Paleoproductivity Changes Research Articles

Benthic foraminifer Ammonia beccarii as a proxy for paleoproductivity changes from West coast of India

We examined three hundred sixty-five (365) sediment subsamples for benthic foraminifera from beach cores sediment in the west coast of India. The study on the recent benthic foraminiferal faunal record, reveals the paleoecological preferences. A total of 38 species belonging to 22 genera were identified from the study. For benthic foraminiferal faunal study, dry samples were sieved over 125 μm-size sieve and split into suitable aliquots to obtain ~250 specimens of benthic foraminiferal species identified and counted. Ammonia beccarii was the most prevalent species found in all subsamples. The high productivity species of Ammonia beccarii can be found in sediments from Puthenthoodu beach ~75 to 60cm and 10 to 5cm depth, from Chellanam beach sediments 100 to 70cm and 30 to 5cm depth, from Azheekal beach sediments around 90 to 70cm and 18 to 5cm depth, 100 to 80cm and 20 to 5cm depth in Anthakaranazhi beach sediments, an indicates of shallow-marine environment. The species of Ammonia beccarii were utilized for biostratigraphy for several years and have also been confirmed very constructive in paleoceanographic and paleoclimatological reconstruction.

Reconstructing hypoxia on the eastern Arabian sea continental shelf during the past ∼7400 years: A multi-proxy approach

Shallow-sea environmental variations in the eastern Arabian Sea (EAS) have been related to monsoon variability, poorly ventilated intermediate water, and primary productivity changes. To understand these conditions, benthic foraminiferal assemblages, oxygen isotope ratios, and total organic carbon content have been examined in sediment Core SK291/GC13 from the EAS. We aim to retrieve decadal to centennial time-scale changes in past oxygenation and paleoproductivity during the last 7400 calibrated years before the present (cal yr BP). Our proxy data suggest that the oxic benthic foraminifera species were dominant during ∼7400–6000 cal yr BP. The change from oxic to dysoxic occurred during ca. 6000–3500 cal yr BP, and transition from dysoxic to suboxic environmental conditions happened at ca. ∼3500 cal yr BP. These changes are contemporaneous with weakening of the Indian summer monsoon beginning at ∼5500 cal yr BP which culminated into a major arid phase so-called ca 4.2 ka event in the Indian subcontinent. The EAS experienced low productivity during the early phase of our studied interval (∼7400–6000 cal yr BP) and higher productivity in the middle Holocene (6000–3000 cal yr BP), as revealed by the total organic carbon and benthic assemblages. Furthermore, productivity increased in the warm period and decreased in the Little Ice Age on a decadal scale. The spectral analysis of total organic content and suboxic species indicates a 23-year Hale (solar) cycle during the last millennium, indicating a close relationship between solar activity and monsoon variability. The strengthening of the El Nino-Southern Oscillation perhaps amplified due to increased solar activity in the last millennium, which influenced the climate of the EAS. The study highlights paleo-oxygenation shifts based on benthic foraminiferal assemblages, which are crucial for understanding changes in the oxygen minimum zones (OMZs) and their potential impacts on marine ecosystems.

Astronomically‐Paced Changes in Paleoproductivity, Winnowing, and Mineral Flux Over Broken Ridge (Indian Ocean) Since the Early Miocene

AbstractA significant shift in Earth's climate characterizes the Neogene, transitioning from a single‐ice‐sheet planet to the current bipolar configuration. This climate evolution is closely linked to changing ocean currents, but globally‐distributed continuous high‐resolution sedimentary records are needed to fully capture this interaction. The Ocean Drilling Program (ODP) Site 752, located on Broken Ridge in the Indian Ocean, provides such a Miocene‐to‐recent archive. We use X‐ray fluorescence (XRF) core scanning to build an eccentricity‐tuned age‐depth model and reconstruct paleoceanographic changes since 23 Ma. We find two intervals of enhanced productivity, during the early and middle Miocene (18.5–13.7 Ma) and late Pliocene/early Pleistocene (3–1 Ma). We also report a mixed eccentricity‐obliquity imprint in the XRF‐derived paleoproductivity proxy. In terms of grain size, three coarsening steps occur between 19.2–16 Ma, 10.8–8 Ma, and since 2.6 Ma. The steps respectively indicate stronger current winnowing in response to vigorous Antarctic Intermediate Water flow over Broken Ridge in the early Miocene, the first transient onset of Tasman Leakage in the Late Miocene, and the intensification of global oceanic circulation at the Plio‐Pleistocene transition. High‐resolution iron and manganese series provide a detailed Neogene dust record. This study utilized a single hole from an ODP legacy‐site. Nevertheless, we managed to provide novel perspectives on past Indian Ocean responses to astronomical forcing. We conclude that Neogene sediments from Broken Ridge harbor the potential for even more comprehensive reconstructions. Realizing this potential necessitates re‐drilling of these sedimentary archives utilizing modern drilling strategies.

