South Atlantic Basin Research Articles

A Lagrangian Identification of the Main Sources of Moisture Affecting Northeastern Brazil during Its Pre-Rainy and Rainy Seasons

This work examines the sources of moisture affecting the semi-arid Brazilian Northeast (NEB) during its pre-rainy and rainy season (JFMAM) through a Lagrangian diagnosis method. The FLEXPART model identifies the humidity contributions to the moisture budget over a region through the continuous computation of changes in the specific humidity along back or forward trajectories up to 10 days period. The numerical experiments were done for the period that spans between 2000 and 2004 and results were aggregated on a monthly basis. Results show that besides a minor local recycling component, the vast majority of moisture reaching NEB area is originated in the south Atlantic basin and that the nearby wet Amazon basin bears almost no impact. Moreover, although the maximum precipitation in the “Poligono das Secas” region (PS) occurs in March and the maximum precipitation associated with air parcels emanating from the South Atlantic towards PS is observed along January to March, the highest moisture contribution from this oceanic region occurs slightly later (April). A dynamical analysis suggests that the maximum precipitation observed in the PS sector does not coincide with the maximum moisture supply probably due to the combined effect of the Walker and Hadley cells in inhibiting the rising motions over the region in the months following April.

An unusual ZZ/ZW sex chromosome system in Characidium fishes (Crenuchidae, Characiformes) with the presence of rDNA sites

Characidium fishes with a sex chromosome system form a monophyletic group. This work presents data of Characidium lanei from the South Atlantic basin (Brazil), including an unknown type of ZW sex chromosome system for the groups including the presence of rDNA sites on sex chromosomes.

Indian Ocean circulation and productivity during the last glacial cycle

The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records from an equatorial Indian Ocean core (SK129–CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (Marine Isotope Stages; MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11–83/TNO57–21 South Atlantic deep Cape Basin Nd isotope record ( Piotrowski et al., 2005). Since the deep Cape Basin is oceanographically upstream of the Indian Ocean from a deep water circulation perspective, the Nd isotopes suggest that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial–interglacial timescales, in particular suggesting higher productivity during MIS 5. Increased equatorial productivity during MIS 5 is supported by benthic infaunal–epifaunal foraminiferal δ 13C gradients, as well as benthic foraminiferal δ 13C gradients along deep-water flow. Concurrent warming, indicated by planktonic foraminiferal Mg/Ca during MIS 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface-ocean hydrographic changes.

Complex rifted continental margins explained by dynamical models of depth-dependent lithospheric extension

Observations from a number of rifted margins (e.g., South Atlantic salt basin, mid-Norwegian margin, Exmouth Plateau) reveal wide regions of extremely attenuated crust and depositional environments that indicate depth-dependent lithospheric extension. Although the one-dimensional thermal-kinematic consequences of depth-dependent extension are understood, no comprehensive process-based explanation for the complex style of these margins exists. Here, we present self-consistent numerical models of passive-margin formation that explain the depth-dependent extension, the width of the margin, its characteristic tripartite nature, and why such margins are prone to deposition of evaporites under appropriate climatic conditions. Some features that are important to reproducing the observed characteristics include decoupling between upper and lower parts of the lithosphere during stretching, contrasting wide and narrow extensional styles above and below the decoupling level, and progressive focusing of crustal extension toward the rift axis.

Subsurface seepage of seawater across a barrier: A source of water and salt to peripheral salt basins

Many of the great Phanerozoic salt basins share a common paleogeography, in which a deep lake or sea is separated from the ocean by a narrow barrier. Examples include the Aptian evaporites of the South Atlantic rift basins and the Messinian salt deposits of the Mediterranean Sea and the North Caspian depression. Marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins. We test an alternate hypothesis, that subsurface seepage of seawater through the barrier was a significant source of water, influencing both water level and water composition. Our model permits flux of water and salt into and out of the basin via (1) seepage through the barrier; (2) evaporation from the water surface; (3) rainfall and/or rivers; and (4) incorporation into basin sediments. Our investigation focuses on three settings, a modern analog in the Gregory Rift of northern Kenya, the Messinian Mediterranean basin, and the Early Cretaceous Aptian salt basins of the South Atlantic. We test our model through a numerical simulation, applying reasonable values for the dimensions of the barrier, differences in water level, and hydraulic conductivities to calculate flow rates through the barrier. Modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant, depending on the climate, size of the peripheral basin, and hydraulic conductivity of the barrier. The flux of salt is significant over the full range of modeled hydraulic conductivities, producing brines in the peripheral lake and/or sea within 0.5 m.y. in most cases. This demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions, and that marine transgressions across the barrier are not necessary to explain the apparent high salinities.

