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Introduction
Keiji Horikawa currently works at Faculty of Science, Academic Assembly, University of Toyama. Keiji does research in Paleoceanography, Climatology and Geochemistry.
Education
April 2003 - June 2006
Publications
Publications (66)
Under sustained global warming, Arctic climate is projected to become more responsive to changes in North Pacific meridional heat transport as a result of teleconnections between low and high latitudes, but the underlying mechanisms remain poorly understood. Here, we reconstruct subarctic humidity changes over the past 400 kyr to investigate the ro...
Dissolved Rare Earth Elements (REEs) concentrations have been widely used in geochemical studies due to their systematic changes in the environment, acting as tracers in various natural processes. In addition to the usefulness of naturally controlled chemical REE fractionations used in the ocean, the extraction and measurement procedures of seawate...
Recent silicoflagellate distributions have been examined in 195 surface sediment samples from the North Pacific to establish the silicoflagellate assemblage dataset from surface sediments across the North Pacific for paleo sea surface temperature (SST) reconstruction. The relative abundance data of all seven silicoflagellate taxa identified in this...
Many studies have applied the multi-element standard (matrix- and non-matrix-matched) and or bracketing standard approach as the calibration strategy to determine Rare Earth Element (REE) concentrations. In this study, we developed an in-house offline preconcentration procedure using Nobias Chelate-PA1 resin to eliminate matrix elements and preconc...
The investigation of triggers causing the onset and intensification of Northern Hemisphere Glaciation (NHG) during the late Pliocene is essential for understanding the global climate system, with important implications for projecting future climate changes. Despite their critical roles in the global climate system, influences of land-ocean interact...
The East China Sea (ECS) is one of the largest marginal seas in the world with high primary productivity and a large area of low dissolved oxygen. This study observed the low‐oxygen bottom water on the outer‐edge shelf of ECS in summer seasons of 2018–2020. The contributions of various water masses to low‐oxygen waters were quantified, and the inte...
Subtropical mode water (STMW) plays a pivotal role in material exchange and influences marine ecosystems in the shallow layers of the western Pacific boundary current area (WPBCA, including the Kuroshio and its adjacent marginal seas), one of the typical western boundary areas in the global ocean. However, since the Kuroshio with high velocity prev...
The Last Interglacial (LIG: 130,000–115,000 years ago) was a period of warmer global mean temperatures and higher and more variable sea levels than the Holocene (11,700–0 years ago). Therefore, a better understanding of Antarctic ice-sheet dynamics during this interval would provide valuable insights for projecting sea-level change in future warmin...
Salinity–oxygen isotope (δ18Osw) relationships in sea surface waters are fundamental to the quantitative interpretation of foraminiferal and coral δ18O records in the analyses of past El Niño–Southern Oscillation (ENSO) variation. However, available δ18Osw data for the western South Pacific are limited to the Geochemical Ocean Sections Study (GEOSE...
The orbital-scale variability of the Indian summer monsoon (ISM) has been influenced by multiple factors, such as atmospheric CO2 concentration, global ice volume, and insolation. Proxies for weathering activity and paleo-productivity provide potential insights into the driving forces of its variability. We documented multi-proxy data at IODP Site...
During Heinrich Stadial 1 (HS1, 18.0–14.7 kyr ago), the Bering and Okhotsk Seas in the western Subarctic Pacific (SAP) exhibited boundary conditions that enhanced intermediate water convection. However, little is known about the quantitative contribution of the intermediate waters from the Bering and Okhotsk Sea (BSIW and OSIW) to ventilation in th...
X-ray fluorescence (XRF) core scanner elemental count data are useful for high-resolution paleoceanographic studies. However, because several factors, such as changes in physical core properties, significantly affect element count intensities, the appropriate calibration of the count data is required. Besides, the existing approaches for calibratio...
During the last deglaciation, the Cordilleran Ice Sheet (CIS) was rapidly melted and supplied a huge amount of meltwater to the coastal areas in the northeast Pacific. The timing of decay of the CIS and its impact on productivity in adjacent coastal areas in the northeastern North Pacific is not well known and has been under investigation. Here, to...
We revisited the stratigraphy at Deep Sea Drilling Project (DSDP) Site 296, which has promise as a reference sequence for the mid-latitude western North Pacific. We constructed the biostratigraphy (calcareous nannofossils, planktic foraminifera, and radiolaria) and chemostratigraphy (strontium isotope ratios, and stable oxygen and carbon isotope ra...
Cyclic changes in volume transport of the Tsushima Warm Current (TWC) have been argued from diatom records in the southern Japan Sea off the Japanese islands during the Holocene. Although this phenomenon has not yet been confirmed by various proxy data, determining whether or not these oceanographic changes occurred is crucial for clarifying the na...
