Late Cretaceous to early Quaternary organic sedimentation in the eastern Equatorial Atlantic

Organic carbon accumulation in the South Atlantic Ocean: its modern, mid-Holocene and last glacial distribution

We investigate the role of sea surface temperature (SST) and land surface temperature (LST) in driving the seasonal cycle of the atmosphere (surface winds and precipitation) in the tropical Atlantic. For this we compare three atmospheric general circulation model (AGCM) experiments for the historical period 1982–2013 forced by different SST: (1) observed daily-climatological SST, (2) globally annual-mean SST, and (3) annual-mean SST in the equatorial Atlantic and daily-climatological SST elsewhere. Seasonal variations in SST strongly influence the seasonal evolution of the West African Monsoon (WAM) and ITCZ over the equatorial Atlantic Ocean. Forcing the model with annual mean SST (globally and in the equatorial Atlantic) considerably reduces the seasonal variance in the atmosphere, except for the zonal winds in the eastern equatorial Atlantic. Equatorial Atlantic SST contributes to the seasonal cycle in precipitation and meridional winds over the entire equatorial Atlantic, but only strongly influences zonal winds in the western equatorial Atlantic and has little influence on the northward penetration of the WAM. The leading modes of coupled SST–LST-atmosphere co-variability are identified by multivariate analysis. The analysis shows that both LST and SST drive seasonal variations in precipitation over equatorial Atlantic, with the LST being a larger contributor to the continental rainfall in West Africa. The coupling between ocean and atmosphere is stronger in the western than in the eastern equatorial Atlantic. The pressure adjustment mechanism is the main driver of the surface meridional wind convergence in the eastern tropical Atlantic.

During Ocean Drilling Program (ODP) Leg 159, four sites (Sites 959-962) were drilled along a depth transect on the Cote d’Ivoire/Ghana Transform Margin. In this study, the Pliocene‐Pleistocene history of carbonate and organic carbon accumulation at Hole 959C is reconstructed for the eastern equatorial Atlantic off the Ivory Coast/Ghana based on bulk carbonate, sand fraction, organic carbon, and other organic geochemical records ( δ 13 Corg, marine organic matter percentages derived from organic petrology, hydrogen index, C/N). Pliocene‐Pleistocene sedimentation off the Ivory Coast/Ghana was strongly affected by low mean sedimentation rates, which are attributed to persistently enhanced bottom-water velocities related to the steep topography of the transform margin. Sand fraction and bulk carbonate records reveal typical glacial/interglacial cycles, preserved, however, with low time resolution. Intermediate carbonate accumulation rates observed throughout the Pliocene‐Pleistocene suggest intense winnowing and sediment redistribution superimposed by terrigenous dilution. “Atlantic-type” sand and carbonate cycles, consistent with records from pelagic areas of the eastern equatorial Atlantic, are encountered at Hole 959C prior to about 0.9 Ma. Total organi c carbon (TOC) records are frequently inversely correlated to carbonate contents, indicating mainly productivity-driven carbonate dissolution related to changes in paleoproductivity. During Stages 22 -24, 20, 16, 12, 8, and 4, sand and carbonate records reveal a “Pacific-type” pattern, showing elevated contents during glacials commonly in conjunction with enhanced TOC records. Formation of “Pacific-type” patterns off the Ivory Coast/Ghana is attributed to drastically increased bottom-water intensities along the transform margin in accordance with results reported from the Walvis Ridge area. Short-term glacial/interglacial changes in paleoproductivity off the Ivory Coast/Ghana are to some extend recognizable during glacials prior to 1.7 Ma and interglacial Stages 21, 19, 13, 9, and 1. Enhanced coastal upwelling during interglacials is attributed to local paleoclimatic and oceanographic conditions off the Ivory Coast/Ghana. Quantitative estimates of marine organic carbon based on organic petrologic and δ 13 Corg records reveal an offset in concentration ranging from 15% to 60%. Highest variabilities of both records are recorded since ~0.9 Ma. Discrepancies between the isotopic and microscopic records are attributed to an admixture of C4 plant debris approaching the eastern equatorial Atlantic via atmospheric dust. Terrestrial organic material likely originated from the grass-savannah-covered Sahel zone in central Africa. Estimated C4 plant concentrations and accumulation rates range from 10% to 37% and from almost zero to 0.006 g/cm 2 /k.y., respectively. The strongest eolian supply to the northern Gulf of Guinea is indicated between 1.9 and 1.68 Ma and during glacial isotopic Stages 22 -24, 20, 14, and 12. The presence of grass-type plant debris is further supported by organic petrologic studies, which reveal well-preserved cell tissues of vascular plants or tube-shaped, elongated terrestrial macerals showing different levels of oxidation.

