Deposition Of Organic-rich Sediments Research Articles

Geodynamic controls on clastic-dominated base metal deposits

Abstract. To meet the growing global demand for metal resources, new ore deposit discoveries are required. However, finding new high-grade deposits, particularly those not exposed at the Earth's surface, is very challenging. Therefore, understanding the geodynamic controls on the mineralizing processes can help identify new areas for exploration. Here we focus on clastic-dominated Zn–Pb deposits, the largest global resource of zinc and lead, which formed in sedimentary basins of extensional systems. Using numerical modelling of lithospheric extension coupled with surface erosion and sedimentation, we determine the geodynamic conditions required to generate the rare spatiotemporal window where potential metal source rocks, transport pathways, and host sequences are present. We show that the largest potential metal endowment can be expected in narrow asymmetric rifts, where the mineralization window spans about 1–3 Myr in the upper ∼ 4 km of the sedimentary infill close to shore. The narrow asymmetric rift type is characterized by rift migration, a process that successively generates hyper-extended crust through sequential faulting, resulting in one wide and one narrow conjugate margin. Rift migration also leads to (1) a sufficient life span of the migration-side border fault to accommodate a thick submarine package of sediments, including coarse (permeable) continental sediments that can act as source rock; (2) rising asthenosphere beneath the thinned lithosphere and crust, resulting in elevated temperatures in these overlying sediments that are favourable for leaching metals from the source rock; (3) the deposition of organic-rich sediments that form the host rock at shallower burial depths and lower temperatures; and (4) the generation of smaller faults that cut the major basin created by the border fault and provide additional pathways for focused fluid flow from source to host rock. Wide rifts with rift migration can have similarly favourable configurations, but these occur less frequently and less potential source rock is produced, thereby limiting potential metal endowment. In simulations of narrow symmetric rifts, the conditions to form ore deposits are rarely fulfilled. Based on these insights, exploration programmes should prioritize the narrow margins formed in asymmetric rift systems, in particular regions within several tens of kilometres from the paleo-shoreline, where we predict the highest-value deposits to have formed.

Sedimentary organic matter applied to paleoenvironmental reconstruction of the Devonian of the Paraná Basin, SW Gondwana

The study of sedimentary organic matter (SOM) is essential for understanding paleoenvironmental conditions and depositional settings of geological formations. This paper explores the Devonian strata of the Paraná Basin, utilizing SOM analysis to reconstruct paleoenvironments and identify global events. The Devonian period is notable for significant geological and biological changes, including mass extinctions and extensive organic-rich sediment deposition. By examining the composition, distribution, and diagenesis of SOM in this region, we aim to elucidate the paleoenvironmental dynamics and depositional history. The Devonian outcrops in the state of Mato Grosso do Sul, Brazil rocks reveal distinct depositional characteristics, when compared to those found on the eastern border of the basin. The palynofacies analysis was conducted on 59 samples from four outcrops (MS-26, MS-29, MS-30 and MS-70), which age spans from Pragian to Givetian. The main groups and subgroups of the SOM were identified: amorphous organic matter (AOM) (“amorphous organic matter” and pseudoamorphous), phytoclasts (opaques, non-opaque-non biostructured, cuticles, membranes and Spongiophyton) and palynomorphs (spores, freshwater algae, scolecodonts, chitinozoan and acritarchs). The SOM was grouped into three palynofacies associations: opaques, land-derived and marine+AOM. In the Pragian—Emsian interval (MS-26, MS-29 and MS-30), a prevalent occurrence of phytoclasts and marine microphytoplankton is noted, with their distribution exhibiting distinct alternations in each section suggesting a deposition in the marine environment ranging from onshore to offshore. In the Givetian (MS-70 outcrop), we detected a progressive increase and diversification of marine elements (e.g., acritarchs), indicating a transgressive phase probably related with the Kačák event.

Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO2 release, marine faunal extinction, sea-level rise, a crisis in carbonate production related to ocean acidification, and elevated seawater temperatures. Despite recognition of the T-OAE as a potential analog for future ocean deoxygenation, current knowledge on the severity of global ocean anoxia is limited largely to studies of the trace element and isotopic composition of black shales, which are commonly affected by local processes. Here, we present the first carbonate-based uranium isotope (δ238U) record of the T-OAE from open marine platform limestones of the southeastern Tethys Ocean as a proxy for global seawater redox conditions. A significant negative δ238U excursion (~0.4‰) is recorded just prior to the onset of the negative carbon isotope excursion comprised within the T-OAE, followed by a long-lived recovery of δ238U values, thus confirming that the T-OAE represents a global expansion of marine anoxia. Using a Bayesian inverse isotopic mass balance model, we estimate that anoxic waters covered ~6 to 8% of the global seafloor during the peak of the T-OAE, which represents 28 to 38 times the extent of anoxia in the modern ocean. These data, combined with δ238U-based estimates of seafloor anoxic area for other CO2-driven Phanerozoic OAEs, suggest a common response of ocean anoxia to carbon release, thus improving prediction of future anthropogenically induced ocean deoxygenation.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial in the Early Silurian

The Late Ordovician and Early Silurian periods experienced glaciation and mass extinctions. However, debates exist regarding the factors that initiated the glaciation and the subsequent delay in biotic recovery. To uncover the relationships between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, temporal variations in Zinc (Zn) isotopic compositions along with other multi-geochemical proxies were analyzed for a carbonate-dominated section in South China. A stratigraphic δ66Zn trend was found in studied section, indicating a widespread disturbance to the marine Zn cycle. A significant negative shift in δ66Zn by up to ∼0.7 ‰ was observed immediately following the Hirnantian Glaciation and HICE, which can be attributed to a combination of reduced burial efficiency of organic carbon, fast weathering of large igneous provinces and increased riverine Zn input. This was followed by a ∼ 0.4 ‰ rise in δ66Zn values, alongside a sharp decrease in δ13C values, indicating an increase in sulfide precipitation (e.g., ZnS, FeS) due to expansions of euxinia. Subsequently, a notable rise in δ66Zn values by up to ∼1.1 ‰ is interpreted as an increase in organic carbon burial, given the concurrent variation of δ66Zn and δ13C during this period. The findings suggest that the increase in organic carbon burial, a reduction in reverse weathering, or a combination of them contributed to the glaciation in Silurian. Furthermore, the weathering-induced high primary productivity led to anoxic water conditions in the latest Hirnantian and Rhuddanian. This prolonged marine anoxia in seawater thus caused the deposition of organic-rich sediments and hindered biotic recovery in the Rhuddanian.

Comprehensive molecular-isotopic characterization of archaeal lipids in the Black Sea water column and underlying sediments.

The Black Sea is a permanently anoxic, marine basin serving as model system for the deposition of organic-rich sediments in a highly stratified ocean. In such systems, archaeal lipids are widely used as paleoceanographic and biogeochemical proxies; however, the diverse planktonic and benthic sources as well as their potentially distinct diagenetic fate may complicate their application. To track the flux of archaeal lipids and to constrain their sources and turnover, we quantitatively examined the distributions and stable carbon isotopic compositions (δ13 C) of intact polar lipids (IPLs) and core lipids (CLs) from the upper oxic water column into the underlying sediments, reaching deposits from the last glacial. The distribution of IPLs responded more sensitively to the geochemical zonation than the CLs, with the latter being governed by the deposition from the chemocline. The isotopic composition of archaeal lipids indicates CLs and IPLs in the deep anoxic water column have negligible influence on the sedimentary pool. Archaeol substitutes tetraether lipids as the most abundant IPL in the deep anoxic water column and the lacustrine methanic zone. Its elevated IPL/CL ratios and negative δ13 C values indicate active methane metabolism. Sedimentary CL- and IPL-crenarchaeol were exclusively derived from the water column, as indicated by non-variable δ13 C values that are identical to those in the chemocline and by the low BIT (branched isoprenoid tetraether index). By contrast, insitu production accounts on average for 22% of the sedimentary IPL-GDGT-0 (glycerol dibiphytanyl glycerol tetraether) based on isotopic mass balance using the fermentation product lactate as an endmember for the dissolved substrate pool. Despite the structural similarity, glycosidic crenarchaeol appears to be more recalcitrant in comparison to its non-cycloalkylated counterpart GDGT-0, as indicated by its consistently higher IPL/CL ratio in sediments. The higher TEX86 , CCaT, and GDGT-2/-3 values in glacial sediments could plausibly result from selective turnover of archaeal lipids and/or an archaeal ecology shift during the transition from the glacial lacustrine to the Holocene marine setting. Our in-depth molecular-isotopic examination of archaeal core and intact polar lipids provided new constraints on the sources and fate of archaeal lipids and their applicability in paleoceanographic and biogeochemical studies.

