Deep-sea Sediments Research Articles

Preliminary Assessment of Environmentally Friendly Mining Options Based on Various Mineral Resources—A Case Study of the Clarion-Clipperton Fracture Zone in Pacific

Deep-sea polymetallic nodules are associated with rich rare substances, such as rare-earth elements (REEs), Mo, Ti, Te, Li, which are currently in demand and are used in various applications. Deep-sea sediments near nodules are another important source of REEs, which will increase the resource potential of polymetallic nodules. Given the similarity of the mining technologies for deep-sea REEs and polymetallic nodules, this study proposed environmentally friendly mining options and developed a technoeconomic evaluation model by combining deep-sea polymetallic nodules and REEs. Using the Clarion-Clipperton Fracture Zone as an example, this study revealed that the development of polymetallic nodules together with REEs of nearby sediments in the form of by-products will improve the economic and environmental benefits. In addition, the effects of discount rate, cost, and price on the economic benefits of nodule mining were discussed, and a technical development direction was proposed based on scientific and technological needs.

Synergetic effects of chlorinated paraffins and microplastics on microbial communities and nitrogen cycling in deep-sea cold seep sediments

Chlorinated paraffins (CPs) and microplastics (MPs) are commonly found in deep-sea cold seep sediments, where nitrogen cycling processes frequently occur. However, little is known about their combined effects on sedimentary microbial communities and nitrogen cycling in these environments. This study aimed to investigate the synergistic impacts of CPs and MPs on microbial communities and nitrogen cycling in deep-sea cold seep sediments through microcosm experiments. Our results demonstrated that the presence of CPs and MPs induced significant alterations in microbial community composition, promoting the growth of Halomonas. Furthermore, CPs and MPs were found to enhance nitrification, denitrification and anammox processes, which was evidenced by the higher abundance of genes associated with nitrification and denitrification, as well as increased activity of denitrification and anammox in the CPs and MPs-treatment groups compared to the control group. Additionally, the enhanced influence of CPs and MPs on denitrification was expected to promote nitrate-dependent and sulfate-dependent anaerobic oxidation of methane, thereby resulting in less methane released into the environment. These findings shed light on the potential consequences of simultaneous exposure to CPs and MPs on biogeochemical nitrogen cycling in deep-sea cold seep sediments.

Assessment of REY resource potential in deep-sea sediments with Fe–Mn (oxyhydr)oxides in the Pacific Ocean

Deep-sea sediments with an abundance bioapatites and Fe–Mn (oxyhydr)oxides in the Pacific Ocean have been considered potential reservoirs of rare earth elements and yttrium (REY). However, comprehensive assessment of the resource potential of REY in deep-sea sediments with Fe–Mn (oxyhydr)oxides throughout the Pacific Ocean is limited due to difficulties in accurately predicting the distribution of extensive Fe–Mn (oxyhydr)oxides and the associated REY. In this study, we predicted the prospective area and resource potential of REY-rich sediments with Fe–Mn (oxyhydr)oxides by considering multiple factors that control REY enrichment based on data from International Ocean Discovery Program (IODP) samples and previous research. According to the distribution map inferred by comprehensively evaluating lithology (clay sediment), hydrothermal fluid influence (δ3He), and water depth (bathymetry), deep-sea sediments with Fe–Mn (oxyhydr)oxides, which have higher than 1000 ppm REY concentration, are distributed in the vicinity of the East Pacific Ridge within a water depth range of 4000–4600 m, and their distribution area is estimated to be approximately 1.1 million km2. If the sedimentation rate (<1.5 m/Myr) is considered, which is a crucial factor influencing REY enrichment, we can achieve a more precise assessment of their distribution area. Assuming a recovery depth of only 1 m, the REY resource amount was estimated to be approximately >450 million tons of REY oxide. Even without accounting for REY resource amount associated with bioapatite, the minimum REY resource amount estimated in this study exceed the world's current land reserves. Furthermore, these sediments contain a significant abundance of industrially important heavy REY, accounting for 53 % of REY resources. This implies that the deep-sea sediments with Fe–Mn (oxyhydr)oxides in the Pacific Ocean are a promising resource of REY. Our findings will serve as essential information for the technological progress required in the exploration and development of REY resources in deep-sea sediments in the future.

