Glacio-eustatic Sea-level Fall Research Articles

Zinc isotope perspective on global carbon cycling during the onset of the late Paleozoic icehouse

The onset of the late Paleozoic icehouse coincided with a large global carbon-cycle perturbation (represented by the mid-Tournaisian carbon isotope excursion [TICE]), although the underlying cause of this isotopic event remains uncertain. Zinc (Zn), an essential micronutrient for plankton, has an isotopic composition (δ66Zn) in seawater that is sensitive to subtle fluctuations in marine carbon (C) cycling. Here, we investigated C cycling during the TICE using paired carbonate δ66Zn-δ13C and organic δ13C records from two widely spaced sections in South China. These records reveal coupling between δ66Zn and δ13C over an ∼4 m.y. interval in the form of two positive excursions (Peaks I and II) separated by a negative shift (interpeak). Peak I is attributed to enhanced marine primary productivity (MPP), which was stimulated by increased micronutrient supply linked to enhanced silicate weathering and/or upwelling. The interpeak may record oxidation of remobilized organic matter from continental shelves exposed by glacio-eustatic sea-level fall, which, in turn, promoted MPP again and resulted in Peak II. Thus, the TICE records complex climate−carbon cycle feedbacks that may have led to large-scale organic carbon burial and driven stepwise cooling, marking the onset of a sustained late Paleozoic icehouse climate.

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.

Biostratigraphic ages and depositional environments of the upper Oligocene to lower Miocene Veldhoven Formation in the central Roer Valley Rift System (SE Netherlands-NE Belgium)

Abstract Discussions on the age and the depositional environments of the Veldhoven Formation and its members are persistent in Belgium and the Netherlands. Uncertainties on stratigraphy and the constructive process of sediment accumulation continue today as a result of lack of data on this succession within the Roer Valley Rift System. The present study provides new information on the bio- and lithostratigraphy and facies from two boreholes based on dinoflagellate cyst taxa. The results were correlated by gamma-ray logs towards other key boreholes in the region and show a good consistency for stratigraphy and depositional environments for the members of the Veldhoven Formation. After marginal to restricted marine conditions in the latest Rupelian (early Oligocene), the start of deposition of the Veldhoven Formation marked the transition towards a higher sea level, expressed by increased glauconite contents and gamma-ray values. The Voort Member in the lower part of the Veldhoven Formation has an early to late Chattian (Late Oligocene) age and comprises predominantly shallow marine (fluctuating restricted to open marine) conditions. The lithology in the lower part of this unit is often very clayey but is coarsening upward into sands. The superjacent Wintelre Member has a latest Chattian to early Aquitanian (early Miocene) age. This member is distinct by its clayey nature which is expressed by relatively high gamma-ray values. Earlier studies suggest a deeper marine facies for the Wintelre Member compared to the Someren and Voort members. However, the dinoflagellate cyst assemblages in this unit are mostly dominated by a single genus indicating a restricted marine setting, including salinities that deviate from normal marine conditions, most probably due to minor ventilation by narrow or lack of connection to the Atlantic Ocean. A glacio-eustatic sea-level fall around the Oligocene/Miocene boundary limited the sea coverage to the strongest subsiding areas, where deposition of the Wintelre Member is recorded, while non-deposition or erosion occurred in the surrounding highs, hence creating an isolated (sub)basin. The superjacent Someren Member was deposited during the late Aquitanian to middle Burdigalian and consists of shallow to open marine clayey fine sands. Increasing clay contents indicate a gradual development towards a higher sea level, which coincide with upward increasing gamma-ray values. The biostratigraphic results of this study suggest that no major hiatuses are present in the differentially subsiding blocks of the Roer Valley Rift System during the late Oligocene to early Miocene.

