Rare earth elements and yttrium (REY) have a distinct distribution pattern in seawater, and this pattern may be faithfully preserved in carbonate sediments and rocks. Anomalous concentrations of ...redox-sensitive cerium (Ce) compared with neighbouring REY originate in oxic water column conditions, and as such, Ce anomalies can provide a potentially useful redox proxy in carbonate-dominated marine settings. The methods used to extract REY from carbonates vary widely, and may suffer from widespread leaching of REY from accessory non-carbonate minerals and organic matter, limiting the application of Ce anomalies for palaeo-redox reconstruction. We have systematically compared different methods on 195 carbonate samples with varying purity (% carbonate) from both modern and ancient environments. We used sequential leaching experiments in nitric acid to identify the most ‘seawater-like’ portion of the carbonate sample where contributions from non-carbonate minerals and organic matter are minimised. We also compared the results of sample dissolution in different types and strengths of acid. Our results confirm that REY concentrations can be inadvertently contaminated by partial leaching of clays and Fe (oxyhydr)oxides during a single-step digestion, and we suggest a pre-leach of 20% of the sample, followed by a partial leach of 40% of the sample to selectively dissolve carbonate. We suggest that REY studies are optimised in carbonates with >85% CaCO3, and show that dolomites behave differently during the leaching process and must be treated separately. We present REY patterns for modern carbonate-rich sediments from a range of environments, and show that seawater REY are faithfully preserved in some non-skeletal carbonate, but modified leaching procedures are necessary for impure, unlithified or organic rich carbonate sediments. We combine REY with Fe-speciation data to identify how Fe oxides and clays contribute to the REY signal and explore how the two proxies can be used together to provide a complex and high-resolution redox reconstruction in carbonate-dominated marine environments.
Extreme climate warmth (hyperthermal) events through deep-time offer prescient insights into how the Earth may respond to present-day warming related to greenhouse gas emissions. This special issue ...deals with Paleoenvironmental changes across the Mesozoic–Paleogene hyperthermal events and comprises 25 interdisciplinary research articles. In this review paper, we summarise the contents of the special issue, placing it into a wider context, and demonstrate that Mesozoic–Paleogene hyperthermal events were among the most devastating and extreme climate modes in the geological record. Key findings are as follows: (1) Multi-proxy geochemical and sedimentological analyses reveal that widespread deoxygenation of oceans and megalakes was a common accompanying feature of most hyperthermals. (2) Evidence exists for complex linkages between volcanism, warm climate conditions, contemporary carbon cycles, aquatic biogeochemical cycles, wildfire activities, and climate-modulated hydrological and terrestrial weathering changes operating at seasonal, orbital and/or tectonic timescales. (3) Pronounced and rapid biological turnovers in the ocean during hyperthermals may have been linked to seawater acidification and shifts in nutrient availability, while promoting significant alterations in primary productivity, biological pump and ecosystem structures. Despite these advances, future interdisciplinary studies are needed to deliver a more comprehensive understanding of the nature and mechanism of complex environmental interactions within the Earth system, as well as the internal and external drivers that may have triggered hyperthermal events.
•Marine and lacustrine deoxygenation was a common feature of Mesozoic–Paleogene hyperthermal events.•The temporal link is often observed between hyperthermals and Large igneous province emplacement and intense volcanism.•Large-scale greenhouse gas emissions induced strong positive climate feedbacks and global–scale biogeochemical extremes.
