Paleoproductivity Proxies Research Articles

Influence of the sedimentary environment of the Wufeng-Longmaxi shale on organic matter accumulation in the Dingshan area, Sichuan Basin

The sedimentary environment and organic matter (OM) accumulation are vital indicators for shale gas exploration. However, research on deep shale gas systems is relatively limited; moreover, the exploration of deep shale gas in the southeastern Sichuan Basin has entered a period of stagnation. In this study, systematic geochemical analysis of Wufeng (WF) and the first member of the Longmaxi (Long-1) deep shale samples from the recently drilled DY7 well in the Dingshan area of the Sichuan Basin is carried out, and the longitudinal variations in major and trace elements are revealed. The differences in the WF, lower section of the Long-1 (Long-11) and upper section of the Long-1 (Long-12) shales are studied in terms of redox conditions, paleoproductivity, terrigenous detrital input, sedimentation rate and paleoclimate, and the different main controlling factors of OM accumulation for these three layers are discussed. The WF shale has a higher TOC content (mean: 5.73%), the Long-11 shale has a high TOC content (mean: 2.89%), while the Long-12 shale has a low TOC content (mean: 1.44%). For the WF shale, due to complex geological events and large fluctuations in element contents, its TOC content is poorly correlated with these indices, redox and paleoproductivity proxies have a positive association with the Long-11 shale’s TOC content, but negatively correlated with terrigenous input and sedimentation rate indices. The formation of these two sets of organic-rich shales (TOC > 2%) is jointly controlled by good preservation conditions. In contrast, the TOC content of the WF shale is higher than that of the Long-11 shale as the result that terrigenous input and sedimentation rate of the Long-11 shale represent the dilution and destruction of OM, which is different from the former. During the Long-12 depositional period, the water column experienced weak reducing conditions and low productivity, and its high terrigenous debris input further diluted the OM, leading to a low TOC content.

Elemental Geochemical Characteristics of Shales from Wufeng-Longmaxi Formations in the Southern Margin of the Qinling Orogenic Belt, China: Implications for Depositional Controls on Organic Matter.

The tectonic background and sedimentary environment during the transition period from the Ordovician to Silurian have been widely studied by many scholars. This study focuses on the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Bajiaokou profile at the southern margin of the Qinling Orogenic Belt in southern China. In order to study the aggregation mechanism of organic matter, geochemical proxies were proposed, including redox proxies (V, V/Al, U, U/Al, Mo, and Mo/Al), paleoproductivity proxies (P, P/Ti, Ba, Ba/Al, and SiXS), paleoclimate proxies (CIA), and terrigenous flux proxies (Al, Zr, and Zr/Al). In addition, Al-Co[EF] × Mn[EF] is used to provide information on paleoenvironmental parameters such as watermass restriction conditions. The redox proxies show that the Wufeng-Longmaxi shale is mainly accumulated under oxic-dysoxic conditions. During the shale deposition period of Wufeng-Longmaxi formations, the marine surface primary productivity in the southern Qinling area is generally low to moderate. The paleoclimate proxies show that from the Late Ordovician to the Early Silurian the southern Qinling area generally had a warm and humid climate. The upwelling current is widely developed in the northern margin of the Sichuan Basin and the southern margin of the Qinling area. Although the upwelling current was highly developed during the deposition of the Wufeng Formation in the Bajiaokou profile, the concentrated accumulation of a large amount of volcanic ash resulted in the low primary productivity of the ocean. During the sedimentary period of the Longmaxi Formation in the Bajiaokou profile, the development of seasonal upwelling currents and a small amount of volcanic ash supply increased the primary productivity to moderate, which provided a good material basis for the enrichment of organic matter, but the high detritus flux and the water body condition of oxic-dysoxic resulted in the slight enrichment of organic matter.