Revising the timing and causes of the Urgonian rudistid-platform demise in the Mediterranean Tethys

Abstract The widespread demise of the Urgonian rudistid-platform ecosystem in the Mediterranean Tethys was so far considered as a progressive phenomenon starting at the Barremian–Aptian transition and culminating in the mid-early Aptian in concert with the Oceanic Anoxic Event (OAE) 1a. The re-assignment of the final demise of the Urgonian-type peri-Vocontian rudistid platforms to the latest Barremian by means of ammonite biostratigraphy forces us to rethink the chronology of events and underlying causes. Re-examination of the best-documented platforms of the Mediterranean Tethys supports a large scale demise of the Urgonian rudist-dominated platforms in the latest Barremian, as demonstrated directly by ammonite occurrences and indirectly by correlations. In most cases, the last rudistid sequences, dominated by caprinid rudists of peri-Vocontian affinity, are topped by an exposure surface. Regional investigation reveals that the duration represented by the exposure surface depends on the regional geodynamics. While the hiatus straddles the middle part of the M. sarasini Zone (latest Barremian) in subsiding platforms, it is of greater duration in quiescent tectonic context spanning the upper M. sarasini Zone up to the D. forbesi–D. deshayesi zones interval (late early Aptian). This exposure event is estimated to have lasted at least 1 Myr and may have resulted from a high-amplitude relative sea-level fall. It is here suggested that a short-lasted, global cooling phase and associated glacio-eustatic sea-level fall may have interrupted rudistid-platform ecosystem in the latest Barremian and subsequently led to hiatus development.

Stratigraphy of the Lungmachi black shales from the Huaying drill core in southwestern China and its palaeogeographic implications

Core samples from the Huaying well, Sichuan Province, have been analyzed for both graptolite biostratigraphy and carbon isotope chemostratigraphy. The result shows a stratigraphic sequence of Lituigraptus convolutus, Stimulograptus sedgwickii and Spirograptus guerichi biozones, and records a strong positive shift of δ13Corg at the base of the Stimulograptus sedgwickii Biozone, which coincides with those from Cornwallis Island (Arctic Canada), Arisaig (Nova Scotia, Canada), and the Barrandian area (Bohemia). Two outcrops were also studied in the Huaying region. The Yanwanggou section shows a continuous, but complicated, stratigraphical sequence, from the upper Katian to the lower–middle Rhuddanian, then a sedimentary hiatus of ∼3.26Ma, and finally a continuous sequence from the upper Aeronian to the lower Telychian, whereas the Liziya section and the Huaying well exhibit significant hiatus, which lasted from Hirnantian to middle Rhuddanian or early Aeronian times. Therefore, the eastern boundary of the Central Sichuan Oldland during the Ordovician and Silurian transition can be precisely constrained to a narrow area between the Huaying well-Liziya section and the Yanwanggou section, which indicates that the Central Sichuan Oldland was much larger than previously considered. The present data also indicate the existing of a so far unidentified stage of the Kwangsian Orogeny, which resulted in the uplift of some portions of the Yangtze Platform region and the consequent delay of the transgression in those places. The multi-stage hiatus, spanning at least from the upper Katian to middle Aeronian in the study area, resulted from a combined effect of the Hirnantian glacio-eustatic sea-level fall and the regional tectonic activity of the Kwangsian Orogeny.

A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia

The Late Ordovician mass extinction (LOME), one of the five largest Phanerozoic biodiversity depletions, occurred in two pulses associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Age. It is widely recognized that environmental disruptions associated with changing glacial conditions contributed to the extinctions, but neither the kill mechanisms nor the causes of glacial expansion are well understood. Here we report anomalously high Hg concentrations in marine strata from south China and Laurentia deposited immediately before, during, and after the Hirnantian glacial maximum that we interpret to reflect the emplacement of a large igneous province (LIP). An initial Hg enrichment occurs in the late Katian Age, while a second enrichment occurs immediately below the Katian-Hirnantian boundary, which marks the first pulse of extinction. Further Hg enrichment occurs in strata deposited during glacioeustatic sea-level fall and the glacial maximum. We propose that these Hg enrichments are products of multiple phases of LIP volcanism. While elevated Hg concentrations have been linked to LIP emplacement coincident with other Phanerozoic mass extinctions, the climate response during the LOME may have been unique owing to different climatic boundary conditions, including preexisting ice sheets. Our observations support a volcanic trigger for the LOME and further point to LIP volcanism as a primary driver of environmental changes that caused mass extinctions.