Macro- and megabenthic assemblages in the bathyal and abyssal Weddell Sea (Southern Ocean)

The assemblages inhabiting the continental shelf around Antarctica are known to be very patchy, in large part due to deep iceberg impacts. The present study shows that richness and abundance of much deeper benthos, at slope and abyssal depths, also vary greatly in the Southern and South Atlantic oceans. On the ANDEEP III expedition, we deployed 16 Agassiz trawls to sample the zoobenthos at depths from 1055 to 4930m across the northern Weddell Sea and two South Atlantic basins. A total of 5933 specimens, belonging to 44 higher taxonomic groups, were collected. Overall the most frequent taxa were Ophiuroidea, Bivalvia, Polychaeta and Asteroidea, and the most abundant taxa were Malacostraca, Polychaeta and Bivalvia. Species richness per station varied from 6 to 148. The taxonomic composition of assemblages, based on relative taxon richness, varied considerably between sites but showed no relation to depth. The former three most abundant taxa accounted for 10‐30% each of all taxa present. Standardised abundances based on trawl catches varied between 1 and 252 individuals per 1000m 2 . Abundance significantly decreased with increasing depth, and assemblages showed high patchiness in their distribution. Cluster analysis based on relative abundance showed changes of community structure that were not linked to depth, area, sediment grain size or temperature. Generally abundances of zoobenthos in the abyssal Weddell Sea are lower than shelf abundances by several orders of magnitude. r 2007 Elsevier Ltd. All rights reserved.

Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge

Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing. This mixing is thought to result primarily from instability of the oceanic internal wave field, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s(-1) characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10(-4) m2 s(-1) at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10(-5) m2 s(-1) that characterize much of the global thermocline and the abyssal ocean. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.

Timing and origin of the South Atlantic pre-salt sag basins and their capping evaporites

Abstract Continental extension between West Africa and Brazil was responsible, directly or indirectly, for the development of the pre-salt sag basins and the evaporites of the South Atlantic salt basin. Subsidence mechanisms to explain these basins and their capping evaporites include: (1) deposition on Barremian-aged ocean crust; (2) rift propagation from east to west across the West African margin such that post-rift subsidence commenced in the east while rifting was still occurring to the west; and (3) depth-dependent lithospheric extension. Predicted thermal subsidence of oceanic crust or rifted lithosphere is inadequate to generate sufficient accommodation for the evaporites. Within the Santos Basin, extensional faulting within the pre-salt sag basin occurs up to the base of the evaporites; extension clearly continued to the late Aptian. Time-equivalent onshore and offshore pre-salt sections across the West African margin, and the inability to generate sufficient subsidence if the sections are considered to be post-rift, disqualifies east to west rift propagation as a mechanism for the observed pre-salt basins and evaporites. Barremian–Aptian depth-dependent extension best explains the general rift and post-rift development of the West African and Brazilian margins and the paucity of syn-rift faulting, the strain balance being achieved by the lateral emplacement of lower crust and continental mantle out from under the adjacent continental lithosphere. Regional exposure and truncation of the top pre-salt sag section attests to a climate-induced lake level drawdown during the mid Aptian, and offers a simple mechanism to generate the shallow water environments for evaporite precipitation across the West African–Brazilian rift system. In the subsequent marine transgression the Gabon and Angolan salts and the evaporites within the conjugate Camamu-Almada, Jequitinhonha and Cumuruxatiba basins were deposited. Santos and Campos basin evaporites are younger. The barrier to southern Atlantic marine incursions and the possible delay in Santos and Campos evaporite deposition relates to the magmatic constructions of a proto-Walvis Ridge and the long-lived anomalous topography of the southeastern Brazilian highlands; Campos and Santos basin extension was necessarily superimposed on a broad, high-relief plateau.