Impact of Chondrites on trace metal distribution in the sapropel S7 (ODP Site 966): Implications for paleoenvironmental and paleoceanographic reconstructions

Organic-rich deposits are valuable paleo-archives, recording significant paleoceanographic changes linked to past climate variations and marine deoxygenation events. The deposition of organic-rich sediments stops when bottom-water reventilation/oxygenation occurs. This impedes organic matter preservation, enabling macro and micro burrowing-organisms to recover and bioturbate the seafloor. In this sense, highly bioturbated sediments require particular attention when sampling is done for geochemical analyses, as sediments can be considerably mixed. While avoiding trace fossils during sampling for geochemical analyses is easy for large traces (e.g. Planolites, Thallasinoides, and Zoophycos), it becomes challenging for smaller and complex traces (< 0.5 cm), as is the case with Chondrites. Chondrites-producers have an opportunistic behavior, being one of the first organisms to colonize the seafloor after an anoxic event. As a result, Chondrites are commonly and sometimes abundantly found in the upper layers of organic-rich sediments, including sapropels and black shales.This study focuses on sapropel S7, which was deposited between ∼198.5 and 191.9 ka at the top of Eratosthenes Seamount and exhibits abundant Chondrites. For this sapropel, we demonstrated that Chondrites-producers can bioturbate significant volumes of organic-rich sediments (over 35%), introducing oxic/dysoxic material downward into anoxic sediments enriched in redox-sensitive trace metals and organic matter. This process can dilute up to 18% the original concentration of redox-sensitive trace metals (e.g. Mo, U and V). This dilution is especially important to consider when calculating trace metals and organic matter burial flux from bulk sediment data. Therefore, Chondrites-producers can play a crucial role in altering the sediment texture and distribution of minerals and organic matter at sub-cm scale, and may thus impact paleoenvironmental and paleoceanographic reconstructions based on geochemical signals. Furthermore, the subsequent redistribution of organic matter and redox-sensitive trace metals (including oligoelements) within the sediment can affect carbon and nutrient cycling, microbial activity, organic matter degradation in deep-marine sediments, and the overall sedimentary biogeochemistry.

Continental depositional record of climate and tectonic evolution around the Eocene-Oligocene transition in southeast France: perpectives from the Vistrenque Basin (Camargue)

Based on detailed sedimentological analyses of cores, interpretation of well logs and a set of geochemical measurements performed on lacustrine sedimentary rocks, the palaeoenvironmental evolution and the sedimentary architecture of the Paleogene continental Vistrenque Basin (SE France) have been reconstructed. The analysis of sedimentary archives revealed three main stages of basin infill evolution: (1) a deep-lake basin (Priabonian-earliest Rupelian) whose sedimentation was dominated by terrigenous gravity-driven deposits during a period of high subsidence rate and strike-slip fault activity and under a prevailing humid climate; (2) an evaporative deep lake (early Rupelian) characterized by a drastic reduction in lake volume (forced-regression), terrigenous supplies and deposition of evaporites in disconnected sub-basins; (3) an overall long-term normal regressive stage (middle Rupelian to earliest Chattian) of lake infill characterized by an increase in terrigenous supplies and a vertical upward transition from deep-lake gravity-driven deposits to marginal lake and floodplain sedimentation. The onset of lake volume reduction and forced regression during the early Rupelian is associated with (1) the reworking of marginal lake carbonates into the deep lake areas, (2) the deposition of organic-rich sediments (TOC &gt; 10%) coupled with sulphate-reduction processes in the deepest areas of the lake, (3) an important decrease in terrigenous supplies and (4) a long-term increase in δ18O of matrix-supported carbonates. This early Rupelian forced regression of the Vistrenque lacustrine system is interpreted to result from a regional decrease in precipitation in response to global cooling during the Eocene-Oligocene Transition (EOT). The final infill of the Vistrenque lake system (late Rupelian-early Chattian) and the onset of a floodplain occurred in more humid conditions during a stage of decreased activity of the Nîmes Fault, prior to or during an early stage of the Liguro-Provençal rifting.

