Junichiro Kuroda

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian “Selli Level” black shale deposited during OAE1a in central Italy reveal two negative excursions in marine 187 Os/ 188 Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Level horizon, suggesting an order-of-magnitude increase in the global fl ux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratifi cation that triggered OAE1a.

Onset of Mediterranean outflow into the North Atlantic

The when of Mediterranean water outflow The trickle of water that began to flow from the Mediterranean Sea into the Atlantic Ocean after the opening of the Strait of Gibraltar turned into a veritable flood by the end of the Pliocene 2 to 3 million years ago. It then began to influence large-scale ocean circulation in earnest. Hernández-Molina et al. describe marine sediment cores collected by an ocean drilling expedition (see the Perspective by Filippelli). The results reveal a detailed history of the timing of Mediterranean outflow water activity and show how the addition of that warm saline water to the cooler less-salty waters of the Atlantic was related to climate changes, deep ocean circulation, and plate tectonics. Science, this issue p. 1244; see also p. 1228 Mediterranean outflow water began to enter the Atlantic and influence global ocean circulation by the late Pliocene. [Also see Perspective by Filippelli] Sediments cored along the southwestern Iberian margin during Integrated Ocean Drilling Program Expedition 339 provide constraints on Mediterranean Outflow Water (MOW) circulation patterns from the Pliocene epoch to the present day. After the Strait of Gibraltar opened (5.33 million years ago), a limited volume of MOW entered the Atlantic. Depositional hiatuses indicate erosion by bottom currents related to higher volumes of MOW circulating into the North Atlantic, beginning in the late Pliocene. The hiatuses coincide with regional tectonic events and changes in global thermohaline circulation (THC). This suggests that MOW influenced Atlantic Meridional Overturning Circulation (AMOC), THC, and climatic shifts by contributing a component of warm, saline water to northern latitudes while in turn being influenced by plate tectonics.

Marine osmium isotope record across the Triassic-Jurassic boundary from a Pacific pelagic site

The Triassic-Jurassic (T-J) boundary ca. 200 Ma represents one of the major mass extinction events of the Phanerozoic; however, the cause of this event remains controversial because of a paucity of geological evidence. In this study we present an isotopic record of osmium extracted from a bedded chert succession across the T-J boundary in the Kurusu section of Japan, deposited within a Paleo-Pacific (Panthalassa) deep basin. The data show a gradual decrease in seawater 187 Os/ 188 Os values during the Rhaetian, followed by a sharp increase in the latest Rhaetian, and a subsequent stable phase across the T-J boundary. The decreasing trend of 187 Os/ 188 Os values during the Rhaetian indicates a gradual increase in the relative supply rate of unradiogenic Os from the mantle associated with emplacement of the Central Atlantic Magmatic Province. The subsequent shift toward radiogenic values reflects an increased supply of radiogenic Os due to enhanced continental weathering. This interval marks more negative isotopic values of organic carbon, the onset of radiolarian faunal turnover, and conodont extinctions, indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the T-J biotic crisis.

Implication of spatiotemporal distribution of black shales deposited during the Cretaceous Oceanic Anoxic Event-2

ABSTRACT Oceanic Anoxic Events (OAEs) mark the contemporaneous deposition of organic-rich marine sediments termed “black shales” in the wide areas of the oceans. An anoxic event that occurred at the Cenomanian-Turonian boundary, OAE-2, has been recognized as one of the largest events in the Cretaceous. Carbon isotopic compositions (δ13C) of sedimentary carbonate and organic matter exhibit a positive excursion across the OAE-2, reflecting an enhanced burial rate of 13C-depleted organic carbon during the event. Here we compile a spatiotemporal distribution of black shales on the basis of their onset timings relative to the δ13C excursion as a time-control reference, and discuss the “spreading patterns” of black shale deposition. The patterns suggest that the deposition of black shales started from marginal regions of the southern North Atlantic and the Western Interior Seaway in North America, and spread to the northern North Atlantic and Tethys Sea. Strangely, the black shales whose onset corresponds to that of the δ13C excursion have not been found in many locations. Furthermore, extensive deposition of black shales in the Tethys and some sites in the North Atlantic occurred significantly after the major shift of the δ13C excursion. Sediments in the largely unexplored Pacific basin may be the missing link in the temporal relationship between the black shale deposition and the isotopic excursion.

Miocene to Pleistocene osmium isotopic records of the Mediterranean sediments

In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375–376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show 187Os/188Os values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have 187Os/188Os values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite 187Os/188Os is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterraneans drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower 187Os/188Os values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).