Petrographic and geochemical characteristics of the Middle Devonian Eifelian Nabiao Formation in the Youjiang Basin, South China: Implications for paleoenvironment and organic matter accumulation mechanism

The Nabiao Formation of the Middle Devonian Eifelian period has been identified as a promising site for shale gas exploration in the Youjiang Basin, South China. And yet, little is known about the accumulation mechanism of organic matter under transgression in the archipelago paleogeography. In this study, the core samples were analyzed by thin section microscopy, field emission-scanning electron microscopy, pyrite morphology, X-ray diffraction, and geochemical analysis (total organic carbon (TOC) content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) to understand changes in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism of the Nabiao Formation. The study indicates that organic-rich sediments were deposited during the two transgressions of the Chotec Event and the Kacak Event. The deposition of carbonaceous mudstone occurred in water columns with weak upwelling, while siliceous mudstone and siliceous shale were developed in a strong upwelling water column. The correlations between multiple geochemical proxies (paleoredox, paleoproductivity, and terrestrial detrital influx) and TOC content indicate that the Chotec Event is more conducive to organic matter enrichment of the Nabiao Formation than the Kacak Event. In stage 1, TOC content is low due to oxic water columns and low productivity. In stage 2, the Chotec Event causes the relative sea-level of Member 2 to rise rapidly and the basin bottom water to quickly change to dysoxic conditions. This is the key to organic matter enrichment. In stage 3, the oceanic upwelling is crucial in organic matter enrichment by increasing paleoproductivity. Under the protection of dysoxic condition, organic matter is accumulated in stage 4. Organic matter of the Nabiao Formation is not significantly affected by the terrigenous detrital flux. By taking into account the changes in paleoproductivity, redox conditions, upwelling events, and terrigenous detrital flux, an evolution model is proposed in this paper to elucidate the paleoenvironmental change and the organic matter enrichment under transgression in the archipelago paleogeography.

Late Pleistocene zinc isotopic indices of paleoproductivity variations in the tropical West Pacific

The tropical western Pacific (TWP) plays a significant role in modulating the global climate, with solar radiation, sea surface temperature, and monsoon circulation. Marine prime productivity fluctuations in the TWP can change the global carbon cycle. However, isotopic geochemical evidence reconstructing the changes of productivity in the western Pacific warm pool (WPWP) is currently lacking. The forcing mechanism that changes productivity in the WPWP remains uncertain. Here, we used the Zinc isotope (δ66Zn) composition in sediment carbonate with Ba/Ti ratios and differences in foraminiferal carbon isotopes between the surface, and thermocline obtained from the Ocean Drilling Program (ODP) Site 807A to reconstruct paleoproductivity and nutricline depth over the last 550 ka period. The Zn isotopic composition signal exhibited a generally fluctuating increase, which is similar in a trend found in the Ba/Ti records. In a periodogram of marine productivity tracing, δ66Zn and Ba/Ti showed periodicities of 33 and 31 ka, respectively, suggesting the influence of long-term El Niño Southern Oscillation (ENSO)-like forcing. Good coherence between the paleoproductivity records and the nutricline suggests that nutricline changes could be the primary controlling factor in changes in paleoproductivity. During El Niño-like conditions, the shallow nutricline in the WPWP can result in high prime productivity accompanied by high δ66Zn values. In addition, the higher productivity and shallower nutricline coeval with the occurrence of low CO2 concentrations in the Antarctic ice core might indicate that biological export production at low latitudes could act as a significant sink in the global carbon cycle and modify atmospheric CO2 concentrations. Our new datasets of sedimentary carbonates in the tropical ocean can provide better insight and constrain on marine Zn biogeochemical cycle and a strong potential tracer in paleoproductivity.

