In-situ mechanical weakness of subducting sediments beneath a plate boundary décollement in the Nankai Trough

© 2018, The Author(s). The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments. [Figure not available: see fulltext.]

Carbon dioxide and coccolithophore physiology in ancient oceans

Coccolithophores form an important and dynamically evolving component of the carbon cycle. These ubiquitous single-celled marine calcifying phytoplankton are re- sponsible for half of the calcium carbonate production in the modern surface ocean, and their adorning calcite plates (coccoliths), produced intracellularly, have con- tributed to sedimentary carbonate for over 200 million years. They constitute a significant control on the partitioning of carbon between the atmosphere, ocean and sedimentary reservoirs on timescales from the instantaneous to the geological. Coc- colithophores are also uniquely placed to record aspects of the carbonate chemistry of the surface ocean, because the carbon isotopic composition of the organic matter (δ13Corg) and calcite (δ13Ccal) that they produce is a function of many parameters, including ambient aqueous carbon dioxide concentration [CO2]. This thesis addresses the bidirectional interaction between coccolithophores and the carbon cycle in the geological past, by asking how cellular carbon fluxes relate to physical evidence that is preserved throughout geological time. First, I present and calibrate a novel rationale for size-normalising coccolith mass, and show that over two glacial-interglacial cycles, coccolithophores appear to calcify more under high [CO2] conditions; a result that is manifest on evolutionary timescales, and is necessarily elusive to experiments. Second, I investigate the parameters controlling δ13Ccal and δ13Corg in coccolithophores through in vivo experimentation, and devel- opment of a model of cellular isotopic fluxes. I show that so called "vital effects" in coccolithophores arise as a result of differences in calcification to photosynthesis ratios. Third, using a combination of novel and established protocols for extraction and isotopic analysis of specific organic molecules from fossils taxonomically separated by size, I show the very first size-specific geologic time series of coccolith-associated δ13Corg, and the first time-series of size-separated coccolith δ13Ccal over a glacial cycle. A novel means of inferring past carbon dioxide concentrations, based on an iterative inverse modelling approach, is presented and tested

Towards the use of the coccolith vital effects in palaeoceanography: A field investigation during the middle Miocene in the SW Pacific Ocean

International audienceRecent culture studies of living coccolithophores have established a biogeochemical framework for the use of the geochemical compositions of their calcite biominerals as proxies in palaeoceanography. Yet, questions remain regarding the transferability of such experimental data to fossil coccoliths. Here we analysed the carbon and oxygen isotopic composition of Miocene coccoliths to assess the suitability of such data for reconstructing the past environment. We found that the oxygen isotopic compositions of the relatively small Noelaerhabdaceae coccoliths gathered in the 3-5 μm fractions appear to be a suitable material to derive temperatures after a correction for a constant vital offset of 0.8‰. The interpretation of the isotopic signal of the relatively large Coccolithales coccoliths (5-8 μm fractions) is more complex, but supports results from cultures. The expression of the carbon and oxygen vital effect in coccoliths appears to be limited during the so-called Miocene Climate Optimum (MCO), a period of relatively elevated atmospheric pCO2. Subsequently, during the Miocene Climatic Transition (MCT; ~14 Ma), which saw a decline in pCO2, large carbon and oxygen vital effects were expressed in coccolith calcite. This phenomenon predates the postulated “Late Miocene Threshold” by approximately 4 Ma, and cannot be reconciled as a temporally-synchronous nor localised feature. Furthermore, we observed a statistically significant correlation between the oxygen and carbon offsets of the small relative to large coccoliths (hence, the vital effect per se) that is likely linked to variations in atmospheric CO2. This biogeochemical correlation further supports a forcing of the environment on the cellular physiology (growth rate and utilisation of intracellular carbon) and ultimately the magnitude of isotopic vital effects in fossil coccoliths