Alpine bogs of southern Spain show human-induced environmental change superimposed on long-term natural variations

Recent studies have proved that high elevation environments, especially remote wetlands, are exceptional ecological sensors of global change. For example, European glaciers have retreated during the 20th century while the Sierra Nevada National Park in southern Spain witnessed the first complete disappearance of modern glaciers in Europe. Given that the effects of climatic fluctuations on local ecosystems are complex in these sensitive alpine areas, it is crucial to identify their long-term natural trends, ecological thresholds, and responses to human impact. In this study, the geochemical records from two adjacent alpine bogs in the protected Sierra Nevada National Park reveal different sensitivities and long-term environmental responses, despite similar natural forcings, such as solar radiation and the North Atlantic Oscillation, during the late Holocene. After the Industrial Revolution both bogs registered an independent, abrupt and enhanced response to the anthropogenic forcing, at the same time that the last glaciers disappeared. The different response recorded at each site suggests that the National Park and land managers of similar regions need to consider landscape and environmental evolution in addition to changing climate to fully understand implications of climate and human influence.

Response of the Pacific inter-tropical convergence zone to global cooling and initiation of Antarctic glaciation across the Eocene Oligocene Transition

Approximately 34 million years ago across the Eocene–Oligocene transition (EOT), Earth’s climate tipped from a largely unglaciated state into one that sustained large ice sheets on Antarctica. Antarctic glaciation is attributed to a threshold response to slow decline in atmospheric CO2 but our understanding of the feedback processes triggered and of climate change on the other contents is limited. Here we present new geochemical records of terrigenous dust accumulating on the sea floor across the EOT from a site in the central equatorial Pacific. We report a change in dust chemistry from an Asian affinity to a Central-South American provenance that occurs geologically synchronously with the initiation of stepwise global cooling, glaciation of Antarctica and aridification on the northern continents. We infer that the inter-tropical convergence zone of intense precipitation extended to our site during late Eocene, at least four degrees latitude further south than today, but that it migrated northwards in step with global cooling and initiation of Antarctic glaciation. Our findings point to an atmospheric teleconnection between extratropical cooling and rainfall climate in the tropics and the mid-latitude belt of the westerlies operating across the most pivotal transition in climate state of the Cenozoic Era.

A new high-precision method for determining stable chlorine isotopes in halite and igneous rock samples using UV-femtosecond laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry

We report a new, rapid method for the high-precision determination of chlorine isotope ratios in halite and AgCl pellets formed from seawater and igneous rock samples. Use of 266 nm ultra violet-femtosecond laser ablation (UV-FsLA) allowed quantitative sampling of halite and AgCl and enabled precise determination of 37Cl/35Cl isotope ratios (δ37Cl) when coupled with a multiple Faraday collector-inductively coupled plasma mass spectrometer (MFC-ICPMS). We used 36Ar+/38Ar+ as an external standard for the mass bias corrections between 39K+–41K+, 36Ar1H+–38Ar1H+–40Ar1H+, and 35Cl+–37Cl+ with isobaric overlap corrections between K+, ArH+, and Cl+ ions. Overlap of the sulfur (36S+) isobar with 36Ar+ was indirectly monitored and corrected by baseline modelling using the 36Ar+/38Ar+ measurement. These procedures collectively helped accomplish an accurate and high-precision measurement of 37Cl+/35Cl+ ratios. Using the proposed analytical method, δ37Cl ratios in natural halite samples were analysed by direct laser ablation. δ37Cl in JB-1a and JB-3 igneous rocks were analysed as AgCl powder pellets produced by pyrohydrolytic separation and co-precipitation of the separated Cl with silver. The external reproducibility of the δ37Cl measurements was ±0.2‰ 2SD (2 standard deviations) for halite and ±0.4‰ 2SD for AgCl precipitate rivalling that of gas source isotope ratio mass spectrometry. The new analytical protocol enabled a precise and rapid δ37Cl analysis of igneous rock samples as AgCl with as little as 4 μg chlorine from the ∼500 μg chlorine separated. This is the first time that high-precision in situ determination of δ37Cl from halite using 4 μg of chlorine has been reported.

An X‐ray spectroscopic perspective on Messinian evaporite from Sicily: Sedimentary fabrics, element distributions, and chemical environments of S and Mg

The Messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the Mediterranean basin at 5.97–5.33 Ma. The interpretation of the facies and stratigraphic associations of the Messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale. In this study, the spatial distributions of Na, Mg, S, O, Si, and Al in a potassic-magnesian salt and a halite layers of Messinian evaporites from the Realmonte mine on Sicily were determined using synchrotron based micro-X-ray fluorescence. The dominant molecular host site of Mg and S obtained by X-ray absorption near edge structure (XANES) is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals, thus shedding light on evaporative concentration processes in the Caltanissetta Basin of Sicily. Mg and S K-edge XANES spectra revealed the presence of highly soluble Mg-bearing sulfates. The massive halite layer “unit C,” contains less soluble minerals, thus did not exceed the stage of halite crystallization. We infer that as evaporative concentration increased, the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values, creating brines that were oversaturated with Mg-sulfate. Density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines. We propose lateral advection of dense Mg-sulfate brines that certainly affected marine biota.