Molar tooth structures (MTS) comprise calcite microspar-filled voids in fine-grained shallow-water carbonate, and were commonly formed in the Mesoproterozoic and early Neoproterozoic. However, the ...origin of MTS and links between the temporal distribution of MTS and contemporaneous seawater chemistry remains poorly understood. Here we report elemental and isotopic data for MTS and host rocks from the Mesoproterozoic Gaoyuzhuang Formation (∼1,600–1,550 Ma), North China Craton. The results reveal similar C, S and Sr isotope signatures between MTS and host rocks, which are close to the isotopic compositions of contemporaneous global seawater, suggesting an early diagenetic, seawater-buffered origin for MTS. A small sulfur isotopic fractionation between seawater sulfate and pyrite (Δ34SCAS-Py) of 4.1 ± 1.5‰ in host rocks is consistent with previously reported data, providing support for sub-millimolar sulfate concentrations in Mesoproterozoic seawater. Our observations suggest that the widespread occurrence of MTS through the Mesoproterozoic to early Neoproterozoic was broadly linked to sulfate scarcity in the ocean. We further propose that in Proterozoic oceans with sub-millimolar seawater sulfate concentrations, where aerobic and anaerobic methane oxidation was likely inhibited, methane produced via methanogenesis may have been more prone to accumulate in sediments, creating voids during escape. The absence of MTS across periods of higher sulfate concentrations during the Palaeoproterozoic and after the mid-Neoproterozoic, suggests that elevated sulfate concentrations promoted consumption of methane via anaerobic methane oxidation, thus preventing methane accumulation and the formation of sediment voids. Rapid lithification of the substrate as a result of elevated carbonate saturation may have also hindered the formation of MTS during these intervals. The link between MTS and changes in both oceanic sulfate levels and benthic methane fluxes gives a new perspective on temporal fluctuations in Earth’s redox state through time.
The late Ediacaran to early Cambrian interval witnessed extraordinary radiations of metazoan life. The role of the physical environment in this biological revolution, such as changes to oxygen levels ...and nutrient availability, has been the focus of longstanding debate. Seemingly contradictory data from geochemical redox proxies help to fuel this controversy. As an essential nutrient, nitrogen can help to resolve this impasse by establishing linkages between nutrient supply, ocean redox, and biological changes. Here we present a comprehensive N-isotope dataset from the Yangtze Basin that reveals remarkable coupling between δ
N, δ
C, and evolutionary events from circa 551 to 515 Ma. The results indicate that increased fixed nitrogen supply may have facilitated episodic animal radiations by reinforcing ocean oxygenation, and restricting anoxia to near, or even at the sediment-water interface. Conversely, sporadic ocean anoxic events interrupted ocean oxygenation, and may have led to extinctions of the Ediacaran biota and small shelly animals.
The early Toarcian of the Early Jurassic was characterized by an interval of global warming, enhanced continental weathering, and an oceanic anoxic event (T-OAE, ∼183 Ma). However, the changes in ...continental weathering intensity in low latitudes still remain poorly constrained. Here, we present strontium isotope (87Sr/86Sr) data of the Pliensbachian–Toarcian (Pl–To) interval from the Tibetan Himalaya, which was formerly located in the tropical/subtropical zone of the southeastern Tethys. An abrupt positive shift in 87Sr/86Sr starting around the Pl–To boundary and continuing during the interval of the negative carbon-isotope excursion (CIE) suggests enhanced continental weathering and increased terrigenous influx. Stratigraphically higher, the 87Sr/86Sr ratios gradually decrease during the recovery phase of the T-OAE CIE, suggesting a decline in continental weathering. Notably, the absolute values and positive-recovery pattern of 87Sr/86Sr data from bulk carbonate (pure micrite) of this study are not consistent with that from belemnites and brachiopods of Europe. Based on strong relationship between 87Sr/86Sr and diagenetic and silicate-derived Sr proxies, the absolute 87Sr/86Sr values here could have been affected to some extent by diagenesis and regional terrestrial input, and thus greatly amplified relative to those of Europe. However, the strontium-isotope positive-recovery pattern broadly corresponds with the pattern and interpretation of other weathering proxies (δ44/40Ca and 187Os/188Os) during the T-OAE. These observations suggest that the 87Sr/86Sr trend still likely reflects the real regional signal but was superimposed by global influence, although the data were biased by diagenesis. Taken together, the 87Sr/86Sr records from Tibetan Himalaya probably provide a useful insight into the regional weathering response, superimposed by the global weathering signal, to the events of the early Toarcian.
•Tibetan 87Sr/86Sr data show the positive-recovery pattern and high absolute values.•The newly reported 87Sr/86Sr data from Tibet are inconsistent with those of Europe.•The 87Sr/86Sr from Tibet offer insight into regional weathering responses to T-OAE.•The mechanism for the Toarcian spatially heterogeneous 87Sr/86Sr remains equivocal.