Research on the shear stress and microscopic deformation mechanism of deep sea sediments under the action of tracked miner

The traction force of the tracked miner is primarily determined by the shear characteristics of deep-sea sediments. The influence of different parameters on the shear characteristics of deep-sea sediments and the particles change law of the soil–track interface are discussed. By constructing the relationship between the particles horizontal displacement and residual shear strength, a novel shear rheological damage model of deep-sea sediments considering the influence of grounding pressure is proposed, and the microscopic mechanism of shear displacement of deep-sea sediments is explained. The results show that the peak shear strength and residual shear strength increase significantly with the increase of grounding pressure, track length, grouser height and shear speed. Moreover, the particles within the soil–track interface move along the lower right of the vehicle operating direction during the shearing process under different working conditions, the change of particle spacing shows a non-linear trend, and a quantitative equation for the horizontal deformation of soil–track interface under the experimental conditions is constructed. Additionally, the new shear model has a high level of accuracy in fitting with the experimental data, the internal particle migration changes on the soil–track interface can be divided into four characteristics: compaction, failure, deformation and translation.

Enhanced deep-water circulation facilitated rare earth elements enrichment in pelagic sediments from the northwestern Pacific Ocean

The lack of knowledge about the enrichment mechanism of rare earth elements and yttrium (REY) in deep-sea sediments is impeding the development of theories and exploration strategies for pelagic REY-rich sediments. Ocean circulation variability seems to be crucial in enriching REY in pelagic sediments, which, however, has not been extensively studied. Here, we examined the Pb-Nd isotopic signals of bottom water recorded by the authigenic ferromanganese oxyhydroxide fractions, as well as the Mn/Al and Mn/Ti ratios of bulk samples from a well-dated sediment core in the northwestern Pacific Ocean. These proxies consistently indicate that enhanced deep-water circulation occurred in the study area at ∼11.5–9.5 Ma, which was most likely to be caused by changes in the flow path of bottom currents. The age of this distinct event consists with the forming age of highly REY-rich sediments. We propose that enhanced deep-water circulation in seamount areas could increase the flux of micronodules and fish debris into the pelagic sediments, facilitating the scavenging of REY from seawater. Our findings establish a connection between enhanced deep-water circulation and the enrichment of REY in pelagic sediments.

Aequorivita flava sp. nov., isolated from deep-sea sediments.

Three Gram-stain-negative, aerobic, non-motile, chemoheterotrophic, short-rod-shaped bacteria, designated CDY1-MB1T, CDY2-MB3, and BDY3-MB2, were isolated from three marine sediment samples collected in the eastern Pacific Ocean. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these strains were related to the genus Aequorivita and close to the type strain of Aequorivita vitellina F4716T (with similarities of 98.0-98.1%). Strain CDY1-MB1T can grow at 15-37 °C (optimum 30 °C) and in media with pH 6-9 (optimum, pH 7), and tolerate up to 10% (w/v) NaCl. The predominant cellular fatty acids of strain CDY1-MB1T were iso-C15 : 0 (20.7%) and iso-C17 : 0 3-OH (12.8%); the sole respiratory quinone was menaquinone 6; the major polar lipids were phosphatidylethanolamine, two unidentified aminolipids and two unidentified polar lipids. The digital DNA-DNA hybridization/average nucleotide identity values between strains CDY1-MB1T, CDY2-MB3, and BDY3-MB2 and A. vitellina F4716T were 24.7%/81.6-81.7%, thereby indicating that strain CDY1-MB1T should represent a novel species of the genus Aequorivita. The genomic DNA G+C contents were 37.6 % in all three strains. Genomic analysis showed the presence of genes related to nitrogen and sulphur cycling, as well as metal reduction. The genetic traits of these strains indicate their possible roles in nutrient cycling and detoxification processes, potentially shaping the deep-sea ecosystem's health and resilience. Based upon the consensus of phenotypic and genotypic analyses, strain CDY1-MB1T should be classified as a novel species of the genus Aequorivita, for which the name Aequorivita flava sp. nov. is proposed. The type strain is CDY1-MB1T (=MCCC 1A16935T=KCTC 102223T).