International Ocean Discovery Program Expedition 355 performed a 1109.4 m penetration drill at Site U1456 in the Laxmi Basin of the Eastern Arabian Sea. The uppermost section (~121 m thick) was analyzed for the paleoceanographic change in response to the orbital-scale Indian monsoon. Correlation of δ¹⁸O values of planktonic foraminifera (Globigerin...
Sr/ ⁸⁶ Sr ratios and εNd values of detrital particles at International Ocean Discovery Program (IODP) Site U1456 in the Laxmi Basin of the Arabian Sea were measured to trace changes in sediment provenance over glacial–interglacial cycles. Based on the correlation of planktonic foraminiferal ( Globigerinoides ruber ) δ ¹⁸ O fluctuations with the LR0...
The Japan Sea in the western North Pacific was connected to the deep Pacific via deep seaways before it became a semi-closed marginal sea. However, the timing of the semi-closure and its cause(s) remain debatable. To further constrain the timing, for the first time we analyzed Nd isotopic compositions of fossil fish teeth and fish debris deposited...
Mg/Ca ratios and oxygen isotope ratios (δ¹⁸O) in biogenic high‑magnesium calcite tests precipitated by reef-dwelling large benthic foraminifers can be used as paleotemperature proxies because they correlated well with the temperature of seawater. However, it is still unclear whether there is an interspecies difference in temperature correlation amo...
Magnetic biomonitoring techniques and in situ topsoil magnetic susceptibility measurements have been shown to be rapid, cost-effective, and useful methods for investigating roadside pollution. However, combustible vegetation in samples makes it very difficult to use them in high-temperature magnetic experiments although the thermal alteration of sp...
The δ¹⁸Osw (seawater δ¹⁸O) and its relationship with salinity are principal information for analyzing mixing of water masses in the ocean, as well as reconstructing paleo-salinity from biogenic carbonate. Although δ¹⁸Osw–salinity equations have been reported in the Sea of Japan, it's unclear whether such equations are applicable in the Tsushima Cur...
Oxygen isotope ratios (δ18O) and Mg/Ca ratios in organically produced calcium carbonate (calcite and aragonite) by marine species have been used as proxies for paleotemperature. The relationship between the components of large benthic foraminiferal tests (high-magnesium calcite) and seawater temperature has been little investigated yet. We conducte...
Background:
Although cis-diamminedichloro-platinum (CDDP) exhibits strong therapeutic effects in cancer chemotherapy, its adverse effects such as peripheral neuropathy, nephropathy, and vomiting are dose-limiting factors. Previous studies reported that chronotherapy decreased CDDP-induced nephropathy and vomiting. In the present study, we investig...
The paired analyses of the Mg/Ca ratio and oxygen isotopic composition ( δ 18Oc) of surface-dwelling planktonic foraminifera have become a widely used method for reconstructing the oxygen isotopic composition of ambient seawater ( δ 18Osw) as a robust proxy for surface salinity. Globigerinoides ruber ( G. ruber) is a mixed-layer dweller, and its fo...
Warming of high northern latitudes in the Pliocene (5.33-2.58 Myr ago) has been linked to the closure of the Central American Seaway and intensification of North Atlantic Deep Water. Subsequent cooling in the late Pliocene may be related to the effects of freshwater input from the Arctic Ocean via the Bering Strait, disrupting North Atlantic Deep W...
Environmental magnetic techniques have been shown to be highly useful for investigating roadside pollution in Europe, North America and Asia. However, no studies have reported such magnetic monitoring in Japan. Here we report environmental magnetic results along the Tateyama-Kurobe Alpine route at the Bijyodaira area of Mt. Tateyama in Toyama, whic...
A novel exopolysaccharide-producing bacterium, designated as strain SE3T, was isolated from Pacific Ocean sediment. The strain was Gram-negative, motile, strictly aerobic, oxidase-positive and catalase-positive, and required Na+ for growth. Its major isoprenoid quinone was ubiquinone-8 (Q-8), and its cellular fatty acid profile mainly consisted of...
Increased emission of acidic pollutants on the Asian continent and long-distance transport of such pollutants have caused soil acidification in eastern Asia. In Japan, Hokuriku district experiences high deposition of acidic pollutants, and soil acidification and acidic fog have been observed at Mt. Tateyama in central Japan. To preserve the Mt. Tat...
We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pa-cific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sedi-ment with a high sedimentation rate. The 14 C data on frag-ile and robust planktic foraminiferal shells were concordant wi...
We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pacific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sediment with a high sedimentation rate. The <sup>14</sup>C data on fragile and robust planktic foraminiferal shells were concor...