A PCR-Based Survey of Methane-Cycling Archaea in Methane-Soaked Subsurface Sediments of Guaymas Basin, Gulf of California.

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, the rapid deposition of organic-rich sediments, steep geothermal gradients, and abundant methane of mixed thermogenic and microbial origin. Subsurface sediment samples from eight drilling sites with distinct geochemical and thermal profiles were selected for DNA extraction and PCR amplification to explore the diversity of methane-cycling archaea in the Guaymas Basin subsurface. We performed PCR amplifications with general (mcrIRD), and ANME-1 specific primers that target the alpha (α) subunit of methyl coenzyme M reductase (mcrA). Diverse ANME-1 lineages associated with anaerobic methane oxidation were detected in seven out of the eight drilling sites, preferentially around the methane-sulfate interface, and in several cases, showed preferences for specific sampling sites. Phylogenetically, most ANME-1 sequences from the Guaymas Basin subsurface were related to marine mud volcanoes, seep sites, and the shallow marine subsurface. The most frequently recovered methanogenic phylotypes were closely affiliated with the hyperthermophilic Methanocaldococcaceae, and found at the hydrothermally influenced Ringvent site. The coolest drilling site, in the northern axial trough of Guaymas Basin, yielded the greatest diversity in methanogen lineages. Our survey indicates the potential for extensive microbial methane cycling within subsurface sediments of Guaymas Basin.

Astrochronology of the late Oligocene-middle Miocene in the Maldives, central Indian Ocean

Substantial work based on biostratigraphy has been done to constrain the chronology of the late Oligocene-middle Miocene in the Maldives (Indian Ocean), but a high-resolution temporal framework for the sedimentary record of this region remains elusive. This hampers a fuller understanding of the evolution of the carbonate platform, which is the second largest modern isolated carbonate platform. Moreover, the sedimentary record of the Maldives platform has the potential to provide insights on the onset of the ‘modern-like’ South Asian Monsoon (SAM) system. Here we present a detailed astrochronology for International Ocean Discovery Program (IODP) Site U1468 through the late Oligocene-middle Miocene in the Maldives. Our cyclostratigraphy of the site is integrated with existing biostratigraphy and statistical evaluation, providing a robust ∼12-Myr absolute astronomical time scale (ATS). Using this time scale, we assign numerical ages of 12.229 ± 0.46 Ma and 11.561 ± 0.46 Ma for the core depths of 450 mbsf and 430 mbsf, respectively. These depths, constrained by biostratigraphy, have previously been suggested to mark the onset of the SAM in the Maldives. Our ATS provides time constraints on the evolution of the platform sequence stratigraphy, and we find evidence of a ∼1.2-Myr obliquity amplitude modulation (AM) control on sea level changes and sequence development. In addition, our results indicate that the deposition of organic-rich sediments towards the base of the record in this area was driven by obliquity cycles and the combined effect of glacio-eustasy and hydrographic changes.

The Guaymas Basin – A hot spot for hydrothermal generation and anaerobic microbial degradation of hydrocarbons

Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading, rapid deposition of organic-rich sediments, and steep geothermal gradients where sedimentary organic material of photosynthetic origin turns into hydrocarbons - potential microbial substrates. After introducing the sources of hydrocarbons in the subsurface, this short overview discusses the phylogenetic diversity, temperature range and occurrence patterns of novel alkane-oxidizing bacteria and archaea in the hydrothermal sediments of Guaymas Basin. Many of these alkane-degrading specialists function as syntrophic consortia, where sulfate-reducing bacteria accept electron from methane- or alkane-oxidizing archaea. As a source of metagenomes, enrichments, and cultures of these methodologically and conceptually challenging microorganisms, Guaymas Basin stands out as an indispensable field site that has consistently catalyzed new insights into the microbiology of anaerobic hydrocarbon oxidation and assimilation.

Effects of redox variability and early diagenesis on marine sedimentary Hg records

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed “steady-state” environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity.