The pH dependent accumulation of PCP in aquatic microcosms with sediment

Summary Standing and migrating sediment waves along the West African continental rise (3000–4500 m water depth) are characterized by a regular spacing of wave length periods that are multiples of 280 m, and by long-term pelagic sediment composition. Interpretations of their origin must account for the following evidence: (1) The wave crests generally run parallel or oblique to the continental rise and also to prevailing (net) currents, as confirmed by SEA BEAM data; (2) Migrating waves along the West African continental margin are directed only upslope, i.e. towards the East. (3) The waves have been accreting for a long time, (>1 Ma). (4) Glacial to interglacial differences in the evolution of sediment accumulation rates on crests and troughs may indicate that the accreting regime was intermittent, and that the waves were more levelled prior to 7500 years bp due to increased deposition of siliciclastic matter in the troughs. (5) Absent patterns of grain-size sorting in the fine-grained muds contradict ‘conventional’ models of current-related sediment transport. A superposition of large tidal components and slow thermohaline net currents (Lonsdale 1978), secondary current processes (Unsöld, pers. comm.), or a superposition of internal waves and thermohaline currents (Kolla et al. 1980) are regarded as the most probable modes of origin.

Black-shale cycles deposited in the late Cretaceous tropical Atlantic at ODP Site 959 were analyzed to reconstruct processes for organic-matter sequestration during the Coniacian–Santonian “oceanic anoxic event” (OAE3). The results from bulk organic and inorganic geochemistry suggest that black-shale accumulation was intimately linked to orbitally forced cycles in the Deep Ivorian Basin (DIB) that alternated between eutrophic conditions stimulating productivity of organic-walled plankton followed by less trophic conditions associated with carbonate production. Results from Rock-Eval Pyrolysis, bulk δCorg analysis, and maceral analysis demonstrate a dominantly marine origin of the organic matter (OM) with only a subordinate proportion from terrestrial sources. Intervals of high organic-carbon (OC) accumulation display high hydrogen indices (HI) up to 720 mg HC/g OC, low oxygen indices (OI) of 20 mg CO2/g OC, and bulk δ Corg varying between –28 to –26.5‰. The enrichment in redox-sensitive trace metals up to 2500 μg/g for vanadium, for example, as well as carbon–sulfur relationships in black-shale intervals suggest intermittently anoxic conditions, on occasion as extreme as during the Cenomanian–Turonian OAE2. We propose that the black-shale cycles were directly linked to the climate development in equatorial Africa via the hydrological cycle. The mechanism for carbon sequestration that operated in the DIB may have worked in a similar way in other equatorial regions of Africa and South America, implying that the tropics acted as a prominent sink for OC, and consequently atmospheric CO2, during the Coniacian–Santonian OAE3.

A 30,000 yr record of land–ocean interaction in the eastern Gulf of Guinea

Annual flux of radiolaria and other shelled plankters in the eastern equatorial atlantic at 853 m: seasonal variations and polycystine species-specific responses

(SST) has a strong influence on regional climate variations, including the strength and onset date of the West African Monsoon (WAM). However, the predictability of the WAM is strongly limited by large biases in coupled climate models. The observational studies presented here are largely based on enhanced observations taken during the TACE period, but also include studies based on historical and remote sensing data and data from the global in situ observing system. The enhanced observations have provided tremendous new insight into physical processes at work shaping the Atlantic climate system. However, while significant progress has been made towards improving coupled climate simulations of the tropical Atlantic, a strong bias in the southeastern tropical Atlantic is a common feature of current generation climate models. As this region still remains underrepresented in the available in situ observational datasets, new observational programs have been developed in the eastern tropical Atlantic, with a particular focus on the coastal upwelling regions. At the same time, the essential ocean observations obtained during TACE, including subsurface moorings along the equatorial wave guide, are being continued. The articles contributed to this volume represent new advances in observing, modeling, understanding and predicting TAV. They encompass a range of topics, including tropical sea surface temperature, salinity, and subsurface oxygen variability, mixed layer heat and freshwater budgets, equatorial circulation and its seasonal to interannual variability, modes of tropical Atlantic variability, tropical Atlantic biases in coupled climate models and their potential causes, and connections between Atlantic and Pacific climate variability. This special issue is coordinated by William Johns, Peter Brandt, and Ping Chang, representatives of the TACE Observations and TACE Modeling and Synthesis working groups. Tropical Atlantic variability (TAV) and coupled model climate biases have been the focus of the recently completed Tropical Atlantic Climate Experiment (TACE, 2006–2011), an international CLIVAR program (http://www.clivar.org/ organization/atlantic/tace). TACE with its backbone, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), was closely linked to other initiatives in the tropical Atlantic, such as the French EGEE (Etude de la circulation oceanique et de sa variabilite dans le Golfe de Guinee) program and the African Monsoon Multidisciplinary Analyses (AMMA). One of the main goals of TACE was to improve the observational database in the equatorial Atlantic and to carry out dedicated process studies enhancing our understanding of the tropical Atlantic climate system. The regional focus of TACE was on the central and eastern equatorial Atlantic, characterized by the development of the Atlantic cold tongue (ACT) during boreal summer. The year-to-year variability of the ACT sea surface temperature

