Tadamichi Oba

Holocene variability of the Kuroshio Current in the Okinawa Trough, northwestern Pacific Ocean

Abstract Cores 255 and B-3GC from the southern and northern Okinawa Trough, respectively, were studied for determining the variability of the Kuroshio Current on centennial scales during the Holocene based on detailed AMS 14 C dates, stable isotope, and planktonic foraminiferal distribution. The main flow of the Kuroshio Current was inferred to re-enter the Okinawa Trough at ∼7.3 calendar (cal.) kyr BP, leading to abrupt changes in sedimentation rate, remarkably increased abundance of the Kuroshio Current indicator Pulleniatina obliquiloculata , increased sea surface temperature (SST) and depth of thermocline (DOT). During ∼4.6–2.7 cal. kyr BP, the abundance of P. obliquiloculata sharply decreased, corresponding to a decreases in SST and DOT, implying that the influence of the Kuroshio Current weakened at that time, possibly as a result of the intensified winter monsoon. Significantly, the Kuroshio Current proxies (e.g. the difference in SST between the southern and northern Okinawa Trough) display periodicities of ∼1500 yr and ∼700–800 yr (ascribed to the second harmonic of the 1500 yr cycle in oceanic thermohaline circulation) during the Holocene. The Holocene events (at ∼0.6, 1.7, 3.3, 4.6, 5.9, 8.1, 9.4 cal. kyr BP) in the Okinawa Trough appear to be the most recent manifestation of the millennial scale climate cycle, recorded in the North Atlantic, Arabian Sea marine sediment cores and Greenland ice cores, suggesting global climatic tele-connections.

Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary

0indices) and d 18 O of planktonic foraminifer calcite (d 18 Ofc) across late Pleistocene sapropel intervals show that d 18 Ofc decreased (between 1 and 4.6%) and SST increased (between 0.7� and 6.7� C). Maximal d 18 Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18 Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ‘‘glacial’’ sapropel S6; its largest d 18 Oseawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 1620 Global Change: Climate dynamics (3309); 4267 Oceanography: General: Paleoceanography; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Mediterranean Sea, sapropels, sea surface temperatures, oxygen isotopes, Quaternary Citation: Emeis, K.-C., et al., Eastern Mediterranean surface water temperatures and d 18 O composition during deposition of sapropels

Caribbean sea surface temperatures: Two‐to‐three degrees cooler than present during the Little Ice Age

We determined the seasonal environmental conditions during the Little Ice Age (LIA) by interpreting isotope proxies in the coral skeleton of Montastrea faveolata from the northeast Caribbean. The oxygen isotope composition was determined for three time intervals during the LIA (1700–1705, 1780–1785, 1810–1815), thought to correspond to the coldest intervals. The period 1984–1989 was used to represent modern calibration conditions. We determined that SSTs from the LIA intervals are nearly 2–3°C cooler than present. LIA cooling in the Caribbean may have resulted from regional oceanic and atmospheric circulation differences, especially in winter. We propose that a trough of cold air from the north Atlantic may have extended farther south than present, into the northern Caribbean. Although there are indications from Pacific corals that seasonality was greater during the LIA, this does not seem to be the case for the Caribbean.

A 36 kyr geochemical record from the Sea of Japan of organic matter flux variations and changes in intermediate water oxygen concentrations

Intervals of organic C- and carbonate-rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal δ18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.

Glacial Holocene environment of the southeastern Okhotsk Sea: evidence from geochemical and palaeontological data

Abstract Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 14 C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 14 C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.

The 1500-year climate oscillation in the midlatitude North Pacific during the Holocene

Suborbital climate variability during the last glacial period is suggested to have involved a 1500-year pacing cycle, but the expression and spatial distribution of the ~1500-year oscillation during interglacials remains unclear. We generated a multidecade resolution record of alkenone sea surface temperature (SST) in the northwestern Pacific off central Japan during the Holocene. The SST record showed centennial and millennial variability with an amplitude of ~1 °C throughout the entire Holocene. Spectral analysis for SST variation revealed a statistically significant peak with 1470-year periodicity. The SST variation partly correlated with the variations of ice-rafted hematite-stained grain content in North Atlantic sediments. These findings indicate that the mean latitude of the Kuroshio Extension has varied on a 1500-year cycle, and suggest that a climatic link exists between the North Pacific gyre system and the high-latitude North Atlantic thermohaline circulation. The regular pacing at 1500-year intervals seen throughout both the Holocene and the last glacial period suggests that the oscillation was a response to external forcing.