The Passage of the Solar System through the Edge of the Local Bubble

The Sun moves through the interstellar medium (ISM) at a velocity of ∼19 pc Myr−1, making the conditions outside the solar system vary with time over millions of years. Today’s solar system is protected from interstellar particles by the heliosphere, the bubble formed by the solar wind as the Sun moves through the ISM, which engulfs the planets. There is geological evidence from 60Fe that Earth was in direct contact with the ISM 2–3 and 5–7 million years ago (MYA). Recent work argues that the Sun encountered a massive cold cloud 2 MYA as part of the Local Ribbon of Cold Clouds that shrunk the heliosphere and exposed Earth to the ISM. Here, we consider the effects of the passage of the Sun through the edge of the Local Bubble occurring at 6.8−0.4+0.5 MYA assuming that the Sun encountered a cloud with a density of 900 cm−3. If we consider additional turbulent motion within the cloud due to shocks, the density encountered can be as low as 283 cm−3. Clouds of this density cover a small but nonzero (≲4.6%) fraction of the surface of the Local Bubble, making an encounter plausible. Using a state-of-the art magnetohydrodynamic model, we show that the heliosphere shrank to a scale smaller than Earth’s orbit, thereby exposing Earth to cold dense ISM, consistent with 60Fe evidence. The timing of the event matches perturbations observed in the paleoclimate record recovered from deep-sea sediments. The passage through the Local Bubble’s surface and contraction of the heliosphere therefore may have impacted the climate and biosphere significantly, suggesting a new driver of major events in Earth’s history.

Organic carbon oxidation and the associated pyrite formation in deep sediments at the Nankai subduction zone

The preservation of organic carbon (OC) in marine sediments is crucial for Earth's climate by sequestering CO2. While OC is mostly remineralized before it reaches the seafloor, a notable fraction is buried in deep-sea sediments, particularly at subduction zones known for their enhanced sedimentation rates that promote OC burial. The mechanisms behind OC recycling and decomposition within subduction zone sediments are governed by biologically mediated redox reactions involving a range of electron acceptors, which facilitate the sequential oxidation of OC. However, direct evidence regarding such effects on OC in deep sediments remains limited. In this study, we examine the contents of pyrite and trace metals of molybdenum (Mo), vanadium (V), and cesium (Cs) in organic-rich deep sediments from the Nankai subduction zone. We find that the pyrite, sulfate, diagenetic illite, and OC contents covary in Unit V, highlighting the important role of organoclastic sulfate reduction and biotic smectite-to-illite reaction at 60–70 °C in OC oxidation and pyrite formation. The profiles of Mo, V, and Cs also align with these processes in the same interval. Our findings contribute to an in-depth understanding of OC dynamics in subduction zones, emphasizing the complex interactions within deep sediments that may influence the global carbon cycle.

Hydrochloric acid leaching of rare earth elements from a novel source of deep-sea sediments and advantage of reduction with H2O2

In this study, the mineralogy of a novel resource of deep-sea sediments containing rare earth elements (REEs) was investigated, along with the leaching of REEs using hydrochloric acid. The results revealed that, apart from 62.0% of the Ce found in the Mn oxides, the other REEs in the deep-sea sediments mostly existed in hydroxyapatite mineral which can be easily decomposed via a hydrochloric acid leaching. An optimized REEs leaching percentage of 89.5% was achieved using 2.0 mol/L hydrochloric acid as the leaching agent, with a liquid-solid ratio of 4:1, at 60 °C for 30 min. However, Mn oxides remained stable during the leaching process, resulting in a low Ce leaching percentage of 30.7%. Under the optimized conditions, Mn oxides could be decomposed by adding 30% H2O2 as a reducing agent, leading to improved leaching percentages of Ce and REEs to 86.6% and 93.2%, respectively. The high leaching efficiency of REEs may further increase the utilization potential of this novel resource.