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

The uppermost Cambrian succession (Stage 10) of the Wa'ergang section (∼300 m of rhythmites, South China) records three globally correlated negative δ13Ccarb excursions (N1, N2, and the HERB- Helenmaria – Red Tops Boundary), overlying the SPICE event (Steptoean Positive Carbon Isotope Excursion), indicating perturbations in the global carbon cycle. The trigger and process of these events still remain elusive. The petrographic and chemical screening of the investigated carbonates proves well preservation of sedimentary fabrics and at least near-primary geochemical signatures. The paleoredox proxy (UEF/TOC, VEF/TOC, and MoEF/TOC) and δ15Norg profiles exhibit positive excursions that correlate with the negative δ13Ccarb excursions, indicating pulses of expanded oceanic anoxia potentially resulted from anoxic deep-water upwelling. The correlated negative shifts in TOC and paleoproductivity proxy (NiEF and CuEF) profiles suggest suppressed primary productivity, possibly by upwelling-induced oceanic toxic events, aligning with mass extinctions particularly during the HERB event. Independently, the paired δ13Ccarb and δ13Corg (Δ13C) variations reveal decreased biological fractionation that aligns with the δ13Ccarb excursions, implying pulses of declining atmospheric O2 levels, which are consistent with the suggested expanded oceanic anoxia and suppressed primary productivity. The peaks of the negative δ13Ccarb excursions are correlated with the base of upward-shallowing sequences, consistent with the evidence from TOC, δ13Corg, δ15Norg, and trace element proxies, support a scenario of upwelling deep toxic waters during sea-level rise that likely resulted in expanded oceanic anoxia, and ultimately led to the negative excursions in δ13Ccarb values and mass extinction.

Pliocene Weakening of Gradients in Temperature but Not in Productivity in the Eastern Equatorial Pacific

AbstractThe modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80° and 140°W. During the globally warmer late Pliocene (3.0–3.6 Ma), the EEP may have experienced permanent El Niño‐like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east‐west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone‐based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east‐west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital‐scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90° and 120°W was reduced from the modern 1.8°C gradient to 0.9°C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X‐ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Niño SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient‐rich subsurface waters in a warmer climate.

Aptian-Albian paleoenvironmental geochemistry: Araripe Basin, Northeastern Brazil

The geochemical record of the early Aptian and the transition to the Albian exposes the presence of Hg/TOC anomalies, revealing the interference of LIPs in paleoenvironmental changes in the Araripe basin. Redox-sensitive (VEF, FeEF, CrEF, MnEF, PbEF, V/Cr, V/V + Ni and V/Ni) and paleoproductivity (CuEF, ZnEF, NiEF and BaEF) proxies in the Santana Group (Barbalha, Crato and Romualdo formations) indicate conditions ranging from oxic to anoxic, and increased paleoproductivity and of trace elements concentrations during LIPs activation. Statistical analyses demonstrated that volcanism was associated with oscillations in paleosalinity (Sr/Ba) and detrital supply (Al). LIPs activity was responsible for the emission of CO2 into the atmosphere, generating paleoclimatic changes in temperature (δ18O), a crisis in the productivity of calcifying organisms (δ13Ccarbonate), and an increase in the load of organic matter (TOC). We suggest that trace elements anomalies found in the early Aptian (Barbalha Formation) are associated with Ontong Java Plateua (OJP), responsible for OAE 1a, while the volcanism's proxies found in the Romualdo Formation occurred due to the Kerguelen South Plateau (SKP) volcanism, triggering the OAE 1 b.