The Occurrence of Recent-Subrecent Seabed Acoustic Anomalies and Its Relationship with Structural Uplift around the Waipoga Trough, West Papua-Indonesia

Marine Geological Institute acquired high-resolution seismic and acoustic data during a survey in the Waipoga Trough-West Papua in 2018. The profiles show acoustically blanking, turbid, and cloudy occur along the trough floor over 3,000 km2. Many accoustic chimneys are breaking through the seabed of recent and subrecent sediment layers. It is suggested due to the local expulsion of methane gas. The pockmarks occur as transparent and turbid acoustic characteristics. They locally have relief down to 2-10 m below the seafloor with horizontal dimensions up to 25 -100 m in width. Methane formation due to the rapid deposition of organic-rich sediment down the uplifted area around the trough, associated with thrust fold tectonics is suggested as a triggering agent for the acoustic anomaly formation.

Intensive peatland wildfires during the Aptian–Albian oceanic anoxic event 1b: Evidence from borehole SK-2 in the Songliao Basin, NE China

The Cretaceous has been considered a “high-fire” world accompanied by widespread by-products of combustion in the rock record. The mid-Cretaceous oceanic anoxic event 1b (OAE1b) is marked by one of the major perturbations in the global carbon cycle characterized by deposition of organic-rich sediments in both marine and terrestrial settings. However, our understanding is still limited on changes in wildfire activity during OAE1b period. Here, we carried out a comprehensive analysis, including organic carbon isotope (δ13Corg), total organic carbon (TOC), coal petrology, trace elements, and pyrolytic polycyclic aromatic hydrocarbons (pyroPAHs), of coal seams of the middle Aptian to early Albian Shahezi Formation from borehole SK-2 in Songliao Basin, Northeast China. Two negative δ13Corg excursions in the Shahezi Formation can be corresponded with the 113/Jacob and Kilian sub-events of OAE1b. Moreover, the intensive peatland wildfires have been identified during the sub-event periods of OAE1b based on the co-occurrence of high abundance of charcoal and pyroPAHs at that time. In addition, Sr/Ba, Sr/Cu and Sr/Rb ratios demonstrate that enhanced peatland wildfires were controlled by dryer climate conditions owing to episodic northward migration of arid zones in East Asia related with rising global temperature during the sub-events of OAE1b. The climate-driven extensive wildfire activity in the mid-latitude terrestrial ecosystems can be a contributing factor for OAE1b through the increased flux of nutrients fuelling primary producers in the lake and marine environments and leading to more speculative anoxia to allow the deposition of organic-rich sediments. Our results provide essential understanding of the importance of wildfires in driving mechanism of oceanic anoxic events (OAEs) in Earth's history.

Hydroclimate variability in the United States continental interior during the early Eocene Climatic Optimum

The early Eocene (56.0 to 47.8 million years ago) was punctuated by a series of transient episodes of rapid global warming superimposed on the long-term early Cenozoic warming trend, culminating in the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 million years ago). Details of the hydroclimate regime operating during the EECO are poorly constrained, especially for continental interior sites. The Green River Formation (GRF) of Utah and Colorado was deposited in a suite of large, unusually productive lakes that offer an ideal opportunity to study the hydrological response to warming. Here we report the hydrogen isotopic composition (δ 2 H) of leaf wax (long-chain n -alkanes) and algal (phytane) lipids preserved in the organic-rich Mahogany Zone (49.3 to 48.7 Ma) and use these data to reconstruct precipitation and lake water δ 2 H records, respectively. We observe large inter-site variations in algal and leaf wax δ 2 H values (~50 to 75‰), suggesting that additional local controls influence precipitation and/or lake water δ 2 H (e.g., salinity). Intriguingly, leaf wax and algal lipid δ 2 H values show little variation through the Mahogany Zone, implying a relatively stable hydrological regime during the latter phase of the EECO. This contrasts with the more variable hydrological regime that prevailed during early Eocene hyperthermals. Unlike the EECO, the early Eocene hyperthermals in the Uinta region do not coincide with the deposition of organic-rich sediments. This suggests that a stable hydrological regime during the EECO may enable the preservation of organic matter within continental-interior lake systems, potentially leading to an important negative climate feedback during the early Eocene and other greenhouse climates. • Gradual variations in leaf wax δ 2 H values indicate a stable hydrological cycle during the latter phase of theEECO. • However, leaf wax and algal δ 2 H values exhibit large lake inter-site variability (~50 to 75‰). • Stable hydroclimate may have promoted organic matter burial within the lake system. • High organic carbon burial may have acted as an important negative climate feedback.