Magnitudes of sea-level falls at lowstands of the past 900,000 years inferred from gravels underlying the Nobi Plain, central Japan

The Nobi Plain alluvial lowland is drained by the Kiso River system, which discharges large volumes of water and sediment into Ise Bay, Japan. The basin is bounded to the west by the Yoro fault and has been tilted down to the west by repetitive movement on the fault. The basin stratigraphy and its stacking patterns suggest uniform and rapid rates of subsidence and tilting in response to movement on the Yoro fault during the middle and late Quaternary. We used lithostratigraphy, tephrochronological and paleomagnetic data, facies analysis, and diatom assemblage analysis of a 601 m sediment core to identify ten marine transgression–regression sequences, characterized by basal fluvial gravel intervals formed by the Kiso River drainage system, and correlated them with glacial–interglacial sea-level changes during the past 900 ky. We inferred that the mean maximum size of clasts in the basal gravels may be proportional to the magnitude of the fall of sea level inferred from MIS curves since MIS 16; that is, clasts inferred to be deposited during MIS 12 and 16 are largest, and those deposited during MIS 14 are the smallest since MIS 16. These observations suggest that the Kiso River system has adjusted its longitudinal profile to accommodate sea-level changes and that a steeper profile was formed during periods of lower sea level. We also found that the size of gravel clasts in the present-day river is proportional to tractive force on the riverbed, which is controlled mainly by the slope of the river profile. Conversely, the facts that larger gravel clasts were deposited during MIS 20 and that gravel intervals deposited during MIS 22 were thicker than those during MIS 2, suggest that a glacio-eustatic sea-level fall comparable in magnitude to that of the Last Glacial Maximum occurred during MIS 22.

Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces

Research Article| January 01, 2015 Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces David S. Jones; David S. Jones Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Roger C. Creel; Roger C. Creel Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Bernardo Rios; Bernardo Rios Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Danielle P. Santiago Ramos Danielle P. Santiago Ramos Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA *Current address: Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA. Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (1): 59–62. https://doi.org/10.1130/G36236.1 Article history received: 02 Sep 2014 rev-recd: 30 Oct 2014 accepted: 05 Nov 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David S. Jones, Roger C. Creel, Bernardo Rios, Danielle P. Santiago Ramos; Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces. Geology 2015;; 43 (1): 59–62. doi: https://doi.org/10.1130/G36236.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (δ13C) to be preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative δ13C anomalies developed in Pliocene–Pleistocene carbonate platforms following glacioeustatic sea-level fall due to remineralization of terrestrial biomass during meteoric diagenesis. These anomalies are similar in structure and magnitude to some Neoproterozoic δ13C records, opening the possibility that the Neoproterozoic δ13C anomalies have a meteoric origin derived from a large terrestrial biosphere. Here we test the hypothesis that a large terrestrial biosphere existed prior to the Silurian–Devonian land-plant radiation by examining δ13C records of subaerial exposure surfaces formed in a shallow-water carbonate platform during the Ordovician–Silurian icehouse. The exposure surfaces include an unconformity at the Ordovician-Silurian boundary with terra rossa and dissolution-collapse breccia, and a lower Silurian quartz sand layer feeding a 50-m-deep system of karst pipes. There is no evidence for δ13C depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10–120 ppm) and covary with δ18O; oxygen isotope ratios, however, do not positively correlate with δ13C. Our results suggest that there was no significant terrestrial biosphere during Ordovician–Silurian time, and by extension, that Neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Organic matter accumulation of Late Ordovician sediments in North Guizhou Province, China: Sulfur isotope and trace element evidences