Surface water mass dynamics at IODP Site U1313 through Principal Component Analysis: Evidence from coccolith assemblages in the ∼25–7 kyr interval

Coccolith assemblages from Integrated Ocean Drilling Program Site U1313 (North Atlantic; 41°0′N, 32°57.4′W) are analysed to assess temporal changes in surface water conditions from the Last Glacial Maximum (LGM) to the Middle Holocene (∼25–7 kyr). We perform Principal Component Analysis (PCA), involving both calcareous nannofossil assemblage data and other paleoenvironmental proxies. We reconstruct sea-surface dynamics using calcareous nannofossil assemblage variations and comparison with other previously published proxy data from Site U1313 including C37:4 alkenone, dolomite/calcite and quartz/calcite ratios, Uk’37 SST, as well as the sedimentation rate.We use variations in paleoproductivity proxies, cold and warm water group abundance, and the PCA results to infer North Atlantic Current (NAC) and North Atlantic Transitional Waters (NATW) migration through the studied interval. These proxies are also used to highlight variations in the Subtropical Gyre dynamics. Warming episodes are recorded during the LGM and Heinrich Stadial (HS) 1 as a result of northward expansion of the Subtropical Gyre. High coccolithophore productivity episodes characterize HS 1 and 2, and are concomitant with increasing terrigenous input. Both processes are considered a consequence of iceberg influence, which transport nutrients and ice-rafted debris to the area. A southward shift of the Subpolar Front and NAC favour the expansion of cold water from 25.3 ka to the end of the Younger Dryas, whereas the early Holocene is characterized by NATW influence resulting from northward migration of the NAC and Azores Current.

Probing surface Earth reactive silica cycling using stable Si isotopes: Mass balance, fluxes, and deep time implications.

Geological reservoir δ30Si values have increasingly been applied as paleoclimate and paleoproductivity proxies. Many of these applications rely on the assumption that the surface Earth Si isotope budget is in mass balance with bulk silicate Earth, such that trends in δ30Si over time can be attributed to changes in flux to or from key silica reservoirs. We compiled δ30Si data from modern reservoirs representing the major sources and sinks of surface Earth reactive silica, to which we applied an inverse model to test assumptions about mass balance. We found that δ30Si values of reverse weathering products must closely match those of diatoms, conflicting with previous assumptions. Model results also revealed that of the 19 to 21 teramoles per year Si released during silicate mineral weathering, ~10 to 18 teramoles per year is stored in terrestrial silica sinks, consistent with assumptions of incongruent weathering reactions. Our results demonstrated that the modern silica cycle summary is in isotopic mass balance.

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.

Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution

High resolution stable isotopic (C and O) and geochemical studies of Paleoproterozoic carbonates were carried out in the Umra sub-basin of the Aravalli Supergroup, Rajasthan, India. All the sections, Umra-Dedkiya (UM), Umra-Jampa (BD), and Matoon (MU and GU), have yielded characteristic Lomagundi type positive values, with δ13Ccarb > 4 ‰ VPDB (maximum up to 6.86 ‰). Samples of two sections (UM and BD) have also yielded several distinct negative δ13Ccarb values (up to −6.14 ‰ VPDB). Geochemical validation of trace element and REE data indicate both these types of excursions to be of pristine character, with no signature of organic-remineralisation or hypersaline depositional condition. Mineralogical assemblages lack hypersalinity indicator minerals, such as Na-tourmaline, Na-Albite or scapolite, in the positive δ13C bearing carbonate samples. This supports the inference regarding the observed positive δ13C excursions mimicking the global Paleoproterozoic Lomagundi event. Chemostratigraphic correlation within the basin and with one of the global strato-type Paleoproterozoic carbonate unit from Paso Severino Formation, Uruguay, indicate interruptions in the initial part of the Lomagundi episode marked by several negative δ13C excursion events before achieving a stable positive δ13C character. This phenomenon is also evident from variation in paleoproductivity proxies, such as TOC, TN, Ba and P values, consistent with δ13C fluctuations. Mathematical model calculations have corroborated such a conclusion. All these observations motivated us to present a modified version of the global Paleoproterozoic δ13Ccarb curve and a six stage basin evolution model to explain the evolution of δ13C vis-à-vis variation in atmospheric oxygen.

