31 and 20.31, between 19.40 and 21.80, and between 19.58 and 21.91 mbsf at holes 994A, 994C, 995A, and 997A, respectively. These Ba fronts coincide with highs in bulk sediment Ba/Al (to 2.5 × 10 −2) and are caused by Ba cycling just above time averaged depths of SO 4 2− depletion. Because the Ba fronts lie immediately above the present-day depths of pore water SO 4 2− depletion, and because no other Ba fronts are found in the upper 25 m at the three sites, the depth of SO 4 2− depletion beneath the seafloor has been nearly constant for considerable time (>18,000 years). Thus, CH 4 fluxes can be determined through SO 4 2− gradients and steady state solutions to diffusion equations. More importantly, F Out through the crest of the Blake Ridge has not varied significantly across major changes in sea level and hydrostatic pressure.

Modes of extinction in two Devonian ostracode faunas of western Canada

The development of a prominent Middle Devonian Eifelian and an Upper Devonian Frasnian ostracode faunal sequence of western Canada is used to trace their evolutionary pathways, to illustrate evolutionary dynamics, and to evaluate the most probable causes and implications of the major and minor changes, which are clearly expressed in two sets of charts. A connection between sedimentary transgressive–regressive and evolutionary cycles is evident and documented in sample-for-sample accounts. Major changes in sea level were the primary cause for the most obvious breaks and trends in the faunal sequences and were responsible for the cyclic development of both the sedimentary and ostracode records. In contrast, numerous subordinate faunal breaks impart a distinctly punctuated pattern to both extinction and speciation pathways. Their regular spacing point to rhythmic influences that are interpreted to reflect primarily temperature fluctuations to which ostracodes are known to react in direct and indirect ways. In turn, the periodicity of the changes falls within the time frame of the Milankovitch Effect or to any one, or a combination, of its three variables. By combining the overall cyclic development with the punctuated progression of both extinction and speciation events in an "event-stratigraphic chart," a blueprint of the evolutionary dynamics emerges, which can be directly applied in sequence-stratigraphic reconstructions.