Analysis of Hurricane Catarina (2004)

Abstract The development of Hurricane Catarina over the western South Atlantic Ocean in March 2004 marks the first time that the existence of a hurricane has been confirmed by analysis and satellite imagery in the South Atlantic basin. The storm undergoes a complex life cycle, beginning as an extratropical precursor that moves east-southeastward off the Brazilian coast and toward the midlatitudes. Its eastward progress is halted and the system is steered back westward toward the Brazilian coast as it encounters a strengthening dipole-blocking structure east of the South American continent. Entering the large region of weak vertical shear that characterizes this blocking pattern, Catarina begins a tropical transition process over anomalously cool 25°C ocean waters above which an elevated potential intensity is supported by the cold upper-level air associated with the trough component of the block. As the convective outflow from the developing tropical system reinforces the ridge component of the dipole block, the storm is accelerated westward toward the Santa Catarina province of Brazil and makes landfall there as a nominal category-1 hurricane, causing extensive damage with its heavy rains and strong winds. The complex evolution of the system is analyzed using a suite of diagnostic tools, and a conceptual model of the tropical transition and steering processes in the presence of a dipole block is developed. Once the essential properties of the upper-level flow are established, an analog study is undertaken to investigate lower-atmospheric responses to similar blocking regimes. Persistent dipole-blocking structures are found to be rare east of South America; however, the evolution of systems occurring during these periods is shown to be complex and to exhibit various subtropical development modes.

Benthic foraminiferal assemblages from Demerara Rise (ODP Leg 207, western tropical Atlantic): possible evidence for a progressive opening of the Equatorial Atlantic Gateway

This paper is based on Santonian–Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian–earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 “black shale” formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards).

Trends and rhythms in global seafloor generation rate

The primary purpose of this paper is to investigate the spreading and production rates of oceanic ridges for the last 180 Myr, based on the detailed analysis of eight oceanic units (North, Central, and South Atlantic basins, Southwest, Central, and Southeast Indian Ridge systems, Somalia basin, and the Pacific plate) and using the most recent timescale for oceanic isochrons. The global study of oceanic ridges presented here shows that (1) the average rate of spreading, which we computed by weighting the rates obtained at each basin by the relevant ridge lengths, is constant since ∼125 Ma at 53.4 ± 5.9 mm yr−1 (full rate), (2) the average surface production rate is 2.7 ± 0.2 km2 yr−1, and (3) the minimum oceanic crust production in volume, or flux, is 18.7 ± 2.9 km3 yr−1. These estimations are in close agreement (within ±10%) with other studies. However, the new results emerging from this analysis are the following: (1) The Cretaceous flux rates (in volume) might be only 10% higher than today over a short period of time (125–100 Myr). (2) The “pulse” of ocean crustal production (120–80 Ma) in the world total is predominantly the result of contributions from mantle temperature and oceanic plateaus but is not linked to the global spreading rate of oceanic ridges, as generally accepted. (3) The rates presented here differ from previously published models for the Cenozoic and show a general increasing trend in the last 50 Myr. (4) We finally suggest a possible ∼25 Myr pseudo‐periodicity of the oceanic production rate (in surface and in volume) at least during the last 75–80 Myr. These data could have a profound impact on a vast number of models including sea‐level changes and more generally on the chemical mass balance between ocean and continent, which is known to be a key parameter in the history of the Earth's climate and ocean chemistry.

Roveacrinids in mid-Cretaceous biostratigraphy of the Sergipe Basin, northeastern Brazil

As part of a study of the Cenomanian–Coniacian sedimentary succession in the Sergipe Basin in northeastern Brazil, microfacies analyses were tested for possible chronostratigraphic application. The common and widespread remains of roveacrinid echinoderms proved useful for intrabasinal biochronostratigraphy. Local dolomitization in the basin, which destroyed most other fossil remains, left the magnesian-calcitic roveacrinid skeletons largely unaffected. Thus, where diagnostic fossils are scarce or lacking, roveacrinids may provide an ancillary means of chronostratigraphic correlation. The forms recorded from Sergipe are described, and a stratigraphic range chart is presented for intrabasinal application, with the possibility of regional or even global correlation either qualitatively (presence/absence patterns) or quantitatively (eco-event patterns). Several roveacrinid abundance horizons have been reported worldwide in relation to sea level changes and associated hypoxic events, during which roveacrinids developed abundant opportunistic populations. The present study of the Sergipe Basin provides a basis for comparisons and correlations of roveacrinid occurrences in the mid-Cretaceous succession of the South Atlantic continental margin basins and ODP/DSDP drill cores.