Diagenetic effect on barium isotope compositions of barites in the lower Cambrian successions

The pelagic barite precipitation in the seawater column is tightly associated with the biological pump, and thus the barite Ba isotope composition (δ138/134Ba) has been advocated as a novel paleo-productivity proxy. However, pelagic barites in sediments are susceptible to diagenetic processes that may modify primary δ138/134Ba signals and hamper the application. To constrain the diagenetic effect, we report δ138/134Ba data of diagenetic barites from the black shales of the lower Cambrian Yurtus Formation in the Aksu area of the Tarim Basin. The δ138/134Ba values vary within a large range from −0.15‰ to +0.49‰ and show a generally positive correlation with the 87Sr/86Sr ratios, indicating the mixing of two end-members. The samples with 87Sr/86Sr ratios close to the coeval seawater value record relatively low δ138/134Ba values from −0.15‰ to +0.14‰, which are inferred to inherit from the primary barites; whereas the samples with 87Sr/86Sr ratios exceeding the coeval seawater value record higher δ138/134Ba values from −0.05‰ to +0.49‰, which may have resulted from the alteration by the host black shales during early diageneses. Based on these investigations, we propose that pore waters below the sulfate-methane transition zone may provide an additional source of Ba (enriched in heavy isotopes) for the formation of diagenetic barites. We suggest to combine barite 87Sr/86Sr ratio with δ138/134Ba value to evaluate diagenetic influence on primary Ba isotope signatures especially for an over-determined system, which is important for applying Ba isotope compositions of barites to understand paleo-oceanic evolution.

Geochemical approach for decoding the paleoenvironmental and depositional evolution of a coastal lacustrine Konservat-Lagerstätte (Early Cretaceous, south-Central Pyrenees)

The lithographic limestone of La Pedrera de Meià (LPM) in south-Central Pyrenees (NE Spain) is considered one of the best preserved lacustrine-coastal successions of the Early Cretaceous in Europe, hosting a taxonomically diverse record of Barremian biota. While this Konservat-Lagerstätte has been extensively surveyed for paleontological purposes, little is known about the paleoenvironmental and depositional conditions prevailing during the formation of this fossil lagerstätte deposit. LPM limestone is made up of a homogeneous, dark gray to pale brown, faintly laminated 50 m-thick succession of micrite-rich intervals. To shed light on the environmental conditions of deposition, 303 rock samples were studied using a multiproxy approach based on different techniques including petrography, SEM-EDX, XRD, ICP-MS, XRF, Rare Earth Element distributions and C and O stable isotope geochemistry. The combination of sedimentological and geochemical data reveals a vertical shift of facies from littoral to profundal environments and then returning to littoral conditions. Profundal facies are made up of filament-rich mudstones originated from in situ precipitation of micrite either through biotically mediated or physico-chemical processes in undisturbed low energy settings, under anoxic conditions, characterized by the presence of transported terrestrial and lacustrine organisms that are exquisitely well-preserved. Littoral areas are characterized by the presence of in situ charophytes, smooth ostracods, and miliolid foraminifera that accumulated in mud-rich facies indicating low/moderate-energy, shallow-water, and relatively oxygenated conditions. Stable isotopic data and elemental geochemistry suggest a hydrologically closed to semi-closed lacustrine system with a restricted water circulation encouraging dysoxic to anoxic conditions in profundal sediments. Paleoproductivity proxies suggest a good correlation between catchment-derived siliciclastic discharge to the lake and primary productivity peaks. Furthermore, a decline in terrigenous input coupled with a reduction of meteoric water entrance to the lake is interpreted based on elemental proxies derived from paleosalinity and paleoredox analysis. Collectively, the strontium/barium (Sr/Ba) ratios, the sparse presence of miliolid foraminifera, and the isotopic signatures indicate an increase in salinity toward the younger intervals of the lake infilling, although their ultimate causes are unclear (i.e., increased evaporation, or a combination of reduced meteoric input and sporadic marine inputs might explain our data). The present work shows the importance of performing detailed multiproxy geochemical-sedimentological studies to better constrain the unique paleoenvironmental and paleohydrological conditions associated with the exceptional preservation of fossil biotas in Konservat-Lagerstätte deposits in analogous depositional settings.