Paleoenvironmental Conditions and Factors Controlling Organic Carbon Accumulation during the Jurassic–Early Cretaceous, Egypt: Organic and Inorganic Geochemical Approach

The Jurassic–Early Cretaceous was a time of variable organic carbon burial associated with fluctuations of marine primary productivity, weathering intensity, and redox conditions in the pore and bottom water at paleo-shelf areas in north Egypt. This time interval characterized the deposition of, from old to young, the Bahrein, Khatatba, Masajid, and Alam El Bueib Formations in the north Western Desert. Although several studies have been devoted to the excellent source rock units, such as the Khatatba and Alam El Bueib Formations, studies on paleoenvironmental changes in redox conditions, paleoproductivity, and continental weathering and their impact on organic carbon exports and their preservation for this interval are lacking. This study presents organic and inorganic geochemical data for the Jurassic–Lower Cretaceous sediments from the Almaz-1 well in the Shushan Basin, north Western Desert. A total of 32 cuttings samples were analyzed for their major and trace elements, carbonates, and total organic carbon (TOC) contents. Data allowed the reconstruction of paleoenvironmental conditions in the southern Tethys Ocean and assessment of the changes in paleo-redox, paleo-weathering, and marine primary productivity, and the role of sediment supply. Additionally, factors that governed the accumulation of organic matter in the sediment were interpreted. Results showed that the Khatatba Formation was deposited during a phase of enhanced marine primary productivity under prevalent anoxia, which triggered enhanced organic matter production and preservation. During the deposition of the Khatatba Formation, significant terrigenous sediment supply and continental weathering were followed by a limited contribution of coarse clastic sediment fluxes due to weak continental weathering and enhanced carbonate production. The Bahrein, Masajid, and Alam El Bueib Formations were deposited during low marine primary productivity and prevalent oxygenation conditions that led to poor organic matter production and preservation, respectively. A strong terrigenous sediment supply and continental weathering predominated during the deposition of the Bahrein Formation and the lower part of the Alam El Bueib Formation compared to the limited coarse clastic supply and continental weathering during the deposition of the carbonate Masajid Formation and the upper part of the Alam El Bueib Formation. Such conditions resulted in the enhanced dilution and decomposition of labile organic matter, and, thus, organic carbon-lean accumulation in these sediments.

Paleoproductivity proxies and alkenone precursors in the Western Mediterranean during the Early-Middle Pleistocene transition

Multidisciplinary analyses (taxonomic analysis of coccolithophore assemblages, stable oxygen isotopes, marine and terrestrial biomarkers) have been carried out on sediments from Ocean Drilling Program Site 975 in the Algero-Balearic basin, through late marine isotope stage (MIS) 20-19 (800–756 ka). The aim is to compare coccolithophore paleoproductivity proxies, such as C37 alkenone concentration and nannofossil accumulation rate (NAR), and understand their relationship with paleoceanographic condition and paleoenvironmental changes, alkenone-producing precursors and unsaturated C37 alkenone compounds. The patterns of C37 alkenones and NAR provide reliable information on past paleoproductivity changes since coccolith dissolution and organic matter preservation were excluded as relevant processes at the site. This is testified by the high values of Nannofossil Dissolution Index and relation between C37 alkenone concentration and Alcohol Preservation Index, the latter used as a proxy of sea bottom ventilation in the basin. A weak mismatching between NAR and C37 alkenone concentration records has been observed and related to paleoenvironmental factors and ecological preferences of alkenone-producing species. Temperature variations mostly controlled the alternating interspecific abundance variations of these taxa through glacial-interglacial and stadial-interstadial climate phases. The percentage abundances of alkenone-producing species, Gephyrocapsa caribbeanica and Gephyrocapsa with open central area (mainly G. margerelii-G. muellerae) strongly co-varied with the percentages of C37:2 and C37:3, respectively during warm and cool-cold periods, suggesting their prominent role in producing these unsaturated C37 alkenone compounds. Moreover, Gephyrocapsa spp. with open central area > 3 μm were likely the main C37:4 producers during the colder late MIS 20 stadial and stadial phases. Other factors in addition to temperature influenced the paleoproductivity proxy patterns. The oceanographic condition established during MIS 20-MIS 19 deglaciation and the more nutrient-rich surface waters during the orbitally-controlled organic-rich layer deposition in the early MIS 19 enhanced primary productivity leading to higher production and preservation of total C37 alkenones.

