Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time

A global reassessment of the controls on iron speciation in modern sediments and sedimentary rocks: A dominant role for diagenesis

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (FeHR/FeT) and the fraction of pyrite iron out of the reactive iron pool (FePYR/FeHR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to ≈70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive FeHR/FeT–FePYR/FeHR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either FeHR/FeT or FePYR/FeHR from the Archean to the present day. The diagenetic FeHR/FeT–FePYR/FeHR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain FeHR/FeT in the local sediment source(s). Hence, we suggest that iron speciation data, together with complementary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns

Multi-phase dolomitization combined with dyke-like breccias in a hyper-extended rift margin. Case study from Jurassic carbonates of the northern Pyrenees (France)

International audienc

Multi-phase dolomitization combined with dyke-like breccias in a hyper-extended rift margin. Case study from Jurassic carbonates of the northern Pyrenees (France)

International audienc

Stable isotopes (δ¹³Corg and d15N) from sediment core PRGL_1-4

In this study we use bulk sediment organic matter stable isotopes (d13Corg and d15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (d13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ~3‰ values in d15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represents 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel depositio

Gondwana break‐up controlled by tectonic inheritance and mantle plume activity: insights from Natal rift development (South Mozambique, Africa)

The breakup of Eastern Gondwana started during the Early Jurassic with the separation of Antarctica and Madagascar from Africa. While margins architecture has been described along the Western Somalia Basin and the central Mozambique, the spatial extent of rifting further south – i.e. the Natal Valley – remains poorly documented. Seismic reflection profiles interpretation, 40Ar/39Ar ages and isotopic data show the existence of a plume related magma-rich margin – the Natal segment – characterized by a large volume of seaward dipping reflectors inferred to be basalts. In particular, the contrast in lithosphere rheology, probably caused by a Meso-Neoproterozoic belt between the Kaapval and Grunehogna Archean cratons, favored extension and upwelling from a deep mantle thermomechanical anomaly, the so-called Karoo superplume that started at around 180 Ma. Further, the presence of stretched continental crust at around 30°S implies a more southerly position of Antarctica which will need to be considered in future kinematic models

Depth, estimated age, and associated geochemical information from sediment core PRGL 1-4

The terrigenous sedimentary budget of passive margin records variations in past sedimentary fluxes, and thus can be used to infer past variations of Earth surface deformation processes, or climate change. Accurate estimates of sediment fluxes over various times scale and space-scale are therefore crucial. Traditionally, offshore sediment sequence volumes take into account only siliciclastic accumulation, the carbonate fraction being considered only as in-situ production (i.e. CaCO3). Here we propose a new geochemical methodology to decipher and quantify the amount of detrital carbonates in comparison to in-situ produced biogenic carbonates. This isotopic approach enables taking into account the export of detrital carbonates, and investigating its effect on sediment budgets. This study, located in the Gulf of Lion, is based on a 300 m long sediment borehole located near the shelf break and covering the last 500 000 years (i.e. 5 glacial-interglacial periods). 86Sr/87Sr isotopic data (0.70809 to 0.70858) are significantly less radiogenic than modern seawater (i.e. 0.7092) and show fluctuations in agreement with stratigraphic and climatic variations. These results suggest an unsuspected high export of detrital carbonates from the catchment area during both glacial (between 55 to 85% of the sedimentary carbonate fraction) and interglacial (between 30 to 50%) conditions. Thus, not only do detrital carbonate fluxes need to be factored into sediment flux calculations, these results suggest that detrital carbonate components could potentially have a strong influence on carbonate 86Sr/87Sr ratios when not obtained from microdrilled biogenic carbonates, such as the entirety of the Precambrian Sr chemostratigraphic record

Continental weathering and climate conditions in southern high latitudes during the Albian-Santonian interval (U1512 and U1513 sites, Exp IODP 369, SW Australia)