Experimental study of mechanical properties of deep-sea hydrate-bearing sediments under multi-field coupling conditions

To ensure the long-term safety of hydrate-bearing sediments (HBSs) in the deep sea, it is necessary to conduct a comprehensive study of the mechanical properties of HBSs, which is closely related to the safe extraction of natural gas hydrates (NGH). In this study, a HBSs preparation method was proposed preparing HBSs artificially to meet the test requirement. A series of triaxial shear tests were carried out by the self-developed HBSs multi-field coupling intelligent triaxial test device to analyze the effects of different hydrate saturation and NaCl concentration on the mechanical properties of HBSs. The results show that HBSs have elastic-plastic properties under multi-field coupling conditions, and the stress-strain curves show a strain-hardening trend. Hydrate saturation affects the shear strength of hydrate-bearing sediment critically. When the hydrate saturation is Sh>35 %, the cohesion and internal friction angle is directly proportional to the hydrate saturation. When the NaCl concentration is low, the cohesion of HBSs is more affected by the NaCl concentration, and the shear strength and yield strength are proportional to NaCl concentration. When NaCl concentration is high, the internal friction angle of HBSs is more affected by NaCl concentration and the shear strength is inversely proportional to NaCl concentration. Under the condition of multi-field coupling, the influence of each factor on the mechanical properties of HBSs has an upper limit, which is not a purely linear relationship.

Separation of microplastics from deep-sea sediment using an affordable, simple to use, and easily accessible density separation device

Microplastics accumulate in the environment but methods to extract particles from sediment for quantification and identification often lack accuracy and reproducibility. Existing methods vary greatly and many do not achieve adequate microplastic separation. During method development for extraction procedures, spike-recovery experiments (positive controls) are essential to ensure accurate and reproducible results from each sample matrix. Furthermore, the large variability in grain size and organic matter can affect the extraction of microplastics from the matrix. Scientists have used density separation to separate microplastics from matrices for decades, but apparatuses are often made of plastic, need to be custom made, and require multiple sample transfers from one apparatus to another. This study presents an affordable, easily accessible, and simple to use Density Separation Device (DSD) to remove plastics from deep-sea sediments. Eight polymers were spiked into replicates of environmental sediment, including six fragments: high density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), nylon (PA6), and crumb rubber (CR) and two fibers: cellulose acetate (CA) and polyester (PEST). Two size classes of polymers were used: 100 μm to 300 μm and > 300 μm. Using a sodium polytungstate solution at a density of 1.9 g/mL and reflectance FTIR microscopy for particle identification, mean recoveries of all fragments exceeded 78% (CR: 92.7% ± 30.8%, PP: 78.4% ± 34.0%, HDPE: 93.8% ± 13.5%, PS: 86.9% ± 25.7%, PA6: 98.4% ± 63.2%, PVC: 100.0% ± 12.4%). Fiber recovery was much lower (PEST: 28.1% ± 28.1% and CA: 25.9% ± 17.3%) because they aggregated, passed through sieves vertically, or were obscured under other particles. The fragment recovery success, accessibility (available online, all parts under $200) and ease of use of this DSD should facilitate widespread use, thus helping to standardize sample preparation methods for microplastic metrology.

Chemical and diatom compositions of dating deep-sea sediment core of the Kuril-Kamchatka Trench, northwestern Pacific

For the first time the grain size, contents of major (Al, Ti, Fe, Mn, Ca, Mg, Na, and K) and trace elements (Mo, Ni, and Cu), total organic carbon, and stable carbon isotope were studied in the sediment cores collected at a water depth of approximately 9500 m from the Kuril-Kamchatka Trench. This study determined the factors of sedimentary environment. The quantitative content and species composition of diatoms were also studied. We have estimated the sedimentation rate based on the excess 210Pb data. According to age models, the sedimentation rate was ∼1.64 mm/year. Paleoconstructions were formed on comparing the distribution of chemical and biological parameters in deep-sea basin sediments. The studied sediments were characterized by the high content of the bioproductivity indicators and chemical elements (Fe, Ca, and Cu) corresponding to a period of increased supply of nutrients that stimulated the production of diatoms. Based on the study of chemical indicators and characteristics of nutrient utilization by phytoplankton, a period of outbreak of diatom productivity due to the introduction of volcanic ash into the sediment in the period 1860–1883 was marked. New data will complement existing information and helpful for further understanding of sedimentary geochemical characteristics in this hadal area, and allow us to relate abiotic conditions to productivity.