We theoretically propose a method to measure the one-dimensional harmonic trap frequency with the Bose–Einstein condensate (BEC) in this trap. In contrast to measuring the frequency using ultracold ...atoms oscillating in a harmonic trap, two Kapitza–Dirac pulses were applied to the BEC. These Kapitza–Dirac pulses act as beam splitters, which divide the BEC into modes with different momenta and evolutionary paths. The weak interatomic interaction causes each mode to have a different distribution of ultracold atoms. According to the density distribution function without any interatomic interactions, the sensitivity of measuring the harmonic trap frequency can reach
10
-
7
, which can be improved by increasing the number of modes. For a system with weak interatomic interactions, the attractive interactions of Bose–Einstein condensation can slightly improve the sensitivity. Increasing the number of modes can reduce the negative influence of weak interatomic interactions on the sensitivity.
The early Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was characterized by marine deoxygenation and the burial of organic-rich sediments at numerous localities worldwide. However, the extent of ...marine anoxia and its impact on the sulfur cycle during the T-OAE is currently poorly understood. Here, stable sulfur isotopes of reduced metal-bound sulfur (δ34Spyrite) and pyrite sulfur concentrations (SPY) have been analyzed across the Pliensbachian-Toarcian boundary (Pl-To) and the T-OAE from the Sakahogi and Sakuraguchi-dani sections (Japan), which were deposited in the deep and shallow Panthalassic Ocean, respectively. Our data reveal marked positive δ34Spyrite excursions of >10‰ across both the Pl-To and the T-OAE at Sakahogi, coincident with increases in SPY, and a positive excursion of >20‰ at the onset of the T-OAE at Sakuraguchi-dani. Whilst the development of deep-water anoxic/euxinic conditions could have resulted in an enhanced burial of pyrite, and also partly contributed to the positive excursion of δ34Spyrite, variations in δ34Spyrite at Sakahogi were most likely controlled by elevated export production and/or preservation. On the shallow shelf generally low and highly variable SPY and the positive shift in δ34Spyrite were likely attributable mainly to elevated sedimentation rates, with redox playing only a minor role in controlling pyrite abundance. Our discovery of a positive δ34Spyrite excursion across the Pl-To at Sakahogi indicates a hitherto unrecognized perturbation to the deep-water sulfur cycle, potentially associated with increased seafloor organic matter flux and pyrite burial at this time, consistent with a transient interval of anoxia.
•First pyrite S-isotope (δ34Spyrite) data across the Toarcian OAE (T-OAE)•Positive pyrite S-isotope excursions across T-OAE from deep and shallow Panthalassa•Excursions mainly attributable to local depositional conditions.•Deep-water sulfur isotope excursion across Pliensbachian-Toarcian boundary
The delivery of potentially reactive iron minerals to the marine environment exerts a major control on ocean and marine sediment biogeochemistry, including the preservation and burial of organic ...carbon. One aspect of the global iron cycle that has received little attention concerns the phase partitioning of iron in highly chemically-weathered sediments from (sub)tropical mountainous regions, where the sediment is commonly deposited directly onto adjacent continental margins. Here, we report the phase partitioning of particulate iron in surface seafloor sediments deposited from oxic bottom waters along the east coast of China, including the Bohai Sea, Yellow Sea and East China Sea, in addition to the continental shelf region adjacent to the (sub)tropical mountainous island of Hainan in the northern South China Sea. We document highly reactive (FeHR) to total iron (FeT) ratios for the Bohai Sea, Yellow Sea and East China Sea that are typical of global marine sediments deposited under oxic water column conditions. By contrast, FeHR/FeT ratios in Hainan offshore sediments are significantly elevated above normal oxic marine sediments. These Hainan offshore sediments display a strong positive correlation between FeHR/FeT ratios and the chemical index of alteration (CIA) weathering proxy, suggesting that the Fe speciation systematics are controlled by the intensity of chemical weathering. This observation is supported by additional geochemical data and Nd isotope analyses, which suggest a common source lithology for all the studied regions. The Hainan sediments thus document a specific scenario, whereby highly chemically weathered sediments from the mountainous Hainan region bypass the inner shore settings that commonly disproportionately trap Fe (oxyhydr)oxide minerals, depositing high concentrations of FeHR on the adjacent continental margin. Such enrichments likely aid in the preservation of organic carbon in marine sediments, suggesting that organic carbon preservation and burial may be enhanced by elevated FeHR in such settings. We also establish a set of essential criteria for potential recognition of chemical weathering-induced enrichments in FeHR/FeT ratios in ancient marine sediments.
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