Global and regional evaluation of the SeaWiFS chlorophyll data set

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll data set was compared to comprehensive archives of in situ chlorophyll data from NASA and NOAA. The global comparison indicated a root mean square (RMS) log error of 31%, with a coefficient of determination ( r 2) of 0.76, using 4168 data points where in situ and SeaWiFS data were coincident and collocated. RMS log error for open ocean (defined as bottom depth>200 m) was 27.7% with r 2=0.72, compared to 33% RMS log error and r 2=0.60 on the coasts, indicating a deterioration of quality of the SeaWiFS data set in coastal regions. All of the Pacific oceanographic basins generally showed very good agreement with SeaWiFS, as did the South Atlantic basin. However, poorer agreement was found in the Mediterranean/Black Seas, Equatorial Atlantic, and the Antarctic. Optical complexity arising from riverine inputs, Saharan dust, and anomalous oceanic constituents contributed to the differences observed in the Atlantic, where an overestimation by SeaWiFS occurred. The Antarctic indicated a pronounced negative bias, indicating an underestimation, especially for chlorophyll concentrations greater than about 0.15 mg m −3. The results provide a comprehensive global and geographic analysis of the SeaWiFS data set, which will assist data users and policy makers in assessing the uncertainty of estimates of global and regional ocean chlorophyll and primary production.

Extreme polar warmth during the Cretaceous greenhouse? Paradox of the late Turonian δ18O record at Deep Sea Drilling Project Site 511

Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ∼2‰ excursion to values as low as −4.66‰ (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer δ18O values suggest that the upper ocean was as warm as 30–32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc δ18O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well‐defined depth distribution in Site 511 planktonics, low sediment burial temperature (∼32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer δ18O suggests brackish water capping as the mechanism for the low planktonic δ18O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2‰ shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower‐than‐normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater δ18O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.

Did the rifting of the Atlantic Ocean cause the Cretaceous thermal maximum?

The Cretaceous thermal maximum was a major turning point in the history of Earth’s climate. This interval of peak warmth in the Turonian has been attributed to very high atmospheric pCO2 resulting from rapid outgassing rates, although crustal cycling rates peaked in the Aptian‐Albian interval. On the basis of coupled ocean-atmosphere model simulations of the middle Cretaceous, we hypothesize that the formation of an Atlantic gateway could have contributed to the Cretaceous thermal maximum. Differences between prerifting and postrifting climate experiments demonstrate substantial regional oceanographic changes in the North and South Atlantic Basins that are consistent with oxygen isotopic evidence used to infer a Cretaceous thermal maximum. The model results help reconcile the paleoclimate record inferred from foraminiferal d 18 O with our understanding of climate dynamics and Cretaceous tectonism.

Variability of the South Atlantic upper ocean circulation: A data assimilation experiment with 5 years of TOPEX/POSEIDON altimeter observations

Dynamical interpolation of five years of TOPEX/POSEIDON altimeter data (October 1992-September 1997) is performed in the South Atlantic basin, through the use of a simple quasigeostrophic model and a simple nudging data assimilation technique. The resulting data set is analysed in order to evidence the role of mesoscale eddies on the interannual variability of the South Atlantic ocean circulations. The annual mean circulations and the eddy fields reveal some significant interannual variability in the Brazil-Malvinas confluence region and in the Aghulas Retroflection region. But this interannual variability spreads out also in large extensions of those previous regions as well as in other areas of the South Atlantic basin. In particular, significant variability shows up in the upper return branch of the subtropical gyre that is interpreted in relation with the drift of some Aghulas rings. The role of those latter rings which are crucial for the Indian-Atlantic interocean exchange processes, is further investigated in the paper, and the fate of five specific rings analysed in detail. The overall influence of topography on the circulation is clearly evidenced, especially through its interaction with eddies.

Transform zones in the South Atlantic rifted continental margins

Abstract Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle. This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuípe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating; (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.