Reconstructing deep-sea oxygenation in the Western Alboran Sea Basin during Late Pleistocene-Holocene (last 37 kyrs): Insights from a multiproxy approach

A deep-sea sediment record from the Western Alboran Basin spanning the last 37 kyr has been analysed using a multiproxy approach, integrating sediment colour, magnetic susceptibility, elemental ratios, benthic foraminiferal assemblages and bioturbation, gaining new insights into Alboran Basin deep-sea environments and palaeoceanographic conditions since the Last Glacial Maximum. The combination of a diverse set of proxies made it possible to recognise the diagenetic signals and reconstruct new low-frequency events in the deep basin, recording the significant fluctuations in deep water oxygenation occurred during the studied period. From the studied data, short-term periodic low-oxygen events are recognised during Heinrich Stadials 2 and 3 (HS2 and HS3) and Greenland Interstadial GI8, showing important responses of the deep sea environments to climate climate change. Furthermore, the presence of a not previously well-defined Organic Rich Layer 2 (ORL 2) can be defined from the observations as a sequence of events transitioning between low oxygen (associated with humid conditions and Heinrich Events) and oxygenated conditions, and not as a singular low oxygenation event. In addition, the Organic Rich Layer 1 (ORL1), associated with the most recent low oxygen event has been also recognised in the record, with low oxygen conditions starting as early as 16 kyr before the present (BP) during the HS1 and spanning to 8 kyr BP with important fluctuations in environmental conditions and oxygenation. Several climate events that have been previously recognised in the Northern Hemisphere and the Mediterranean Basin can be linked to the observed changes in oxygen conditions during ORL1 deposition, for instance reoxygenation that occurred during the pre-ORL1, between the demise of Heinrich Event 1.1 (HE1.1) and the end of HS1 and the inception of the Bølling-Allerød (BA); the start of ORL1 deposition during BA with the onset of low oxygen conditions. Interestingly, the BA is punctuated by short duration reoxygenation events that can be recognised in the record and correlated to cold events observed in the North Atlantic records. In addition, there is a partial discontinuous reoxygenation during the Younger Dryas (YD), observed also in the Eastern Mediterranean and in other Mediterranean sites which was followed by the return to lower-oxygen conditions after the YD, coinciding with the start of S1 deposition. Finally, the final reoxygenation event occurs at around 8 kyr BP along with the demise of the ORL1.

Globally distributed marine Gemmatimonadota have unique genomic potentials

BackgroundGemmatimonadota bacteria are widely distributed in nature, but their metabolic potential and ecological roles in marine environments are poorly understood.ResultsHere, we obtained 495 metagenome-assembled genomes (MAGs), and associated viruses, from coastal to deep-sea sediments around the world. We used this expanded genomic catalog to compare the protein composition and update the phylogeny of these bacteria. The marine Gemmatimonadota are phylogenetically different from those previously reported from terrestrial environments. Functional analyses of these genomes revealed these marine genotypes are capable of degradation of complex organic carbon, denitrification, sulfate reduction, and oxidizing sulfide and sulfite. Interestingly, there is widespread genetic potential for secondary metabolite biosynthesis across Gemmatimonadota, which may represent an unexplored source of novel natural products. Furthermore, viruses associated with Gemmatimonadota have the potential to “hijack” and manipulate host metabolism, including the assembly of the lipopolysaccharide in their hosts.ConclusionsThis expanded genomic diversity advances our understanding of these globally distributed bacteria across a variety of ecosystems and reveals genetic distinctions between those in terrestrial and marine communities.-srP_8yBftBuaKffV-GvkTVideo

The sources and transport model of deep-sea sediment in the Southwest Sub-basin of the South China Sea

The sources and transport model of deep-sea sediment in the Southwest Sub-basin of the South China Sea

Balancing the oceanic Zn isotope budget: The key role of deep-sea pelagic sediments

Abstract Oxygenated deep-sea pelagic sediments with Fe-Mn–oxide particles represent a key oceanic oxic sink for transition metals in the modern ocean. However, the isotopic composition of authigenic Zn in the pelagic zone remains poorly constrained, which hampers our understanding of the global budget of Zn isotopes. Here, we analyzed the Zn isotopic compositions of two deep-sea pelagic sediment columns collected from the Pacific Ocean. The results show that authigenic Zn in deep-sea sediments is primarily hosted by the Fe-Mn (oxyhydr)oxides. The light Zn isotopic signatures (δ66Zn: −0.02‰ to 0.34‰, n = 42; computed as the per mille deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) observed in deep-sea sediments are completely different from the previously assumed values of ~1.0‰ based on the Zn isotopic compositions of Fe-Mn crusts and nodules. Based on this observation (Zn flux of deep-sea sediments = 5.3 × 108 mol yr–1), we propose a new, balanced global budget for Zn isotopes.