The Environment Effect on Differential Organic Matter Enrichment of the Lower Cambrian Shale in the Tarim Basin, NW China

AbstractThe early Cambrian shale is of an important petroleum source rock on the Earth. Due to few drilling data and poor seismic data, the organic matter enrichment of the Lower Cambrian shale is still an enigma in the Tarim Basin. Total organic carbon (TOC), major and trace element data of shale samples from five boreholes were experimented to decipher the mechanism of the organic matter enrichment in the early Cambrian shales. The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents (3.2%–19.8%, on average 11.0%) than those from the eastern basin (2.2%–10.2%, on average 4.5%). The paleoproductivity proxies (Ba, Ba/Al, P/Al) in the western restricted platform are much higher than those in the eastern basin. The trace element indicators such as V/Cr, Ni/Co, Mo‐TOC and MoEF‐UEF suggest an anoxic environment across the basin, but a more restricted environment in the western intraplatform. It is concluded that the paleoproductivity rather than anoxic condition and hydrothermal activity have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales, and restricted environment is favorable for paleoproductivity and preservation of organic matter.

EA SURFACE PALEOPRODUCTIVITY RECONSTRUCTION BASED ON FORAMINIFERAL ACCUMULATION RATE IN THE WESTERN SAVU STRAIT SINCE THE LAST GLACIAL MAXIMUM (~23 KA BP)

Despite its importance, sea surface paleoproductivity of the western Savu Strait is not well studied. Results from previous studies at the nearby Southwest Sumba and Sumba Strait might not be applicable due to the oceanographic difference. Foraminiferal proxies from gravity core ST10 were applied to generate sea surface paleoproductivity and thermocline depth reconstruction. Foraminiferal Accumulation Rate and Benthic Foraminiferal Accumulation Rate were used as paleoproductivity proxies while the thermocline dwellers’ relative abundance was applied as the thermocline depth proxy. This study suggested paleoproductivity increase during the Last Glacial Maximum (LGM)–~16 ka BP and Holocene (after ~11.65 ka BP) in the western Savu Strait. Thermocline depth was relatively shallower during the LGM–Last Deglaciation and became deeper afterwards. Paleoproductivity increase at LGM–~16 ka BP was caused by the Australian-Indonesian winter monsoon (AIWM)-like condition, characterized by intense coastal upwelling while the Holocene paleoproductivity increase was related to the abrupt rainfall increase which enhanced terrestrial/riverine input. Thermocline depth variability in the western Savu Strait is in-phase with thermocline depth variability in the Java upwelling region, characterized by a shallower thermocline during the LGM–Last Deglaciation (before ~11.65 ka BP) and a deeper thermocline during the Holocene (after ~11.65 ka BP). This thermocline depth shifting indicates a strong Australian-Indonesian Monsoon (AIM) influence on the paleoceanography of the western Savu Strait since LGM.

Sedimentary paleoenvironment and organic matter accumulation model of the Lower Silurian Gaojiabian Formation shales in the Lower Yangtze region, South China