Paleoproductivity variations and implications in the subarctic northwestern Pacific since MIS 7: Geochemical evidence

Knowledge of changes in marine primary productivity and export production in high-nutrient, low-chlorophyll regions of the subarctic NW Pacific Ocean is vital for understanding glacial-interglacial pCO2 variations induced by the biological pump mechanism. We conducted a multi-proxy of geochemical and biological productivity study of sedimentary core LV63-4-2, from the subarctic NW Pacific Ocean, spanning the last ~230 kyr (since ~MIS 7). The measurements included total organic carbon (TOC), opal, biogenic barium, and CaCO3 content as indicators of biological productivity; and the stable carbon isotope composition of TOC and trace elemental abundances to indicate sources of organic matter and redox conditions. The results reveal cyclic changes in paleoproductivity, with higher export production corresponding to periods of low Earth orbital obliquity and high winter insolation. Opal accumulation in core LV63-4-2 was higher during MIS 7, MIS 5d, and MIS 4, which differs from records from the Bering Sea, Okhotsk Sea, and the region to the south of the site LV63-4-2, but it is consistent with records from the Gulf of Alaska. We suggest that export production in the region is controlled by the combined effects of regional vertical convection associated with the formation of North Pacific Intermediate Water (NPIW), and the local iron supply sourced from the Gulf of Alaska. High export production is promoted by the coupling of weakened NPIW formation and an increased iron supply caused by enhanced upwelling in the Gulf of Alaska due to a strengthened Aleutian Low during warmer winters. By contrast, stronger NPIW formation dominated reduced export production during MIS 6 and MIS 2, despite an increased iron supply. Our results highlight the heterogeneity of export production in the subarctic North Pacific and the significant role of the local environment on productivity.

Organic matter heterogeneity and shale oil significance in Triassic Zhangjiatan shale, Ordos Basin, China

Organic matter heterogeneity exerts an important impact on the generation, evaluation, and exploitation of shale oil resources. In the past, only a limited number of analytical samples that represented the contribution or influence of high or low organic matter abundance intervals were used to represent an entire set of thick source rocks, and based on this limitation, occasionally researchers have reached incorrect conclusions. Here, the heterogeneity of organic matter and its significance to shale oil in the Triassic Zhangjiatan shale of the Ordos Basin were discussed based on sedimentary and geochemical analyses. Our results indicate that (1) the Chang 73 shale within the study area is characterized by high abundance, good quality, strong heterogeneity, and segmented enrichment in the vertical profile. The primary organic matter type was type II1, and this was followed by types I, II2, and III that exhibited three changing trends in the vertical profile. Organic matter maturity is primarily at the immature stage. (2) Additionally, we observed that the organic matter heterogeneity of Chang 73 shale was related to changes in lithology, sedimentary environment, paleoproductivity, terrigenous influence, and event action (such as volcanism and hydrothermal processes). The abundance of organic matter is the result of the coupling control of these processes. In the process of organic matter deposition, if volcano or hydrothermal activities occurred, the organic matter was abnormally enriched, and the total organic carbon content exceeded 20% and could even be as high as 40%. (3) Finally, we observed at least nine enrichment layers that were controlled by the coupling of different geological factors, and there were four potential target resource layers corresponding to the hydrothermal sedimentary area.

Long term natural and anthropogenic forcing on aquatic system - evidence based on biogeochemical and pollen proxies from lake sediments in Kashmir Himalaya, India

Freshwater aquatic systems are subjected to rapid deterioration driven by multiple stressors such as climate change and human activity. The understanding of the long-term history of eutrophication and their trends provides an opportunity for developing relevant management strategies. In this study, we examine the natural versus anthropogenic impacts on Ahansar Lake using a multiproxy approach (total organic carbon (TOC), total nitrogen (TN), amino acid composition, δ15N, grain size and pollen data) on a210Pb/137Cs dated sediment core spanning the last 200 years. The amino acid datasets clearly show that the organic matter in Ahansar sediment core is less degraded and can be utilized to understand the paleoproductivity changes. Organic matter (OM) within this core is mostly derived from aquatic sources as deduced from C/N (6–11) and δ15N (0–3.2‰) values. The aquatic productivity gradually increases from 1880s, becoming accelerated after the 1930s, and peaked between 1970 and 2016 AD. This enhancement of primary productivity in the lake indicating the increased eutrophication through time due to anthropogenic activities in the recent decades. The results provide baseline information for policymakers and environmentalists to develop strategical framework for future environmental changes in an aquatic system subjected to anthropogenic stressors.