Acoustic Rapid Detection Technology and Its Application for Rare Earth Element (REE)-Rich Sediments in the Pigafetta Basin of the Western Pacific

This study aims to investigate the stratigraphic features and rare earth element (REE) mechanisms of deep-sea REE-rich sediments in the West Pacific Pigafetta Basin using acoustic rapid detection technology. Through an analysis of sub-bottom profile data and synthesis of existing studies, this study reveals the acoustic properties and thickness distribution of the REE-rich sediments. Acoustic spectral records identify three distinct acoustic facies: opaque (O), transparent (T), and laminated (L). This study maps the thickness and spatial distribution of the REE-rich sediment layer in the research area, ranging from approximately 6 to 36 m in thickness. Regions with REE-rich sediments exceeding 30 m in depth are identified, showing concentrated distribution along the northwest–southeast axis and a contiguous zone in the southwest corner of the study area. The method employed in this study can determine the potential bottom boundary of the REE-rich layer by assessing the thickness range of the sedimentary layer, overcoming limitations of traditional sampling methods. Furthermore, the thickness distribution characteristics of the REE-rich sedimentary layer in the study area provide valuable insights for future research on resource evaluation and estimation.

Biogenic opal δ18O in marine deep-sea sediments through the Cenozoic Era and implications for ocean cooling

Benthic foraminifera δ18O increased by 5.4 ‰ through the Cenozoic Era, the past 65 million years of Earth's history. This increase is attributed to growth of global ice sheets and global ocean cooling. On the other hand, δ18O measurements of planktonic foraminifera do not exhibit significant change. These measurements initially were interpreted to suggest cool tropical and subtropical temperatures and introduced the concept of the “cool tropics paradox”, in which global bottomwater temperatures were significantly warmer but tropical surface temperatures were not during the early Cenozoic. This lack of change in δ18O of planktonic foraminifera was explained by secondary postburial alteration of the primary planktonic isotope signal.We provide an alternative record of sea surface water characteristics of Cenozoic Era by measuring δ18O of siliceous microfossils. This scarce phase of marine silica, unaltered biogenic opal-A, composed of diatoms, radiolaria and sponge spicules, was separated and purified from deep-sea Cenozoic sediments. We measured δ18O of siliceous microfossils from a set of drilled cores: DSDP 278 from the South Pacific, DSDP 513 from the South Atlantic, and ODP 1050–1053 and DSDP 391 from the subtropical North Atlantic. The purity of the biogenic opal and its mineralogical phase was assessed with SEM, SEM/EDS, XPS, and XRD.The δ18O of biogenic opal-A fluctuates around an average value of 43.0 ‰ through the Cenozoic Era, comparable to the Southern Ocean average of 43.2 ‰ of diatoms and radiolaria from the last 30,000 years which represent the last glacial and interglacial period and suggesting cold surface temperatures of 2–7 °C, both in the subtropical and polar latitudes. Taking into consideration changes in δ18O associated with bottomwater interaction with the lower crust and changes in sea level associated with waxing and waning of ice sheets, we conclude lack of or minimal change in the surface water temperature through the Cenozoic Era. This revives the original idea of the “cool tropics” paradox with a carbonate independent δ18O record and without issues associated with diagenesis of benthic and planktonic foraminifera.