A set of organic-rich shales developed in the lower part of the Lower Silurian Gaojiabian Formation in the Lower Yangtze region, South China. The paleoenvironment and the organic matter (OM) accumulation mechanism in the Lower Yangtze region are rarely studied, restricting the in-depth understanding of the shale gas exploration prospects of this region. This study described the petrological, mineralogical characteristics and geochemical properties of the organic-rich shales of the Gaojiabian Formation in the Lower Yangtze region (Well TD2) using multidisciplinary approaches. The purpose is to study the palaeoenvironment evolution of organic rich shales, and reveal the controlling factors and accumulation mechanism of OM. Two shale sections with high TOC content developing in the lower part of the Gaojiabian Formation were identified through petrological observations and geochemical analysis, namely the lower shale section (LSS) and the upper shale section (USS) with TOC content of 0.79–4.62% and 0.65–2.92%, respectively. The two organic-rich shale sections were formed in a suboxic-anoxic environment. The Cd/Mo, Co × Mn, Mo/TOC values show that the USS and the LSS were formed in a strongly restricted water environment and were not affected by upwelling, and the environment of the USS was more restricted than that of the LSS. The paleoproductivity proxies indicate the two sections have a high-productivity background, and that the LSS has a higher paleoproductivity level than the USS. The terrigenous input shows a long-term growth trend. Compared with the LSS, the USS had more terrigenous input and was closer to the source area. The chemical index of alteration (CIA) indicates that the shale was formed in a warm and humid climate. The index of compositional variability (ICV) shows that the USS has a more mature composition than the LSS and was likely dominated by recycling. The relationships between TOC content and several geochemical proxies indicate that the OM accumulation was mainly controlled by redox condition, paleoproductivity and terrigenous input, whereas these three factors are determined by the combined impact of the sea level and regional tectonic movement. On this basis, this study reconstructed the paleoenvironment and the OM accumulation model. The OM accumulation of the LSS was mainly controlled by relatively high regional sea level affected by global transgression, the anoxic-suboxic water environment, the high paleoproductivity, and a small quantity of terrigenous input, while the OM accumulation of the USS was primarily controlled by the suboxic condition affected by decreased relative sea level resulting from the strengthened regional tectonic movement, the relatively weakened paleoproductivity, and the increased recycled terrigenous input. The above understanding provides important enlightenment for deepening the study of OM enrichment and shale gas exploration direction of the Gaojiabian Formation in the Lower Yangtze region.

Paleoenvironment-driven organic matter accumulation in lacustrine shale mixed with shell bioclasts: A case study from the Jurassic Da'anzhai member, Sichuan Basin (China)

The sedimentary environment governs the depositional processes, ecological environments, and hydrodynamics, which affect the accumulation of organic-rich sediments. Some particular issues are unsolved about the organic matter accumulations of lacustrine shale mixed with shell bioclasts due to their alternating deposition. Freshwater bivalve remains are a familiar constituent of the Da'anzhai lacustrine calcareous shale, which indicates enhanced activities of benthic organisms. Under this background, favorable environmental conditions for the deposition and preservation of abundant organic matter may be different from classical models. Total organic carbon and elemental concentration analyses of lacustrine calcareous shale samples from the Jurassic Da'anzhai Member in the Sichuan Basin were carried out to reconstruct the paleoenvironment and reveal the organic enrichment mechanism. Results show that the samples are notably enriched in strontium, phosphorus and biogenic calcium (indicated by excess calcium concentrations) and reveal the mass death event of benthic organisms (including freshwater mussels and gastropods) in paleolakes under the control of climatic transformation. Such enrichment strongly supports the hypothesis that CaO is considered to be a paleoproductivity proxy to some extent. The paleolake was dominated by a warm and humid climate and rapid sedimentation rates and experienced intense chemical weathering, which are characteristic of freshwater input and a high flux of detrital fractions. The variations in the redox state and paleoproductivity are due to climatic shifts, hydrographic restrictions and biological behavior. Furthermore, the organic matter was enriched during both oxygenated and oxygen-deficient conditions. The combination of high sedimentation rates and high sinking influxes of particulate organic carbon reduced the chances of decomposition and thereby facilitated the efficiency of organic matter accumulation in an oxic environment. The low degree of organic matter degradation, moderate sedimentation rates and enhanced phosphorus recycling were responsible for the organic carbon accumulation and preservation in sediments for dysoxic bottom conditions.