Early-Middle Pleistocene productivity changes of the Northern Cascadia Margin, Pacific Ocean

A total of 362 marine core samples from 200 meters below the sea-floor (mbsf) were examined through the International Ocean Drilling Program (IODP), Hole U1325B, Northern Cascadia Margin. The diversity parameters increased at ~200 mbsf during the Early Middle Pleistocene Transition (EMPT) period and gradually decreased towards ~165 mbsf. The diversity pattern is also notable, which projects the termination of EMPT due to increased orbital cyclicity (~41,000 to ~100,000 years). The δ18O (Uvigerina peregrina) at regular intervals ~150 mbsf, ~110 mbsf, ~50 mbsf and ~20 mbsf certainly agrees with the abundance of Cibicides wuellerstorfi, indicating the intrusion of California Cold Current in the region. The relative abundance rate of Cassidulina laevigata inversely proportional to the peaks of C. wuellerstorfi strongly indicating the circulation of Alaskan warm current in the Cascadia Margin. The paleoproductivity and oxygenation proxies strongly suggest the maximum primary productivity throughout ~85 mbsf towards the recent interval and the study region are showing oxic to suboxic environment further, the correlation matrix and the principal component phenomenon also strongly mirror the relationship and driving factors of the paleoceanographic condition. The present study aimed to understand the connections between each proxy liable to the paleoceanographic and paleoproductivity changes during the Early-Middle Pleistocene, Northern Cascadia Margin.

CHANGES OF THERMOCLINE DEPTH AT THE SUMBA ISLAND OFFSHORE BASED ON PLANKTONIC FORAMINIFERAL ASSEMBLAGES AND ITS IMPLICATION TO EUTROPHICATION SINCE THE LAST DEGLACIATION (~18 ka BP): A PRELIMINARY STUDY

Changes of the thermocline depth (DOT) at the Sumba Island offshore are not well-known compared to the DOT changes in the Timor Sea, the main exit passage of the Indonesian Through-flow (ITF). Planktonic foraminiferal assemblages in cores collected from the southwest Sumba offshore (ST08) and Sumba Strait (ST12, ST13, and ST14) were used as a tool to infer the DOT and paleoproductivity changes at the Sumba Island offshore. The DOT changes were indicated from the thermocline and mixed layer dwellers’ relative abundance while the paleoproductivity changes were indicated from the relative abundance of Neogloboquadrina dutertrei. This study suggests a contrast between the DOT pattern at the Sumba Island offshore and the DOT pattern in the Timor Sea during the Last Deglaciation–Holocene. The contrast DOT pattern indicated that the multi-millennial changes of the Australian-Indonesian monsoon (AIM) during the Last Deglaciation– Holocene were the main factors behind the DOT changes in this region while the effects of El Niño Southern Oscillation (ENSO) –like, Indian Ocean Dipole (IOD) –like, and ITF to DOT changes were minimal. Paleoproductivity enhancement at the Sumba Island offshore was not solely related to the monsoon-driven coastal upwelling intensification, which resulted in the DOT shoaling and eutrophic condition. The increase of nutrient availability in surface water due to river runoff increase and changes in the lifted water mass nature were also able to enhance productivity in this region.

Paleoceanographic changes in the Southern Ocean off Elephant Island since the last glacial period: Links between surface water productivity, nutrient utilization, bottom water currents, and ice-rafted debris

To understand past changes in ocean–cryosphere interactions in the Southern Ocean off the Antarctic Peninsula, multi-proxy analyses of three sediment cores located off Elephant Island were used to reconstruct changes in paleoproductivity, nutrient utilization, bottom current intensity, and iceberg calving since the last glacial period. The glacial period was characterized by low surface water productivity with high nutrient utilization, indicating surface water stratification. During the deglaciation, surface water productivity increased with decreasing nutrient utilization, implying that the increase is associated with increased nutrient supply from the subsurface water by enhancing Antarctic Circumpolar Current (ACC) influence as fronts migrate southward with warming. Abundant occurrence of grains >1 mm during the deglacial period indicates rapid ice sheet retreat with large-scale melting and calving. During the glacial period, however, coarse silt–fine sand-sized fraction represented ice-rafted debris (IRD). The different IRD grain size characteristics are thought to be related to the IRD source material characteristics. Regardless of IRD input, the running downcore correlation (5 to 9-point) between sortable silt mean grain size and percentage showed that sediments are well sorted by bottom current. However, the cross plot of them showed different temporal relationships. Sediments were sorted by the ACC and southwestward flowing bottom current. Along with southward migration of fronts and the ACC, southwestward flowing bottom current influence diminished, whereas the ACC influence increased particularly from 7 ka. Our results indicate that the sedimentary processes in the Scotia Sea largely depend on the regional interactions between the ocean and the cryosphere.