The organic-rich Upper Ordovician sediments (Wufeng and Guanyinqiao Formations) on the Yangtze platform are considered to be one of the main source rocks. Here we present geochemical proxies, including redox indicator (S/C ratios and sulfur isotopes) and productivity indices (TOC, Mo and Ba contents), from Nanbazi section in North Guizhou province, South China, in order to investigate the mechanism of organic matter accumulation. The geochemical data suggest a stagnant and anoxic environment predominated the Yangtze Sea during the Wufeng period, whereas ventilated and oxygenated marine conditions pervaded the Yangtze Sea during the Guanyinqiao period. Variations in the concentration of TOC, Mo and Ba indicate that higher organic carbon export in the Wufeng intervals than those in the Guanyinqiao intervals. These variations in redox and productivity during the late Ordovician were associated with different mechanisms and forcing processes. The abrupt change from anoxic to oxygenated condition at the beginning of the Guanyinqiao was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. The productivity variations were related to the change of nutrient supply, which is consistent with volcanic activities and runoff to the Yangtze Sea. Redox changes, together with primary productivity fluctuations could have played a significant role on the variation of organic matter accumulation during the late Ordovician in South China.

Sea-level and environmental changes around the Devonian–Carboniferous boundary in the Namur–Dinant Basin (S Belgium, NE France): A multi-proxy stratigraphic analysis of carbonate ramp archives and its use in regional and interregional correlations

The paper focuses on high-resolution multidisciplinary research on three Devonian–Carboniferous boundary sections in shallow-water carbonate rocks in the Namur–Dinant Basin (Belgium, France). The aim of the study is to provide palaeo-environmental reconstructions and correlations supported by several independent quantitative proxies. We describe several correlative horizons and provide their sequence-stratigraphic interpretation based on facies analysis, spectral gamma-ray data, element concentrations (XRF) and δ13Ccarb, with foraminifer-biostratigraphy age control. The most prominent surface is a basal surface of forced regression, which is indicated by a sharp basinwards facies shift and a drop in clay-gamma-ray values and Al concentrations at the base of the Hastière and Avesnelles formations in more distal settings. In proximal settings, this surface merges with a hiatus at the Devonian–Carboniferous boundary inferred from foraminifer biostratigraphy. This hiatus can be correlated with the global Hangenberg sandstone event, which indicates a glacioeustatic sea-level fall. Increasing values of Zr/Al, K/Al, Sr/Al and Mn/Al coincide with the proximal facies of the falling stage system tract and lowstand system tract in the Hastière and Avesnelles formations as a consequence of the enhanced input of siliciclastics and nutrients during low sea levels. The top of the middle Hastière member is interpreted as the maximum regression surface, which is overlain by transgressive system tract of the upper Hastière member. The patterns of gamma-ray, δ13Ccarb, Th/K, Al and Zr/Al curves are well correlated between the studied sections. The δ13Ccarb excursions are correlated with the unnamed excursion in the Upper expansa conodont zone (Carnic Alps) and with the global Hangenberg event s.l. excursion in the kockeli conodont zone. This sequence-stratigraphic framework is used for correlations with deltaic successions from the Tafilalt Basin, Morocco.The basal surface of the forced regression equivalent to the Hangenberg sandstone event, which is typical for deeper-water settings, is easily recognisable and correlatable with gaps in more-shallow water settings. We suggest that it should be taken into account as a possible candidate for the “natural solution” of the Devonian–Carboniferous boundary in discussions concerning its redefinition.

