Hiroyuki Takata

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.

Abyssal Benthic Foraminifera in the Eastern Equatorial Pacific (IODP Exp 320) During the Middle Eocene

Abstract We report on the faunal transition of benthic foraminifera during the middle Eocene at Site U1333 (4862 m water depth, 3,560–3,720 m paleo-water depth) of Integrated Ocean Drilling Program Expedition 320 in the eastern equatorial Pacific Ocean. During the period ∼41.5–40.7 Ma, which includes carbonate accumulation event 3 (CAE-3), the benthic foraminiferal accumulation rate (BFAR) increased gradually and then it declined rapidly. In contrast, BFAR was considerably lower during ∼40.7–39.4 Ma, corresponding to the middle Eocene climatic optimum (MECO), and then it increased during ∼39.3–38.4 Ma, including CAE-4. Diversity (E [S200]) was slightly lower in the upper part of the study interval than in the lower part. The most common benthic foraminifera were Nuttallides truempyi, Oridorsalis umbonatus, and Gyroidinoides spp. in association with Globocassidulina globosa and Cibicidoides grimsdalei during the period studied. Quadrimorphina profunda occurred abundantly with N. truempyi, O. umbonatus, and G. globosa during ∼39.4–38.4 Ma, including CAE-4, although this species was also relatively common in the lower part of the study interval. Virgulinopsis navarroanus and Fursenkoina sp. A, morphologically infaunal taxa, were common during ∼38.8–38.4 Ma, corresponding to the late stage of CAE-4. Based on Q-mode cluster analysis, four sample clusters were recognized and their stratigraphic distributions were generally discriminated in the lower and upper parts of the study interval. Thus, there was only a small faunal transition in the abyssal eastern equatorial Pacific during the middle to late-middle Eocene. The faunal transition recognized in this study may be related to recovery processes following intense carbonate corrosiveness in the eastern equatorial Pacific during MECO. To reference Supplemental Data please click here.

Factors controlling typhoons and storm rain on the Korean Peninsula during the Little Ice Age

Documenting multi-decadal typhoon and storm-rain variability is useful to prevent future typhoon and flood disasters. We present the history of typhoon and storm-rain activity in East Asia inferred from multi-proxy analyses of Lagoon Hwajin-po sediments along the eastern coast of Korea. Anthropogenic effects were enhanced in Lagoon Hwajin-po since ca. AD 1900, by increasing farming in the catchment. To avoid these human-induced effects, we reconstructed the history of typhoon and storm-rain activity only for the interval AD 1400–1900. The record indicates that typhoon frequency throughout the Korean Peninsula varied in response to the state of the El Niño/Southern Oscillation. Typhoon variability was likely modulated further by the state of the East Asia summer monsoon (EASM) pattern, associated with variation in the magnitude of solar irradiance. During periods of minimum solar activity, such as the early Maunder Minimum (AD 1650–1675), typhoons struck the east China coast and Korean Peninsula more frequently because of a strengthened EASM.

Correlation between faunal transitions of benthic foraminifera and ballasting of particulate organic carbon by siliceous plankton during the Holocene off San-in district, southwestern Japan

We investigated the biotic response of benthic foraminifera (Rhizaria) to Holocene paleoceanographic changes in a upper bathyal environment by examining fossil benthic foraminifera younger than ~13 ka from core GH87-2-308 (316-m water depth) off San-in district, southwestern Japan. A non-metric multidimensional scaling (nm-MDS) analysis enabled the distinction of three main benthic foraminiferal assemblages and two faunal transitions. The phase 1 to 2 transition at 11.3 ka was characterized by a rapid decrease in species diversity followed by its gradual recovery and a shift toward present-day fauna. The phase 2 to 3 transition at ~7 ka was characterized by continuation of the shift toward present-day fauna as the Tsushima Warm Current developed and approached its present-day form. The nm-MDS axis 1 was negatively correlated with the radiolarian accumulation rate at this site and with the radiolarian and diatom accumulation rates of neighbor core D-GC-6, suggesting that benthic foraminifera responded to ballasting of particulate organic carbon (POC) with siliceous plankton skeletons. Furthermore, a decrease in the benthic foraminiferal accumulation rate during late phase 2 (~9–7 ka), when warm-water planktonic foraminifera were common, can be attributed to a decreased food supply from the surface ocean because of reduced paleoproductivity beneath a warmer, more stratified surface ocean, possibly accompanied by enhanced remineralization of POC, in response to strengthening of the Tsushima Warm Current. Thus, the benthic foraminifera at upper bathyal depth off San-in district appear to have responded to surface oceanographic changes, suggesting coupling of surface and bottom biota.

Latitudinal change in benthic foraminiferal fauna by ITCZ movement along the ~131°W transect in the equatorial Pacific Ocean

Modern and fossil benthic foraminifera were examined from nine surface sediments and two piston cores along the ~131°W transect in the equatorial Pacific Ocean. This study was conducted to clarify the biotic response of abyssal benthic foraminifera during the last 220 ka to changes in the seasonal extent of the Intertropical Convergence Zone (ITCZ). The abundance of modern benthic foraminifera was high at stations between the equator and 6°N, whereas it was low at stations north of 6°N, which is generally consistent with the latitudinal CaCO3 distribution of surface sediments. The northward increase of Epistominella exigua from the equator to ~6°N is similar to the seasonal variations in chlorophyll-a concentrations in the surface water and ITCZ position along ~131°W. This species was more common at core PC5103 (~6°N) than at core PC5101 (~2°N) after ~130 ka, when the Shannon-Wiener diversity (H’) between the two cores started to diverge. Hence, the presentday latitudinal difference in benthic foraminifera (E. exigua and species diversity) between ~2°N and ~6°N along ~131°W has been generally established since ~130 ka. According to the modern relationship between the seasonality of primary production and seasonal ITCZ variations in the northern margin of the ITCZ, the latitudinal divergence of benthic foraminiferal fauna between ~2°N and ~6°N since ~130 ka appear to have been induced by more distinct variations in the seasonal movement of ITCZ.

Distribution of living (stained) benthic foraminifera (Protista) in the Ohashi River, southwest Japan: a clue to recent faunal change in the Lake Shinji-Nakaumi system.

The modern distribution of benthic foraminifera in the Ohashi River (southwest Japan) was compared with the prevalent types of bottom substrates. Bottom substrates in the upper ranges of the river were characterized by relatively coarse-grained sediment, low total organic carbon (TOC) and total sulfur (TS) contents, whereas those in the lower ranges were characterized by relatively fine-grained sediment, high TOC and TS contents. Two macrobenthic species, Corbicula japonica and Musculista senhousia, were associated with both the coarse- and fine-grained bottom substrate areas, respectively. Ammonia beccurii forma 1 was the dominant foraminiferal species in the Ohashi River and Haplophragmoides canariensis was common in the middle reaches of the river. Variable salinity and biological substrate disturbance, caused especially by M. senhousia, accounted for this foraminiferal distribution. A transition from the H. canariensis assemblage to the A. beccarii forma 1 assemblage has been found in the 20 th century sedimentary record not only in the Ohashi River but also in Lake Shinji. Although the habitat of H. canariensis has not been specified well in the Lake Shinji-Nakaumi system, our result of the modern distribution of benthic foraminifera in the Ohashi River suggests that the faunal transition in Lake Shinji can be explained by the replacement of the dominant foraminiferal species in the Ohashi River.