The controlling factors of lacustrine shale lithofacies in the Upper Yangtze Platform (South China) using artificial neural networks

Abstract The influence of paleoenvironments on the lithofacies of lacustrine mudstone is important for both paleoenvironment reconstruction and hydrocarbon exploration. Compared with the machine learning approaches, the methods to find coupling relationships are too qualitative to be convincing. This study used the artificial neural network (ANN) approach to better understand the primary factors that control lacustrine shale lithofacies development and how paleoenvironments influence depositional processes. The neural network was trained with the back propagation training algorithm based on identified shale lithofacies (desired output) and geochemical indicators (input). The lithofacies deposited under different paleoenvironments were identified from an 80-m long core based on observations of the sediment texture and structure and biota and analyses of the mineralogy and trace elements. To confirm the primary factors of lithofacies development in J1d of the Upper Yangtze Platform, five groups of elements were selected as geochemical indicators (Sr/Ba, Zr/Rb, Sr/V, V/V + Ni, and Ba/Ca) to monitor the changes in paleosalinity, paleohydrodynamic condition, paleoclimate, paleoredox, and paleoproductivity, respectively. The element groups were selected and parameterized as inputs to constrain the lithofacies (output) boundaries using artificial neural networks (ANNs). The results of the ANN models indicate that the lacustrine lithofacies development of J1d was controlled by paleosalinity, paleohydrodynamic condition, paleoclimate, and paleoredox but not by paleoproductivity. The results of this study indicate that the primary factors of shale lithofacies development can be effectively assessed via geochemical, sedimentological, and ANN analyses.

Quantitative Middle Eocene benthic foraminiferal biofacies from west-central Sinai, Egypt: Implications to paleobathymetry and sequence stratigraphy

Abstract The basinal depositional architecture, paleobathymetry and paleoproductivity assessment of west central Sinai (Egypt) lacks robust sequence stratigraphic framework inferred in conjunction with changes in benthic foraminiferal biofacies. Sinai is an economically important region for hydrocarbon exploration, hence understanding its depositional architecture is fundamental. For this, the planktic foraminifera biostratigraphy is done first to date the strata exposed at Wadi Nukhul, and then, the quantitative benthic foraminiferal biofacies are evaluated to infer changes in paleobathymetry and paleoproductivity. Five Eocene planktic biozones identified (E7b to E11), spanning 48.6 to 40.4 Ma are identified along with five 3rd order depositional sequences bounded by four sequence boundaries. Quantitative (Rotated factor analysis) benthic foraminiferal analysis revealed seven benthic foraminifera biofacies (A to F) suggesting a meso- to eutrophic, outer neritic, moderate to low oxygenated bottom waters, dominated largely by benthic foraminifera taxa, Bulimina, Uvigerina, Bolivina and Lenticulina. Exception to this trend is the presence of biofacies B (Discorbis ciperensis-Lenticulina cuvillieri) at the top of the Thebes Formation that reflects well‑oxygenated bottom waters straddling the middle to outer neritic depth boundary. The inferred relative sea level gradually decreases throughout the studied section (from bottom to top) and is consistent with a gradual global decline. Of interest is the possible identification of the Middle Eocene Climatic Optimum (~40.5–40 Ma), a global warming event that in the present study, encompasses the Uvigerina mediterranea-Spiroloculina esnaensis biofacies (Biofacies A) suggesting the presence of mesotrophic but moderately well‑oxygenated bottom waters at outer neritic depths. The base of the Thebes Formation is marked by the presence of an Oxygen Minimum Zone.

Tracing paleoredox conditions across the Devonian–Carboniferous boundary event: A case study from carbonate-dominated settings of Belgium, the Czech Republic, and northern France