Studying monsoon dynamics during past warm time periods such as the Miocene Climatic Optimum (MCO; ∼16.9–14.5 Ma) could greatly aid in better projecting monsoon intensity, in the context of future greenhouse warming. However, studies on regional MCO temperature change and its effect on the monsoons during this time period are lacking. Here, we present the first high‐resolution, low‐latitude record of sea surface temperature (SST) and paleoceanographic change covering the Miocene Climatic Optimum, in the eastern equatorial Atlantic, at Ocean Drilling Program Site 959, based on TEX86 paleothermometry. SSTs were ∼1.5°C warmer at the onset of the MCO (16.9 Ma) relative to the pre‐MCO (∼18.3–17.7 Ma). This warming was accompanied by a transient increase in %total organic carbon. Prior to the MCO, sediment composition, geochemical proxy data as well as dinoflagellate cyst assemblages imply a productive surface ocean at Site 959. Immediately following the MCO onset (∼16.9–16.5 Ma), we record an intensification of the West African Monsoon (WAM) characterized by higher amplitude variability in all proxy records on precession to obliquity timescales. We interpret increased orbital‐scale SST, biogenic Ba and dinocyst assemblage variability to represent intensification of equatorial upwelling, forced by the WAM strength. Furthermore, higher SSTs during eccentricity maxima correlate to increased relative abundances of the warm and stratification‐favoring dinocyst Polysphaeridium zoharyi, during periods of low WAM intensity. Finally, while long‐term SSTs decline toward the middle Miocene, maximum SSTs and Polysphaeridium zoharyi abundances occur during MCO peak warming at ∼15.6 Ma.

The Devonian Woodford Shale is a prolific unconventional resource shale for oil and gas. Like many such shales, the Woodford sits atop an unconformity on the surface of underlying carbonate rocks (mainly the Hunton Group in this case). There is variable topographic relief on the unconformity surface due to incised valleys, cave collapse, and/or karst formation during periods of subaerial exposure resulting from eustatic sea-level fluctuations. Anomalously high thicknesses of the Woodford, with relatively high total organic carbon (TOC), can form within topographic depressions on the unconformity surface, giving rise to potential “sweet spots” as drilling targets. It is likely that the topographic relief that formed during subaerial exposure created areas of restricted marine circulation (or possibly hypersaline lakes) during an early fall in the sea level, and thus, localized anoxic depositional environments conducive to preservation of organic matter (TOC). Seismic analysis, calibrated with well logs and cuttings, of two areas on the Cherokee Platform in Oklahoma were completed to test the discontinuous and isolated distribution, vertically and horizontally, of the TOC. In one area, the TOC ranged up to 10 wt.% and in the other area, up to 13 wt.%. Seismic inversion and attribute analysis demonstrated the patchy distribution of the TOC vertically and laterally in both areas. These patchy, discontinuous distribution spotlights areas where TOC was preserved (in the minibasins), and point to potential sweet-spot locations. The delineation of organic-rich sweet spots was accomplished by integrating geologic, geochemical, and geophysical data in probabilistic neural networks obtaining seismic impedance-derived TOC that was mapped across different locations in the Cherokee Platform.