Paleoclimatic and paleoenvironmental records of the Oligocene–Miocene transition, central Jylland, Denmark

A multidisciplinary study of the Oligocene/Miocene (O/M) transition was carried in two boreholes (Harre-1 and Horn-1) from the Danish land area in order to improve the understanding of the paleoclimatological and environmental changes across the Mi-1, the earliest Miocene cooling event. Dinoflagellate cyst (dinocyst) biostratigraphy, supported by re-evaluation of the foraminiferid biostratigraphy, a new set of magnetic data and stable carbon isotope (δ13C) records were applied to improve the age model for the studied succession; as well as the positioning of the O/M boundary. These data further enabled a correlation of the two investigated profiles with the well-established stratigraphical framework for the Danish area. The BIT index (an organic proxy indicating the relative input of soil-derived organic matter), dinocyst assemblages and palynofacies have been applied for establishing the paleoenvironmental changes across the Oligocene/Miocene (O/M) boundary. Our data indicate a shallowing upward trend in the latest Oligocene, resulting in the establishment of a very restricted marine setting in the earliest Miocene. Our study confirms that the O/M boundary is located at a sequence boundary (the local sequence boundary B — SB B) and corresponds to an increase in terrestrial organic matter input. A mean air temperature record based on the MBT′/CBT proxy reveals an ~ 2 °C drop in temperature at the O/M and the sequence boundary. Our findings confirm previous suggestions that this sequence boundary is a result of the glacio-eustatic sea level fall related with the Mi-1 cooling event. The unconformity at the sequence boundary probably correlates with the coldest part of Mi-1, and thus no deposits from this period have been preserved. This suggests that the temperature drop might have been greater than 2 °C.

Evidence of climatic control on hydrocarbon seepage in the Miocene of the northern Apennines: The case study of the Vicchio Marls

The Vicchio outcrop in the Tuscan Apennines contains excellent exposures of a Miocene methane-derived carbonate system, made up of more than 80 carbonate bodies enclosed in marly sediments. Facies analysis, composition and a detailed biostratigraphic study of the carbonates and enclosing Vicchio Marls allowed us to document the role of climatic changes and eustasy on seepage in these ancient deposits. Results of our study indicate that the stratigraphic horizon bearing seep-carbonates is constrained by two planktonic foraminiferal events, the Acme End (AE) of Turborotalita cf. T. quinqueloba (13.75 Ma) and the Acme1 Beginning (A1B) of Paragloborotalia siakensis (13.32 Ma), encompassing about 400,000 years. The AE of T. cf. T. quinqueloba approximates the mid-Miocene global cooling event (Mi3b), as defined by the δ18O maxima (13.78 Ma). The glacio-eustatic sea level drop associated with this cooling event is estimated to be approximately 60 m. Assuming that the highest rates of glacio-eustatic sea level fall coincide with δ18O maxima, the timing of the seepage onset (at about 13.75 Ma) is concomitant with the phase of sea level-lowering. The ascent and emission of methane-rich fluids may have been triggered by pressure drop due to the eustatic fall associated with the Mi3b event. The moderate intensity of fluid expulsion is suggested by the lack of brecciation, by the scarcity of detrital particles in the carbonates and by the pervasive occurrence of carbonate in the enclosing marls. This type of seepage differs from focused fault-confined fluid flows, typical of other tectonically controlled Apenninic seep-carbonates.

Climate change and the selective signature of the Late Ordovician mass extinction

Selectivity patterns provide insights into the causes of ancient extinction events. The Late Ordovician mass extinction was related to Gondwanan glaciation; however, it is still unclear whether elevated extinction rates were attributable to record failure, habitat loss, or climatic cooling. We examined Middle Ordovician-Early Silurian North American fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per-taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk. Genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the Katian stage were particularly hard hit. The duration of the subsequent sampling gaps had little effect on extinction risk, suggesting that this extinction pulse cannot be entirely attributed to rock record failure; rather, it was caused, in part, by habitat loss. Extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled, a macroecological trait linked to thermal tolerance. A model trained on the relationship between 16 explanatory variables and extinction patterns during the early Katian interval substantially underestimates the extinction of exclusively tropical taxa during the late Katian interval. These results indicate that glacioeustatic sea-level fall and tropical ocean cooling played important roles in the first pulse of the Late Ordovician mass extinction in Laurentia.