The Devonian–Carboniferous boundary was coeval with the Hangenberg Crisis, which is regarded as a first-order mass extinction event related with large sea-level changes and widespread anoxia. This study aims to trace the geochemical paleoproductivity and paleoredox proxies across the Devonian–Carboniferous carbonate-dominated successions of the Namur-Dinant Basin (Belgium, northern France) and the Moravo-Silesian Basin (Czech Republic), located in the Variscan Rhenohercynian Zone. The research was focused on the distribution of oxides (SiO2, Al2O3, Fe2O3, CaO, Na2O, K2O, TiO2, MnO) and trace elements (Th, U, V, Zr, Mo, Zn, Pb, Ni, Cu) measured by ICP-OES and ICP-MS, respectively. The enrichment factors of Mo and U and their ratios were used as the main paleoredox proxies, whereas the enrichment factors of Zn, Cu, Pb, and Ni were employed to trace the paleoproductivity changes. High values of Zr/Al2O3 (>0.001), TiO2/Al2O3 (>0.06), and Fe/Ti (>20) and low values of Al/(Al + Fe + Mn) (<0.35) coinciding with an increase of paleoredox proxies suggest a possible volcanic and hydrothermal source of nutrients, related to Late Devonian and Early Carboniferous extensional magmatism in the Rhenohercynian domain, although a relationship between higher paleoproductivity and enhanced continental runoff of volcanic material cannot be excluded. The studied Devonian–Carboniferous boundary sections from the Namur-Dinant Basin (Gendron-Celles, Ardennes quarry) reveal three levels with similar vectors of Mo and U enrichments. The pre-crisis Famennian pattern is typical for weakly restricted basins with Fe-Mn redox cycling accompanying vertical fluctuations of the oxic/anoxic chemocline close to the sediment/water interface with the influence of particulate shuttle. The MoEF and UEF patterns corresponding to the unrestricted or weakly restricted basin with alternating suboxic to anoxic conditions are considered to represent the transgressive Hangenberg Black Shale Event. The lower Tournaisian MoEF and UEF patterns fall along a vector in the direction of the strongly restricted marine conditions with prevailing suboxic conditions. In the Moravo-Silesian Basin (Lesní lom section), the pre-crisis Famennian MoEF and UEF vector indicates an unrestricted marine trend which converges with that of restricted systems. Redox conditions range from suboxic to euxinic. In the Tournaisian, MoEF and UEF oscillate between oxic and anoxic conditions and fall along a vector of strongly restricted marine conditions. Increased isolation of both studied basins in the Early Tournaisian is regarded as a reflection of a significant eustatic fall during the Hangenberg Crisis, linked to a severe episode of the Gondwana glaciation. These paleohydrographic patterns have correlative potential that should be considered for a revised Devonian-Carboniferous Boundary definition.

Continuous accumulation of organic matter-rich sediments associated with Oceanic Anoxic Event 1a in the El Pujal section, Organyà Basin, Catalunyà Spain and its relation to episodic dysoxia

A detailed lithostratigraphic and geochemical analysis was performed on a 13.77 m sequence consisting of a succession of limestones, argillaceous limestones and marlstones at the El Pujal section in the Organyà Basin of the south-central Spanish Pyrenees. Biostratigraphic analysis reveals the presence of the foraminifer Leupoldina cabri positioning the sequence within OAE 1a. Corroborating evidence from stable carbon isotope data permits the correlation of the El Pujal section with other Tethyan sections, namely Roter Sattel (Switzerland), Cismon (Italy) and El Pui (Spain) to within carbon isotope segment C5. Geochemical results reveal periodic increase of redox sensitive trace elements (RSTEs: V, Ni, Co, Cr, Cu, Mo), biolimiting (P, Fe) and terrestrially derived elements (Al, Si, Ti) at argillaceous limestone and marlstone intervals, concurrent with episodes of enhanced organic matter preservation (TOC >1.18%). Further evidence from microfacies characterized by higher pyrite concentration, impoverished benthic fauna and lower degree of bioturbation are in accord with geochemical proxies. Biomarker data shows n-alkanes distributions ranging from nC10 to nC31, suggesting that the organic matter (OM) is a mixture of in-situ produced and terrestrially derived. Calculated average chain length (ACL nC25-nC33) shows lower values in intervals associated with argillaceous limestones and marlstones, suggesting wetter climate during these periods. The results imply that fluvial influences controlled by regional hydrologic variability related to climatic fluctuations probably exacerbated the already sluggish basinal conditions through the development of density stratification in the water column. Integration of the data from the El Pujal section shows that changes in paleoproductivity, terrigenous input, and redox conditions characterize the deposition of OAE 1a in the Organyà Basin without the hallmark of an anoxic phase. Conditions remained similar to those recorded in the preceding sequence at El Pui, thus revealing continuous deposition of OM under impoverished oxygen conditions, punctuated by episodes of dysoxia.