Characterizing Ag uptake and storage in the marine diatom Thalassiosira pseudonana: Implications for Ag biogeochemical cycling

Silver is a non-nutrient element that is readily taken up by diatoms. It exists in the ocean in pM concentrations and is known to accumulate in marine sediments underlying surface waters that support diatom blooms. Laboratory studies and open-ocean water column profiles of Ag versus Si have led to the hypothesis that diatoms accumulate Ag within their silica frustules, subsequently delivering Ag to underlying sediments as the organisms die and sink to the seafloor. Through this delivery mechanism, sedimentary Ag concentrations might serve as a useful record of past export production. However, the actual partitioning of Ag between diatom organic and frustule parts is unknown, and as such represents a considerable source of uncertainty in the development of Ag as a proxy for diatom paleoproductivity and in fully describing Ag biogeochemical cycling. In this study, we use a synchrotron XRF microprobe to map the location of Ag within the laboratory-grown diatom Thalassiosira pseudonana and show that diatoms primarily store Ag in their soft tissue, not in their frustules. Intracellular Ag appears to be concentrated in vacuoles, although the majority of Ag is widely distributed and may be associated with the cell membrane and/or cytoplasm. These results imply an alternate mechanism for Ag delivery to sediments whereby Ag is associated with decomposing organic particles rather than skeletal remains. Silver therefore continues to show promise as a qualitative paleoproductivity proxy. Quantitative use is unlikely, however, given variable uptake rates by diatoms in response to environmental conditions and the significant potential for remineralization with organic matter in the water column.

The genetic relationship between paleoenvironment, mineral compositions and lithofacies in the Ordovician–Silurian Wufeng–Longmaxi sedimentary succession in the Sichuan Basin, SW China

• Five intervals (I–V) showing changes in minerals, lithofacies and paleoenvironment were identified. • High productivity and low detrital influx lead to the enrichment of nondetrital quartz in siliceous shale in Intervals I-II. • The enrichment of carbonate in calcareous shale in Interval III was due to high detrital influx. • High detrital influx and strong chemical weathering lead to an abundance of silt and clay minerals in silty–argillaceous shale and argillaceous shale in Intervals IV-V. Changes of sedimentary environments recorded in the Late Ordovician–Early Silurian sedimentary succession can be reflected in mineral composition and lithofacies of shales. Outcrops and cores from this set of shales in the Sichuan Basin, China, are analyzed. The results show that quartz, clay minerals, and carbonate are the main minerals. Five major lithofacies are developed. Based on the provenance proxies (Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc ratios), chemical index of alteration (CIA), paleoproductivity proxies (Cu/Al and Ni/Al ratios), redox condition proxies (U/Th, V/Cr, and Ni/Co ratios), proxies of detrital influx (Ti and Al contents), and sedimentation rate (SR), the paleoenvironment is analyzed. The source rocks are dominated by felsic igneous rock. Comparison between five lithofacies revealed that the relatively low degree of chemical weathering, anoxic conditions, maximum paleoproductivity probably associated with the high frequency of volcanic activities, and minimum detrital influx and SR (0.60–5 m/Ma) led to the enrichment of nondetrital quartz and organic matter, which eventually formed siliceous shale. The moderate degree of chemical weathering, dysoxic–oxic conditions, decreasing paleoproductivity, and increasing detrital influx and SR (10–16.67 m/Ma) resulted in the increase in detrital quartz, clay minerals, and carbonate content, and the development of mixed shale and calcareous shale. Relatively strong chemical weathering, oxic conditions, minimum paleoproductivity, and maximum detrital influx and SR (100.74 m/Ma) favored the enrichment of silt and clay minerals, forming silty–argillaceous shale and argillaceous shale. This study provides an insight into the relationship between the environmental changes and shale mineral composition and lithofacies.

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.