An Early Jurassic (Sinemurian–Toarcian) stratigraphic framework for the occurrence of Organic Matter Preservation Intervals (OMPIs)

Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ 13 C record. This survey combines an in-depth analysis of literature on the distribution of organic-rich facies in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco. Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ 13 C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressive in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ 13 C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ 13 C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ 13 C (recovery) trend following the δ 13 C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of organic matter (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval. The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian (not taking into account the T-OAE related OMPIs) regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).

Facies associations and sequence stratigraphy of the Middle-Late Eocene Pila Spi Formation in the High Folded Zone, Kurdistan Region, northeastern Iraq

Facies analysis and sequence stratigraphy of the Pila Spi Formation (Middle-Late Eocene) was studied from two outcrop sections (Bekhma and Gomaspan) within the High Folded Zone, Iraqi Kurdistan Region. The formation consists of light brown dolomitic, crystallized, chalky, limestones, and dolostone, alternating with multicolored marl and marly limestones. Chert nodules are present in the upper part of the formation. The petrographic analysis was based on 65 thin sections, and the results showed that the carbonate rocks are partially dolomitized. The distinguished bioclasts consist of benthic and planktonic foraminifera, micro-gastropods, calcispheres, ostracods, pelecypods, and bryozoans. Non-skeletal grains such as peloids, intraclasts, and extraclasts were also observed. X-ray diffraction analysis revealed that abundant clay minerals are palygorskite with the rare existence of chlorite. Non-clay minerals include dolomite, calcite, and quartz with low pyrite contents. According to field observations, four different lithofacies were recognized. They are dolomitic limestone, chalky limestone, marl-marly limestone, and cherty limestone. Based on detailed microfacies analysis of limestones, four main microfacies and eleven submicrofacies types are observed. These microfacies are subdivided into two groups of facies associations; they are semi-restricted and open lagoon. The results of stable carbon and oxygen isotopes of the studied carbonate samples show negative values of δ18O, and this indicates that the seawater was warm with lower salinity. However the positive δ13C values reflect the deposition of organic-rich sediments with transgression of the sea and deepening of the basin. Two third-order depositional sequences are observed. Each of them is subdivided into a TST and HST, reflecting the transgression and stillstands of the relative sea level during Middle-Late Eocene.

Late Pleistocene and Holocene climate and environmental evolution of a subantarctic fjord ingression basin in the southwest Pacific

Subantarctic islands are located at a critical latitude for reconstructing past changes in ocean-atmosphere interactions. Currently, there is a lack of high-resolution records from the Pacific sector of the Southern Ocean that can be used to reconstruct climate since the Last Glacial Maximum (LGM). Here, we present a reconstruction of environmental change from a fjord ingression basin at New Zealand’s subantarctic Auckland Islands (50.5°S) over the last ∼19 ka cal BP. Using sedimentological and geochemical analysis of cores collected along a depth transect in Norman Inlet we find evidence for four different stages of environmental history: deglacial, lacustrine, marine transgression, and marine. Glaciers retreated from their maximum extent before 19.4 ka cal BP during a period of ice retreat that appears synchronous across the Pacific basin, likely due to southward migration of the Subtropical Front. However, in contrast to other glaciated regions, we see no evidence for ice re-advance, and a proglacial lake was present in the basin until 15.7 ka cal BP. Following deglaciation, organic-rich sedimentation dominated the lacustrine environment until post-glacial sea-level rise flooded the depositional basin at 8.8 ka cal BP. Deposition of organic-rich sediment continued during the Antarctic Cold Reversal and we find no evidence for a glacial re-advance. Mixing of terrestrial and marine organic matter during the marine transgression (8.8–6.7 ka cal BP) indicates significant erosion and re-working of sediment in the catchment as seawater overtopped the sill. Modern fjord circulation commenced ∼6.7 ka cal BP when sea level reached its maximum in the early Holocene. This well-dated, high-resolution record constrains the timing of deglaciation, sea-level rise, and subantarctic environmental change following the LGM that expands our understanding of the key drives of climate change in the middle to high southern latitudes.