Evaluation of the fidelity of isotope records as an environmental proxy in the coral Montastraea

Abstract. Many studies of climate variability in the Tropical Ocean have used high-resolution chemical tracer records contained in coral skeletons. The complex architecture of coral skeletons may lead to the possibility of biases in coral records and it is therefore important to access the fidelity of coral geochemical records as environmental proxies. Coral skeletal records from the same coral colony, and even the same corallite, may show large variation due to differing extension rates, formational timing of the skeletal elements, colony topography, and sampling resolution. To assess the robustness of the skeletal record, we present δ13C and δ18O data based on different sampling resolutions, skeletal elements, and coral colonies of Montastraea faveolata species complex, the primary coral used for climate reconstruction in the Atlantic. We show that various skeletal elements produce different isotopic records. The best sampling rate to resolve the full annual range of sea surface temperature (SST) is 40 samples per year. This sampling frequency also consistently recovered SST variability measured at weekly intervals. A sampling rate of 12 times per year recovered 84% of the annual range recording average monthly SST changes through the year. Six samples per year significantly decreased the ability to resolve the annual SST range. The δ18O recorded from two adjacent colonies were very similar, suggesting that this isotope can be trusted to record environmental changes. The δ13C, on the other hand, remained highly variable, perhaps as a result of the activity of symbiotic algae (zooxanthellae).

Paleoclimatic Significance of Eolian Carbonates Supplied to the Japan Sea during the Last Glacial Maximum

Laminated sediments deposited under anoxic bottom waters in the Japan Sea during the last glacial maximum (LGM) contain extremely well preserved calcareous microfossils and eolian carbonates. The radiocarbon age-difference between bulk sediment and monospecific planktonic foraminifera in discrete laminae from a core in the southern Japan Sea implies that ∼40% of the total carbonates in the sediments at the LGM are of eolian origin. Extrapolation of this result yields a rate of supply of eolian carbonates of ∼2800 tons d−1 to the entire Japan Sea during the LGM. The climatic significance of this flux potentially lies in its broader geographic extension, particularly in the interaction of the carbonate-bearing dust with shallow, corrosive North Pacific waters and with rain in the atmosphere. By increasing the alkalinity of such waters and by enhancing the biological pump the dust flux could have increased CO2 absorption by both the ocean and rain during the LGM.

Isotopic composition and morphology of living Globorotalia scitula: a new proxy of sub-intermediate ocean carbonate chemistry?

Abundance, isotopic composition and morphological imprints of the planktonic foraminifera Globorotalia scitula (Brady) were closely examined for possible use as a novel reconstruction tool of chemical environments in sub-intermediate depth seawater in the past. Based on the MOCNES plankton tow observation of dwelling depths of G. scitula and the isotopic compositions together with hydrochemistry data, the empirical relations between isotopic disequilibria in carbon (Δδ13C=δ13CG. scitula−δ13CDIC) and oxygen (Δδ18O=δ18OG. scitula−δ18Ow) isotopes in the carbonate tests and the seawater δ18O and δ13C of dissolved inorganic carbon (DIC), respectively, are introduced. The morphological information such as pore density and porosity is also examined for significant relations to carbonate chemistry. Shell porosity is strongly correlated saturation state of calcite. The dissolution of living G. scitula tests may promote the observed isotopic differences as well as the increases in porosity. Δδ18O of G. scitula is found effectively to be linear function of both water temperature and calcite saturation state (Ω), and thereby temperature equation for G. scitula is provided, while Δδ13C of G. scitula is a linear function of only calcite saturation state. The equation was validated by using Globorotalia scitula collected by a sediment trap in intermediate water depths. Satisfactory agreements were found between observed and calculated Δδ18O from the empirical equations based on temperature and hydrochemistry data at sediment trap deployment site, indicating that the equation may be useful in paleo-environmental reconstruction of sub-intermediate water. The sediment trap observation further suggests that the abundance of G. scitula does not necessarily correspond to surface water productivity and to POC flux, but instead, it correlates well with the supply of fine organic matter, which appears to be a result of water convection. Thus, G. scitula may be an unambiguous and excellent paleo-environmental recorder for carbonate chemistry and for fine organic matter transport to the depths, if isotopic and morphological observations are combined.

Glacial‐interglacial migration of an upwelling field in the Western Equatorial Pacific recorded by sediment 15N/14N

Nitrogen isotope ratios (δ15N) of surface sediment in the equatorial ocean reflect the mode of upper water circulation pattern. We applied this knowledge to reconstruct glacial-interglacial variations of an upwelling field in western equatorial Pacific using δ15N from sediment cores. Distinct minima in δ15N values were found corresponding to glacial periods on the northern and eastern edges of the upwelling field (Mindanao Dome), indicating glacial enlargement of upwelling due to enhanced winter monsoon activity. However, at a site on southern side of the upwelling region, minima in δ15N values were obtained during interglacial periods. These opposing variations in δ15N values could be interpreted as a northward shift of the upwelling zone, during glacial periods. The glacial-interglacial migration of the circulation pattern in this region may reflect a modal change in the global thermohaline circulation (conveyor belt) during glaciation.