Delineation of terrestrial and marine productivity in the southwestern continental margin of India

• Terrestrial/marine paleoproductivity proxies show paleosummer monsoon fluctuations. • Pollen is more reliable and coherent climate proxy than marine ones. • Summer monsoon weakened along SW India during Younger Dryas . • Monsoon intensified from early to late Holocene with a break during 6.7–4.0 ka. Although there are numerous investigations on the reconstruction of palaeomonsoon, there are inconsistencies between the terrestrial and marine archives. In this study, we have made an attempt to unravel this problem from a multi-proxy study of a sediment core from the south-west continental margin of India (SWCI). Here, we characterize pollen and dinocyst, and compare it with the marine primary productivity indicators (organic carbon and CaCO 3 ) in an AMS 14 C-dated sediment core collected off the SWCI. The sedimentation rate decreases consistently from late-glacial through early Holocene to the late Holocene (31.16 cm/ka, 24.8 cm/ka and 15.4 cm/ka respectively). The down-core distribution of pollen shows the presence of Poaceae and Cyperaceae without mangroves and other terrestrial pollen around 13.2 ka BP − 11.7 ka BP, suggesting a weakened summer monsoon during the Younger Dryas. Increase in the abundance of mangroves and tropical moist/dry deciduous forest pollen along with the dinocyst during ∼ 11.7 ka to 8.5 ka BP indicates the growth of vegetation in the hinterland as a result of the intensification of monsoon during the early Holocene. This was followed by a maximum marine transgression phase along with a strengthened summer monsoon between ∼ 8.0 ka BP and 6.7 ka BP, as evidenced by the dominance of mangroves and presence of terrestrial pollen taxa. Further, a significant decline of vegetation during 6.7 ka BP to 4.0 ka BP indicates weakened summer monsoon. In contrast to the pollen profiles, however, a significant increase in the mangroves and herbaceous pollen from 4.0 ka BP to 3.2 ka BP suggests a gradual revival of the summer monsoon during the late Holocene. Above mentioned paleoclimatic reconstructions are in contrast to the marine CaCO 3 and organic carbon, as these are lower and fairly constant from Younger Dryas upto 8.0 ka BP, then gradually increased till 6.5 ka BP, and show little variations during the late Holocene. This study suggests that the pollen preserved in the marine sediments are not only reliable indicators, but they are also consistent with other palaeomonsoon proxies measured in archives from the South Asia for the reconstruction of climate.

Sedimentary geochemistry of Late Cretaceous-Paleocene deposits at the southwestern margin of the Anambra Basin (Nigeria): Implications for paleoenvironmental reconstructions

The Campano-Maastrichtian deposits of the Nkporo and Mamu formations of the Anambra Basin represent prospective hydrocarbon resources in southern Nigeria. In order to reveal the paleoclimate, sediment recycling and paleoenvironment during the deposition of these formations in the southwestern portion of the Anambra Basin, a detailed geochemical analysis was performed using major elements, trace elements and rare earth element data, as well as the mineralogical characterization of 20 samples from two wells. The combination of geochemical ratios (Rb/Sr, Th/U, and C-value) and weathering indices (CIA, CIW, and PIA) demonstrates that the region experienced a warm, humid tropical paleoclimate throughout the Late Cretaceous. Furthermore, West Africa experienced strong precipitation during the Cretaceous, leading to intense chemical weathering in the region, as documented by the geochemically matured sediments.The paleosalinity (Sr/Ba) and paleoproductivity (Ba/Al and P/Ti) proxies show that the studied mudrocks were deposited primarily in brackish to shallow-marine settings, with poor primary productivity due to terrestrial clastic influx and hydrodynamic influence. Using multiple paleoredox indicators, deposition under oxic conditions is detected. The low authigenic U enrichment and depleted V and Cr values normalized to UCC standards further support oxic bottom water conditions. Wavy laminations observed in the mudrocks indicate retreating seawater and high hydrodynamic conditions. Ultimately, the southwestern Anambra Basin reveals Late Cretaceous-Paleocene deposits of a shallow sea controlled by a strong hydrodynamic regime.