Neodymium (Nd) isotopes have become an accepted water mass tracer for paleoceanographic reconstructions; however, high rates of radiogenic Nd input from volcanic islands/arcs in the western equatorial Pacific (WEP) and the subarctic North Pacific (SANP) can significantly alter the isotopic composition of ambient seawaters, leading to questions abou...
During the last deglaciation, atmospheric carbon dioxide concentrations rose at the same time that the Delta14C of that CO2 fell. This has been attributed to the release of 14C-depleted carbon dioxide from the deep ocean, possibly vented through the Southern Ocean. Recently, a sediment record from the eastern North Pacific Ocean spanning the last d...
The n-alkane C31/(C29+C31) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or arid...
Changes in flux and locality of the overturning circulations may have large effects on marine ecosystem and CO2 exchange between ocean and atmosphere. However, unlike the Atlantic, little is known about ocean circulation status under glacial boundary conditions in the North Pacific (NP), especially in regard to intermediate water circulation and it...
Across-shelf variations in thickness, grain size, and frequency of sandstone beds in a transgressive outer-shelf succession were investigated from the Middle Pleistocene (ca. 0.7Ma) Kakinokidai Formation on the Boso Peninsula, Japan. The transgressive deposits are generally muddy and contain slumps and slump scars. The intercalated sandstone beds a...
By measuring nitrogen isotopic compositions (d15N) of nitrate, particulate organic matter, and sediments in the ocean, the behavior of oceanic nitrogen and related characteristics of d15N values have been clarified. Based on these findings in the moder ocean, we have inferred the past changes in N2 fixation, denitrification, and relative nitrate ut...
Paleoceanographic studies in the southern hemisphere high latitudes are important for understanding the global climate system. These detailed records of the oceanic response to various climate conditions are well- documented in marine sediments. The main goal of this study is to construct the precise chronostratigraphy and to reveal the paleocenogr...
This study reconstructs productivity evolution in the North Equatorial Countercurrent (NECC) in the eastern equatorial Pacific and verifies its association with ocean-atmospheric changes modulated by precession and other controlling factors on productivity. Alkenone abundance and total organic carbon are regarded as reliable proxies of marine produ...
N2 fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic
nitrate inventory. However, it is not known how, when, and where N2 fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition
(δ15N) over the last 83 k...
We present the new alkenone-based sea-surface temperature (SST) records from the eastern equatorial Pacific over the past 150 kyr. Core HY04 (4°N) at the North Equatorial Countercurrent (NECC) registered the invariant SSTs (25.8–27.2°C), whereas core HY06 (0°N) registered cyclic glacial-interglacial SST variations (22.4–26.1°C). Although HY06 alken...
We collected three piston cores on a south-north transect crossing the equator in the eastern equatorial Pacific (EEP) to reconstruct paleoceanographic environments (e.g., SST, nutrient state, productivity, etc) in the EEP. We have generated organic carbon and bulk nitrogen stable isotopic compositions and alkenone-derived SST records at HY04, HY06...
Sequence-stratigraphic signatures of hemipelagic siltstones were investigated using profiles of the magnetic susceptibility and selected chemical composition of the Early Pleistocene deep-water successions of the Kiwada and Otadai Formations on the Boso Peninsula, Japan. In the context of an independently developed sequence-stratigraphic framework...
Millennial-, century-, and decadal-scale stratigraphic cyclicities are described from transgressive shelf successions of the Middle Pleistocene Ichijiku and Kakinokidai Formations (ca. 0.7 Ma) in the Boso Peninsula, Japan. The Ichijiku Formation (ca. 400 m thick) represents a shelf sandridge complex developed under the influence of the large meande...
The Plio-Pleistocene (ca. 2.4-0.45Ma) Kazusa Group on the Boso Peninsula is an infill of the Kazusa forearc basin and is characterized by a thick succession (ca. 3, 000m thick) of shallow marine and deep-water siliciclastic sediments intercalated with many volcanic ash beds. Here, we briefly summarize the significance of the Kazusa Group as a geolo...
Millennial-, century-, and decadal-scale cyclicities were identified in a transgressive shelf succession of the Middle Pleistocene (ca. 0.7 Ma) on the Boso Peninsula, Japan. The shelf succession consists mainly of a sandridge complex and associated outer shelf deposits. The deposition was interpreted to have been controlled by fluctuation in speeds...
ABSTRACT BODY: The IODP Expedition 323 in the Bering Sea was the first expedition to recover continuous marine sediment sections to investigate high-resolution paleoceanography at the most northern region of the North Pacific during 5 Ma. Drill sites explored in the Bowers Ridge region (Sites U1340 and U1341) provide complete sequences for the last...