International audienceThe Albian-Santonian interval (113-83 Ma) is considered as a transitional period between the Early Cretaceous times, marked by a succession of short climatic variations associated with volcanism episodes and the Late Cretaceous times, marked by a progressive decrease of temperatures. This 30 Myr-longed interval is characterized by a gradual increase of temperature in oceanic domain, which culminates during the Cretaceous thermal Maximum, at the end of the Turonian (~ 90 Ma). Although the evolutions of continental weathering and climatic conditions are well documented in oceanic domain of low to middle latitudes, especially in Atlantic and Tethyan oceans, their record are less well known in high latitudes, especially in the proto-Indian Ocean. Thanks to the Exp IODP 369, two new boreholes, U1512 and U1513, drilled respectively in the Bight Basin (Southern Australia) and in the Mentelle Basin (Southwestern Australia), provide the opportunity to study the Albian to Santonian deposits at high latitudes (~60°S). Cores of the site U1513 recovered a sedimentary sequence from Albian to Santonian whereas the site U1512 record a continuous sequence from Turonian to Santonian. An integrated study, coupling mineralogical determination (XRD analyse and SEM observation) and isotopic analyses of neodymium on clay fraction was done on both sites in order to determine climatic and weathering conditions in these southern high latitude zone.</p><p>Our study reveals that the clay fraction are dominated by smectites (>85% in average) with lower proportions of kaolinites (< 25%) and traces of illites (<5%) associated with opal-CT and clinoptilolites. SEM observations have demonstrated a negligible impact of both burial diagenesis and authigenesis on clay assemblage. They are thus interpreted as the products of the alteration of rocks and pedogenic blankets from adjacent landmasses. At Site U1513, the Albian clay fraction contains noticeable proportions of kaolinites (5 to 25%), which progressively decrease during the Cenomanian and disappear at the Cenomanian-Turonian boundary (~94 Ma). Turonian to Coniacian deposits are almost exclusively composed of smectites. The decrease in kaolinite proportions is coeval with a decrease in ε<sub>Nd</sub> values, which indicates a probable diminution in the erosion of Australian Archean rocks. At Site U1512, clay mineral assemblages, show slight variations along the borehole, which reflects stable weathering conditions during the 10 Myr of the Turonian-Santonian interval.</p><p>The dominance of smectites and to a lesser extent of kaolinites seem to indicate a warm to temperate and humid climate for high latitude zone during the Albian-Santonian interval. The decrease in kaolinite proportions from Albian to early Turonian in U1513 reflect probably a decrease of hydrolysis conditions associated with increasing temperatures and sea-level rise in southwestern Australian margins. The absence of noticeable variations from the Turonian to the Santonian in both sites would be the result of a stable continental climate for several million years after the Cretaceous thermal maximum (~ 90 Ma). The persistent presence of kaolinites in U1512 (southern Australia) could be due to the proximity of the Bight Basin with Australian Western Highlands

Geochemistry of sediment core PRGL1-4

The sulfur biogeochemical cycle plays a key role in regulating Earth's surface redox through diverse abiotic and biological reactions that have distinctive stable isotopic fractionations. As such, variations in the sulfur isotopic composition (d34S) of sedimentary sulfate and sulfide phases over Earth history can be used to infer substantive changes to the Earth's surface environment, including the rise of atmospheric oxygen. Such inferences assume that individual d34S records reflect temporal changes in the global sulfur cycle; this assumption may be well grounded for sulfate-bearing minerals but is less well established for pyrite-based records. Here, we investigate alternative controls on the sedimentary sulfur isotopic composition of marine pyrite by examining a 300-m drill core of Mediterranean sediments deposited over the past 500,000 y and spanning the last five glacial-interglacial periods. Because this interval is far shorter than the residence time of marine sulfate, any change in the sulfur isotopic record preserved in pyrite (d34Spyr) necessarily corresponds to local environmental changes. The stratigraphic variations (>76 per mil) in the isotopic data reported here are among the largest ever observed in pyrite, and are in phase with glacial-interglacial sea level and temperature changes. In this case, the dominant control appears to be glacial-interglacial variations in sedimentation rates. These results suggest that there exist important but previously overlooked depositional controls on sedimentary sulfur isotope records, especially associated with intervals of substantial sea level change. This work provides an important perspective on the origin of variability in such records and suggests meaningful paleoenvironmental information can be derived from pyrite d34S records