Iron species and sulfur isotopic compositions of authigenic pyrite in deep-sea sediments at southern Hydrate Ridge, Cascadia margin (ODP Leg 204): implications for non-steady-state depositional and diagenetic processes

Two accretionary sediment sequences from Sites 1245 and 1252 recovered during Ocean Drilling Program (ODP) Leg 204 at southern Hydrate Ridge were investigated to explore the response of geochemical partitioning of iron and sulfur isotopic composition of authigenic pyrite to non-steady-state depositional and diagenetic scenarios. Five iron species were characterized by a modified sequential extraction procedure that covers almost all iron-bearing minerals in sediment cores, including: (1) iron-bearing carbonates, mainly siderite; (2) ferric (hydr)oxides, probably ferrihydrite and/or lepidocrocite; (3) magnetite; (4) iron-bearing silicates; and (5) pyrite. Highly reactive iron has been accumulated for a long-term steady-state history and its pyritization, to varying degrees, is limited by availability of dissolved sulfide. This causes pyrite and siderite occurred in the same sedimentary layer and shows an inverse relationship between their concentrations. From this, their proportions to highly reactive iron can be chosen for evaluating the degree of sulfidization. A significant change in sulfur isotopic composition of pyrite (-42.4 to +16.8‰ VCDT) indicates that the steady-state conditions are dramatically limited, where the δ34S values higher than -20‰ may result from an upward shift of SMT zone close to the seafloor or a sudden, massive depositional event. To explain the downcore sulfidization effects and pyrite δ34S values, we developed two categories of conceptual scenarios based on variations in sedimentation rate and methane flux. The geochemical features similar to those derived from each scenario were searched in the sediment columns and the non-steady-state events behind the scenarios were proved to be consistent with the real observations. Thus, iron species and pyrite δ34S values can be regarded as a proxy to differentiate different non-steady-state depositional and diagenetic controls on the sedimentary record.

Re-evaluating Marine Isotope Stage 5a paleo-sea-level trends from across the Florida Keys reef tract

Unraveling how Global Mean Sea Level (GMSL) fluctuated during past warm periods can improve our understanding of linkages between sea-level fluctuations, orbital forcing, and ice-sheet dynamics. Current estimates of GMSL for Marine Isotope Stages (MIS) 5a and 5c — two warm intervals following the relatively well-documented MIS 5e — contain meters of uncertainty and fewer data due to several challenges. These challenges include concealment of datable in-situ coral facies by MIS 1 deposits and inaccessibility due to submergence by modern sea level. We present a comprehensive dataset based on U–Th dating and stratigraphic correlation of 23 cores totaling over 170 m of recovered coral-reef deposits across the tectonically stable Florida Keys Reef Tract (FKRT). Following detailed facies descriptions, 34 in-situ, minimally altered aragonitic coral samples (≤2.7% calcite) below the Holocene-Pleistocene boundary were targeted for U–Th geochronology. Fourteen closed-system coral U–Th ages from MIS 5a include the commonly used sea-level indicator Acropora palmata, but also the massive coral taxa Pseudodiploria strigosa, Siderastrea siderea, Orbicella spp., and Porites astreoides. Dating yielded ages in the range of 88–81 ka (average 2σ uncertainty of less than 200 years). These ages suggest MIS 5a reef initiation at ∼88 ka BP, a peak near 83 ka with minimum elevations between −6.0 ± 0.5 and −5.6 ± 0.5 m MSL (2σ uncertainty and subsidence-corrected), and reef termination and sea-level fall by ∼81 ka BP. Notably, the range of peak MIS 5a relative sea-level estimates of −6.5 to −5.1 m MSL are more than 2 m shallower (higher) than previous estimates of −11 to −9 m. Our higher resolution regional sea-level reconstruction across four subregions of the Florida Keys reef tract aligns with changes in July insolation at 65° N: a trend that most other records, such as deep-sea sediments, do not have the accuracy and precision to resolve. Three massive coral samples from MIS 5c, consisting of Pseudodiploria clivosa, and Orbicella spp., yielded ages in the range of 104 to 99 ka (average 2σ uncertainty less than 200 years); however, because only one sample met the closed-system criteria, our ability to estimate MIS 5c sea level is relatively limited. More empirical estimates of sea-level from the MIS 5a and MIS 5c intervals based on numerical dating of reliable local sea-level constraints are critical for GMSL calculations and relating changes in sea-level amplitude and timing to global ice volume modeling and glacio-isostatic effects, all of which can improve predictions of future sea-level changes in coastal regions.