NATURE AND EXTENT OF A LATE EDIACARAN (CA. 547 MA) GLACIGENIC EROSION SURFACE IN SOUTHERN AFRICA

Erosional surfaces within the late Ediacaran upper Kuibis and lower Schwarzrand Subgroups of the Nama Group, that extend from the Klein Karas Mountains in Namibia eastward to the Namibian-South African border and from southern Namibia to Nigramoep (near Springbok) in South Africa, are interpreted to form one discontinuous regional unconformity termed the Vingerbreek Unconformity (VGU). This unconformity formed by glacial, proglacial and fluvial erosion. The palaeochannels (palaeovalleys) associated with this unconformity in southern Namibia and northwestern South Africa are interpreted to have formed distant from major glaciers as a result of predominantly glacio-eustatic sea level fall. Our findings are consistent with a late Ediacaran (ca 547 Ma) glaciation, that is also recorded in South America and Eurasia. The evolutionary implications of a glacial event just before the Ediacaran/Cambrian are considerable, because climate deterioration could have acted as an evolutionary bottleneck and thus promoted the Cambrian explosion in its aftermath.

Predominance of stratified anoxic Yangtze Sea interrupted by short-term oxygenation during the Ordo-Silurian transition

The Ordo-Silurian transition was a critical interval in geological history, during which profound biotic, climatic and oceanic changes occurred. In order to explore the oceanic palaeoredox changes, multiple geochemical proxies, including ratios of S/C, FeHR/FeT, FeP/FeHR and DOP values, are presented here from three sections (Wangjiawan, western Hubei, Sanjiaguan, northern Hunan, and Nanbazi, northern Guizhou) across the Ordo-Silurian boundary on the Yangtze Platform. These palaeoredox data indicate a predominance of stratified, anoxic (ferruginous) ocean on the Yangtze block during this transition, which was interrupted by a brief episode of oceanic oxygenation in the early Hirnantian. This oxygenation, temporally coinciding with the end-Ordovician glaciation and global glacio-eustatic sea level fall, likely resulted from enhanced circulation of polar cold, dense oxygen-rich water onto the low-latitude shelf. The prior and subsequent longer-term episodes of anoxic ocean, particularly the later one which started in the late Hirnantian, occurred in parallel with the rapid climatic warming and sea level rise, which could have slowed down oceanic circulation, thereby enhancing oceanic stratification, anoxia and organic preservation. Oceanic redox changes, together with rapid climatic and sea-level fluctuations, were likely responsible for the stepwise massive demise of the Ordo-Silurian biotic crisis.

Chemostratigraphy of the Tamengo Formation (Corumbá Group, Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve

The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids ( Vendotaenia, Eoholynia), soft-bodied metazoans ( Corumbella) and skeletal fossils ( Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostratigraphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive δ 13C excursion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated with a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau characterized by δ 13C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the δ 13C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age ( Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excursion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although δ 13C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the origin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out.

δ18O composition of conodont apatite indicates climatic cooling during the Middle Pridoli

Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian (Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont colour alteration index of the biogenic apatite did not exceed 1.5 reflecting only a minor thermal alteration. Conodont δ 18O apatite values range from 17.7 to 19.2‰ V-SMOW, with the average values around 18.3‰. Fossil apatite of fish exoskeleton microremains from the same samples have lower δ 18O apatite values ranging from 15.2 to 17.4‰ V-SMOW, with the average values around 16.3‰ V-SMOW. Palaeoseawater temperatures calculated from conodont apatite δ 18O range from 24.8–31.5 °C, those derived from fish δ 18O give approximately 10 °C higher temperatures ranging from 32.6 to 38.1 °C. The lower δ 18O apatite values of fish apatite and thus the unrealistic high palaeotemperatures are interpreted as a result of diagenetic alteration. A significant positive shift in conodont apatite δ 18O of + 1.1‰ at the boundary between the Vievis and Lapės formations is observed, coinciding with a major facies change. It is interpreted as a mid-Pridoli event within the O zarkodina eosteinhornensis Bizone, or the O zarkodina remscheidensis Biozone reflecting a major cooling event, which may have resulted in the formation of an ice sheet in high latitudes and a glacio-eustatic sea-level fall.