Carbon dioxide and coccolithophore physiology in ancient oceans

Coccolithophores form an important and dynamically evolving component of the carbon cycle. These ubiquitous single-celled marine calcifying phytoplankton are re- sponsible for half of the calcium carbonate production in the modern surface ocean, and their adorning calcite plates (coccoliths), produced intracellularly, have con- tributed to sedimentary carbonate for over 200 million years. They constitute a significant control on the partitioning of carbon between the atmosphere, ocean and sedimentary reservoirs on timescales from the instantaneous to the geological. Coc- colithophores are also uniquely placed to record aspects of the carbonate chemistry of the surface ocean, because the carbon isotopic composition of the organic matter (&delta;13Corg) and calcite (&delta;13Ccal) that they produce is a function of many parameters, including ambient aqueous carbon dioxide concentration [CO2]. This thesis addresses the bidirectional interaction between coccolithophores and the carbon cycle in the geological past, by asking how cellular carbon fluxes relate to physical evidence that is preserved throughout geological time. First, I present and calibrate a novel rationale for size-normalising coccolith mass, and show that over two glacial-interglacial cycles, coccolithophores appear to calcify more under high [CO2] conditions; a result that is manifest on evolutionary timescales, and is necessarily elusive to experiments. Second, I investigate the parameters controlling &delta;13Ccal and &delta;13Corg in coccolithophores through in vivo experimentation, and devel- opment of a model of cellular isotopic fluxes. I show that so called "vital effects" in coccolithophores arise as a result of differences in calcification to photosynthesis ratios. Third, using a combination of novel and established protocols for extraction and isotopic analysis of specific organic molecules from fossils taxonomically separated by size, I show the very first size-specific geologic time series of coccolith-associated &delta;13Corg, and the first time-series of size-separated coccolith &delta;13Ccal over a glacial cycle. A novel means of inferring past carbon dioxide concentrations, based on an iterative inverse modelling approach, is presented and tested.</p

Organic carbon burial during OAE2 driven by changes in the locus of organic matter sulfurization

The mechanisms responsible for the burial of vast quantities of organic matter during Ocean Anoxic Event remain unclear. Here, the authors combine biogeochemical analysis and modeling and show that sulfurization could play a critical role in facilitating globally elevated burial of organic matter

Organic carbon burial during OAE2 driven by changes in the locus of organic matter sulfurization.

Ocean Anoxic Event 2 (OAE2) was a period of dramatic disruption to the global carbon cycle when massive amounts of organic matter (OM) were buried in marine sediments via complex and controversial mechanisms. Here we investigate the role of OM sulfurization, which makes OM less available for microbial respiration, in driving variable OM preservation in OAE2 sedimentary strata from Pont d'Issole (France). We find correlations between the concentration, S:C ratio, S-isotope composition, and sulfur speciation of OM suggesting that sulfurization facilitated changes in carbon burial at this site as the chemocline moved in and out of the sediments during deposition. These patterns are reproduced by a simple model, suggesting that small changes in primary productivity could drive large changes in local OM burial in environments poised near a critical redox threshold. This amplifying mechanism may be central to understanding the magnitude of global carbon cycle response to environmental perturbations

Organic carbon burial during OAE2 driven by changes in the locus of organic matter sulfurization

The mechanisms responsible for the burial of vast quantities of organic matter during Ocean Anoxic Event remain unclear. Here, the authors combine biogeochemical analysis and modeling and show that sulfurization could play a critical role in facilitating globally elevated burial of organic matter

Expedition 370 Preliminary Report: Temperature Limit of the Deep Biosphere off Muroto.

International Ocean Discovery Program (IODP) Expedition 370 aimed to explore the limits of life in the deep subseafloor biosphere at a location where temperature increases with depth at an intermediate rate and exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Drilling Site C0023 is located in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174 at the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m. ODP Leg 190 in 2000, revealed the presence of microbial cells at Site 1174 to a depth of ~600 meters below seafloor (mbsf), which corresponds to an estimated temperature of ~70°C, and reliably identified a single zone of higher cell concentrations just above the décollement at around 800 mbsf, where temperature presumably reached 90°C; no cell count data was reported for other sediment layers in the 70°–120°C range, because the limit of manual cell count for low-biomass samples was not high enough. With the establishment of Site C0023, we aimed to detect and investigate the presence or absence of life and biological processes at the biotic–abiotic transition with unprecedented analytical sensitivity and precision. Expedition 370 was the first expedition dedicated to subseafloor microbiology that achieved time-critical processing and analyses of deep biosphere samples by simultaneous shipboard and shore-based investigations. Our primary objectives during Expedition 370 were to study the relationship between the deep subseafloor biosphere and temperature. We aimed to comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor to ~120°C at the sediment/basement interface and thus likely encompasses the biotic–abiotic transition zone. We also aimed to determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex. To address these primary scientific objectives and questions, we penetrated 1180 m and recovered 112 cores across the sediment/basalt interface. More than 13,000 samples were collected, and selected samples were transferred to the Kochi Core Center by helicopter for simultaneous microbiological sampling and analysis in laboratories with a super-clean environment. Following the coring operations, a temperature observatory with 13 thermistor sensors was installed in the borehole to 863 mbsf