Enhanced productivity led to increased organic carbon burial in the euxinic North Atlantic basin during the late Cenomanian oceanic anoxic event

Three Cenomanian/Turonian (C/T, ∼93.5 Ma) black shale sections along a northeast‐southwest transect in the southern part of the proto‐North Atlantic Ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in δ13C values of both bulk organic matter (OM) and molecular fossils of algal chlorophyll and steroids. All three sites show an increase in marine organic carbon (OC) accumulation rates during the C/T Oceanic Anoxic Event (OAE). The occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria, lack of bioturbation, and high abundance of redox sensitive trace metals indicate sulfidic conditions, periodically reaching up into the photic zone before as well as during the C/T OAE. During the C/T OAE, there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and Mo/Al ratios. The presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the C/T OAE. However, bottom water conditions did not dramatically change as indicated by more or less constant V/Al and Zn/Al ratios at site 367. This suggests that the increase in OC burial rates resulted from enhanced primary productivity rather than increased anoxia, which is supported by stable carbon isotopic evidence and a large increase in Ba/Al ratios during the C/T OAE. The occurrence of the productivity event during a period of globally enhanced organic carbon burial rates (i.e., the C/T OAE) points to a common cause possibly related to the formation of a deep water connection between North and South Atlantic basins.

Strontium-isotope stratigraphy and oxygen- and carbon-isotope variation during the Middle Jurassic–Early Cretaceous of the Falkland Plateau, South Atlantic

87Sr/ 86Sr data of belemnites are presented from a Middle Jurassic–Early Cretaceous succession from the Falkland Plateau (Deep Sea Drilling Project Sites 511 and 330) that was deposited in a periodically anoxic, semi-enclosed shallow water basin. Diagenetically screened strontium-isotope values of 0.706789 rise to 0.707044 before increasing sharply to 0.707428 in the uppermost part of the sampled succession. Comparison with published strontium calibration curves suggests that the oldest samples were Callovian to Oxfordian in age, whilst the remainder of the Jurassic part of the succession consisted of Kimmeridgian and Early Tithonian age sediments. The nannofossil, dinoflagellate and molluscan assemblages provide comparable age determinations. The strontium-isotope analysis of the youngest belemnites points to a Hauterivian–Barremian age, whilst age interpretations based upon the fauna provide a wide age range from the Barremian to early Albian. Strontium-isotope stratigraphy of this succession hence offers increased age resolution providing data regarding the timing of episodes of bottom water anoxia which have been recorded throughout the South Atlantic Basin. Well-preserved belemnite specimens display an oxygen-isotope range between +0.08 and −2.22‰ (PDB, Peedee belemnite international standard) and a carbon-isotope range from +2.35 to −1.33‰ (PDB). δ 13C values become increasingly negative through the Late Jurassic–Early Cretaceous and in concert with the 87Sr/ 86Sr data reveal a trend that could be accounted for by increasing levels of weathering and erosion. The oxygen-isotope data if interpreted in terms of palaeotemperature are consistent with warm palaeotemperatures in the Kimmeridgian and slightly cooler temperatures for the Tithonian and Early Cretaceous parts of the succession. The proposed relative Kimmeridgian warmth (based upon strontium-isotope age assignments) is thus in good agreement with other published palaeotemperature records.

Response of the Mid‐Cretaceous global oceanic circulation to tectonic and CO2 forcings

The mid‐Cretaceous was a period of unusually active tectonism that drove enhanced volcanic outgassing and high seafloor spreading rates. This intense tectonic activity is coincident with dramatic events in the marine environment, including oceanic anoxic events 1 (Aptian‐Early Albian) and 2 (Cenomanian/Turonian boundary), high biological turnover rates, and a thermal maximum. In this study, a series of mid‐Cretaceous ocean general circulation model experiments were completed using the Parallel Ocean Climate Model. These experiments demonstrate the effect of enhanced atmospheric CO2 concentrations and paleogeographic change on the global oceanic circulation. The experiments reveal that paleogeography, specifically the presence/absence of a marine connection between the North Atlantic and South Atlantic basins, may have governed the nature of the mid‐Cretaceous global oceanic circulation. In the absence of this connection, an Albian simulation is characterized by extremely warm, saline conditions throughout the North Atlantic and northern South Atlantic Oceans. With a gateway in a Turonian simulation, Antarctic Bottom Water ventilates the Atlantic basins. In both Albian and Turonian simulations the Pacific‐Indian basins are dominated by thermohaline circulation with deep water sources in the Southern Ocean. While atmospheric CO2 concentrations influence the global temperature and salinity, an increase from present‐day to 4 times present‐day levels alters the global circulation very little. Differences between the Albian and Turonian numerical simulations agree well with aspects of the marine record, supporting speculation that the climatic and oceanographic changes surrounding the Cenomanian‐Turonian boundary were driven by the initiation of a connection between the Atlantic Oceans.