Glacioeustatic sea level fall marking the base of supercycle TA-1 at 66.5 Ma recorded by the kaolinization of the Whitemud Formation and the Colgate Member of the Fox Hills Formation

The Maastrichtian continental Whitemud Formation in the Cypress Hills of southwestern Saskatchewan, Canada, has been kaolinized by in situ leaching to a depth of at least 20 m, produced by a lowered water table. The first stage of leaching took place during accumulation of the coal zone in the middle of the 20 m-thick Whitemud. A second stage took place after deposition of the lower few meters of the overlying organic-rich Battle Formation. The U-Pb age on bentonite zircon in the lower Battle is 66.5 ± 0.2 Ma, indicating the last stage of sea fall. Using an average compacted sediment accumulation rate of 72 m/m.y., based upon the thicknesses of magnetozones 29r and 30r, the time spanning the two stages of lowered sea level is 200,000 years or less. The Colgate Member of the Fox Hills Formation in Montana is an exact correlative of the Whitemud, and kaolinization of the Colgate is due to the same sea level fall from the International Boundary to central North Dakota. The age correlates to the sea level cycle marking the base of supercycle TA-1, and the short duration indicates that this sea level cycle was glacioeustatic.

Bioturbation levels during the end-Ordovician extinction event: a case study of shallow marine strata from the Welsh Basin

ABSTRACT: The Ordovician-Silurian succession of the Llandovery area, mid-Wales, preserves shal-low marine sediments deposited during the Hirnantian (end-Ordovician) extinction event. Ichno-logical analysis of the rocks shows that the degree and depth of bioturbation in this part of the WelshBasin was often low across this interval, both during glacioeustatic sea level fall and the early part ofthe subsequent transgression. This pattern suggests that stratigraphic dilution was not the primarycontrol on trace fossil abundance: an increase in sediment supply during the regression could explainlow abundance in early Hirnantian units, but not in the post-glacial transgressive strata. A significantdecrease in oxygen levels during sea level rise might be invoked instead, but this cannot have beenthe sole cause, as the occurrence of burrows up to 20 mm in diameter in the Bronydd Formation (lateHirnantian–Rhuddanian) shows that seafloor oxygen levels were at least intermittently high. Anabsence of vertical bioturbation through much of the succession indicates that sessile, suspension-feeding organisms were generally scarce. The overall pattern probably reflects a decline in benthicinfauna during the extinction event, but the 2 pulses of extinction described previously in body fossils are not evident ichnologically in this part of the Welsh Basin.KEY WORDS: Ichnofabrics · Benthos · Infauna · Mass extinctions · Trace fossils

Eocene/Oligocene ocean de-acidification linked to Antarctic glaciation by sea-level fall

One of the most dramatic perturbations to the Earth system during the past 100 million years was the rapid onset of Antarctic glaciation near the Eocene/Oligocene epoch boundary (approximately 34 million years ago). This climate transition was accompanied by a deepening of the calcite compensation depth--the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution. Changes in the global carbon cycle, rather than changes in continental configuration, have recently been proposed as the most likely root cause of Antarctic glaciation, but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear. Here we use a global biogeochemical box model to test competing hypotheses put forward to explain the Eocene/Oligocene transition. We find that, of the candidate hypotheses, only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor. In our simulations, glacioeustatic sea-level fall associated with the growth of Antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves, leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth. At the same time, fresh limestones are exposed to erosion, thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater delta13C. Our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the Cenozoic era.