We analyzed the value of diatoms as a proxy of export production from the surface waters in the tropical and equatorial Atlantic. Species composition and downward flux of diatom assemblages was determined from time-series sediment traps off Cape Blanc in the Mauretanian upwelling zone; south of Cape Verde Island within the Guinea Dome; along a N-S transect in the high productivity belt of the eastern equatorial Atlantic in the Guinea Basin; in the less productive western equatorial Atlantic around 25°W; and at the Walvis Ridge within the Benguela upwelling system. Diatom fluxes were highest at the sites located adjacent to or influenced by coastal upwelling (off Cape Blanc and at the Walvis Ridge), south of Cape Verde Islands, and north of the equator in the Guinea Basin. Lowest values were recorded in the oligotrophic western Atlantic. Regardless of the percent contribution to the total flux, diatoms dominated (in numbers per m-2 d-1) the biogenic opal fraction at all sites and, in general, showed significant correlation with organic carbon flux. At all sites, downward fluxes revealed strong coupling with surface water production, and with the atmospheric and oceanic circulation systems related to the Trade Winds and the seasonal migration of the Intertropical Convergence Zone (ITCZ): Diatom flux maxima were observed in early spring and early summer off Cape Blanc; in fall and winter at Cape Verde; in spring and summer in the N-Guinea Basin, and only in spring in the S-Guinea Basin; and in austral fall and spring at the Walvis Ridge. No distinct seasonal pattern was seen in the western Atlantic. Diatom assemblages varied with the water mass characteristics influencing each trap location. Throughout the sampling periods, small specimens of Nitzschia bicapitata accompanied by other taxa typical of open-ocean conditions dominated the diatom assemblage in the eastern and western equatorial Atlantic, and at Cape Verde. At Cape Blanc and Walvis Ridge, on the other hand, a coastal upwelling assemblage was characterized by the colonial diatom Thalassionema nitzschioides var. nitzschioides and members of the chain-forming genus Chaetoceros, with a minor contribution of pelagic forms. In addition to the marine assemblage, freshwater diatoms and phytoliths (of continental origin) were present in the traps. Their fluxes coincided with the seasonal changes in Saharan dust transport patterns, and the geographical extension of the dust plume across the Atlantic. Flux values decreased as a function of distance from the African continent. Discrepancies between diatoms trapped and diatoms accumulating on the seafloor were seen in the eastern and western equatorial Atlantic, and at Walvis Ridge, and are attributed to strong dissolution at the sediment/water interface. At these sites, surface sediment assemblages are enriched in robust diatom species representative of the low productivity season of the surface waters. In contrast, diatoms in the trap off Cape Blanc, indicators of persistent offshore spreading of the coastal upwelling, possess moderately robust frustules readily preserved in the underlying sediments. Here, diatom species-specific selective dissolution was minor in sediment-trap samples. Despite this loss, the preserved diatom assemblage carries generalized information which can be related to the hydrographic conditions of the surface waters.

Nature of organic matter in surface sediments from the Pakistan continental margin and the deep Arabian Sea: amino acids

Marine carbon burial flux and the carbon isotope record of Late Cretaceous (Coniacian–Santonian) Oceanic Anoxic Event III

This study examines the relationship between depositional environment and sedimentary organic geochemistry in Lake Malawi, East Africa, and evaluates the relative significance of the various processes that control sedimentary organic matter (OM) in lacustrine systems. Total organic carbon (TOC) concentrations in recent sediments from Lake Malawi range from 0.01 to 8.80 wt% and average 2.83 wt% for surface sediments and 2.35 wt% for shallow core sediments. Hydrogen index (HI) values as determined by Rock-Eval pyrolysis range from 0 to 756 mg HC g−1 TOC and average 205 mg HC g−1 TOC for surface sediments and 228 mg HC g−1 TOC for shallow core samples. On average, variations in primary productivity throughout the lake may account for ~33% of the TOC content in Lake Malawi sediments (as much as 1 wt% TOC), and have little or no impact on sedimentary HI values. Similarly, ~33% to 66% of the variation in TOC content in Lake Malawi sediments appears to be controlled by anoxic preservation of OM (~1–2 wt% TOC), although some component of the water depth–TOC relationship may be due to physical sediment transport processes. Furthermore, anoxic preservation has a minimal effect on HI values in Lake Malawi sediments. Dilution of OM by inorganic sediment may account for ~16% of variability in TOC content in Lake Malawi sediments (~0.5 wt% TOC). The effect of inputs of terrestrial sediment on the organic character of surface sediments in these lakes is highly variable, and appears to be more closely related to the local depositional environment than the regional flux of terrestrial OM. Total nitrogen and TOC content in surface sediments collected throughout the lake are found to be highly correlated (r2 = 0.95), indicating a well-homogenized source of OM to the lake bottom. The recurring suspension and deposition of terrestrial sediment may account for significant amounts of OM deposited in offshore regions of the lake. This process effectively separates denser inorganic sediment from less dense OM and allows terrestrial OM to preferentially be transported farther offshore. The conclusion is that for the organic carbon content in these regions to be elevated a mixed terrestrial-lacustrine origin is required. The hydrodynamic separation of mineral and organic constituents is most pronounced in regions with shallow bathymetric gradients, consistent with previous findings from Lake Tanganyika.