Recurrent deposition of organic-rich sediments in Early Triassic pelagic Panthalassa and its relationship with global oceanic anoxia: New data from Kyoto, Southwest Japan

Abstract The Early Triassic experienced one of the most extensive episodes of oceanic oxygen depletion in the Phanerozoic, which was considered to be reflected in the deposition of black organic-rich claystone and general absence of radiolarian chert in pelagic deep-water areas of Panthalassa. However, while studies of peri-continental sections demonstrate that oceanic anoxia repeatedly occurred throughout the Early Triassic, black claystone deposition was thought to be restricted to earliest Triassic times. In this paper, the lithostratigraphy, conodont biostratigraphy and chemical composition are investigated at two Olenekian–Anisian sections in Japan, deposited in pelagic deep-water areas of Panthalassa. Sedimentation rates and major element compositions support published findings that the absence of chert is a consequence of increased clastic input and not radiolarian extinction due to oceanic anoxia. Biostratigraphic correlation with previously described sections reveals that the deposition of extensive black claystone occurred in the Induan, early Spathian (early late Olenekian) and across the Olenekian–Anisian boundary. The three black claystone lithofacies are associated with sedimentary and geochemical evidence of anoxia in the water column and bottom waters. Organic-rich deposition is generally coeval with episodes of expanded anoxic seafloor as indicated by uranium isotopes, although they are decoupled in Smithian (early Olenekian) times. Black claystone of end-Olenekian to early Anisian age represents the first finding of a laterally extensive deposit associated with oceanic anoxia during this time interval. Results imply that the deposition of black organic-rich claystone in pelagic deep Panthalassa reflects global trends in the spatial extent of bottom water anoxia during the Early Triassic, probably controlled mainly by nutrient input to the oceans.

The mechanism of sapropel formation in the Mediterranean Sea: insight from long-duration box model experiments

Abstract. Periodic bottom-water oxygen deficiency in the Mediterranean Sea led to the deposition of organic-rich sediments during geological history, so-called sapropels. Although a mechanism linking the formation of these deposits to orbital variability has been derived from the geological record, physics-based proof is limited to snapshot and short-time-slice experiments with (oceanic) general circulation models. Specifically, previous modelling studies have investigated atmospheric and oceanographic equilibrium states during orbital extremes (minimum and maximum precession). In contrast, we use a conceptual box model that allows us to focus on the transient response of the Mediterranean Sea to orbital forcing and investigate the physical processes causing sapropel formation. The model is constrained by present-day measurement data, while proxy data offer constraints on the timing of sapropels. The results demonstrate that it is possible to describe the first-order aspects of sapropel formation in a conceptual box model. A systematic model analysis provides new insights on features observed in the geological record, such as the timing of sapropels as well as intra-sapropel intensity variations and interruptions. Moreover, given a scenario constrained by geological data, the model allows us to study the transient response of variables and processes that cannot be observed in the geological record. The results suggest that atmospheric temperature variability plays a key role in sapropel formation and that the timing of the midpoint of a sapropel can shift significantly with a minor change in forcing due to nonlinearities in the system.

Trace metal elements as paleoenvironmental proxies: Why should we account for sedimentation rate variations?

Abstract Trace metal elements (TMEs) are commonly used to reconstruct the environmental conditions present during the deposition of organic-rich sediments. For example, TME concentrations controlled by changes in primary productivity and redox conditions are widely used in paleoenvironmental studies. Recently, these proxies have undergone a resurgence of interest and are commonly used in large-scale (10–1000 km) studies. However, applying these geochemical proxies at basin scale while ignoring variations in sedimentation rates (SR) may lead to misinterpretation of paleoenvironmental conditions. Here, we show how SR can affect the geochemical records and may lead to incorrect interpretations of TME evolution. Accounting for SR, we computed the authigenic fraction accumulation rates of key TMEs in the Upper Montney Formation and Doig Phosphate (Triassic, western Canada), and we correct the concentration of these elements in the Vaca-Muerta Formation (Jurassic–Cretaceous, Argentina). Our SR-corrected TME proxies require a different interpretation of paleoenvironmental conditions (e.g., primary productivity, basin restriction) compared to conventional TME results and highlight that elementary enrichments commonly interpreted as indicative of anoxic depositional environments may reflect low SR and the formation of condensed intervals. This work also introduces a new workflow to account for SR in paleoenvironmental studies at basin scale and over long time periods.