Meridional Contrasts in Productivity Changes Driven by the Opening of Drake Passage

AbstractChanges in atmospheric pCO2 are widely suggested to have played a major role in both the long‐term deterioration of Cenozoic climate and many superimposed rapid climate perturbations such as the pivotal Eocene‐Oligocene transition. Changes in marine productivity affecting the biological oceanic carbon pump represent one possible cause of past CO2 variability. Here we explore the relationship between ocean gateway change and marine biogeochemistry. Specifically, we use a fully coupled atmosphere‐ocean‐biogeochemical model (IPSL‐CM5A) to examine global ocean paleoproductivity changes in response to the opening of Drake Passage. In our simulations, we find that Drake Passage opening yields a spatially uniform decrease in primary productivity in the low‐latitude oceans while the high‐latitude response is more spatially heterogeneous. Mechanistically, the low‐latitude productivity decrease is a consequence of a fundamental reorganization of ocean circulation when Drake Passage opens driven by the isolation of the Southern Ocean from low‐latitude water masses. Nutrient depletion in the low latitudes is driven by a marked decrease in the intensity of deep convection in the Southern Ocean, which drives the accumulation of nutrients at depth and their depletion in the intermediate and upper ocean, especially away from sites of subduction. In the high latitudes, the onset of the Antarctic Circumpolar Current in the model exerts a strong control both on nutrient availability and on regions of deep‐water formation. The qualitative agreement between geographically diverse long‐term paleoproductivity records and the simulated variations suggests that Drake Passage opening may contribute to the long‐term paleoproductivity signal.

Testing the applicability of a benthic foraminiferal-based transfer function for the reconstruction of paleowater depth changes in Rhodes (Greece) during the early Pleistocene.

We present paleo-water depth reconstructions for the Pefka E section deposited on the island of Rhodes (Greece) during the early Pleistocene. For these reconstructions, a transfer function (TF) using modern benthic foraminifera surface samples from the Adriatic and Western Mediterranean Seas has been developed. The TF model gives an overall predictive accuracy of ~50 m over a water depth range of ~1200 m. Two separate TF models for shallower and deeper water depth ranges indicate a good predictive accuracy of 9 m for shallower water depths (0–200 m) but far less accuracy of 130 m for deeper water depths (200–1200 m) due to uneven sampling along the water depth gradient. To test the robustness of the TF, we randomly selected modern samples to develop random TFs, showing that the model is robust for water depths between 20 and 850 m while greater water depths are underestimated. We applied the TF to the Pefka E fossil data set. The goodness-of-fit statistics showed that most fossil samples have a poor to extremely poor fit to water depth. We interpret this as a consequence of a lack of modern analogues for the fossil samples and removed all samples with extremely poor fit. To test the robustness and significance of the reconstructions, we compared them to reconstructions from an alternative TF model based on the modern analogue technique and applied the randomization TF test. We found our estimates to be robust and significant at the 95% confidence level, but we also observed that our estimates are strongly overprinted by orbital, precession-driven changes in paleo-productivity and corrected our estimates by filtering out the precession-related component. We compared our corrected record to reconstructions based on a modified plankton/benthos (P/B) ratio, excluding infaunal species, and to stable oxygen isotope data from the same section, as well as to paleo-water depth estimates for the Lindos Bay Formation of other sediment sections of Rhodes. These comparisons indicate that our orbital-corrected reconstructions are reasonable and reflect major tectonic movements of Rhodes during the early Pleistocene.

Paleoceanographic changes through the last 130 ka in the western South Atlantic based on planktonic Foraminifera

This work reconstructs Late Quaternary paleoceanographic changes in the western South Atlantic Ocean based on sedimentary core GL-77, recovered from the lower continental slope in the Campos basin, off shore SE Brazil. The studied interval comprises the last 130 ka. Changes in sea surface temperature (SST) and paleoproductivity were estimated using the total planktonic foraminiferal fauna and oxygen isotope analyses. The age model was based on the oxygen isotope record, biostratigraphic datums and AMS 14C dating. It was observed that the Pleistocene/Holocene transition occurs within Globorotalia menardii Biozone Y, and is not coeval with the base of Biozone Z. The range between summer and winter SST estimates is larger during the glacial period compared to interglacials. Three peaks of low SST around 70, 50-45 and 20 ka coincided with periods of enhanced SE trade winds. Despite faunal diff erences between the last interglacial (MIS 5e) and the Holocene, our SST estimates suggest that SSTs did not diff er signifi cantly between these intervals.