Osamu Seki

Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr

[1] Marine- and terrestrial-derived biomarkers (alkenones, brassicasterol, dinosterol, and long-chain n-alkanes), as well as carbonate, biogenic opal, and ice-rafted debris (IRD), were measured in two sediment cores in the Sea of Okhotsk, which is located in the northwestern Pacific rim and characterized by high primary productivity. Down-core profiles of phytoplankton markers suggest that primary productivity abruptly increased during the global Meltwater Pulse events 1A (about 14 ka) and 1B (about 11 ka) and stayed high in the Holocene. Spatial and temporal distributions of the phytoplankton productivity were found to be consistent with changes in the reconstructed sea ice distribution on the basis of the IRD. This demonstrates that the progress and retreat of sea ice regulated primary productivity in the Sea of Okhotsk with minimum productivity during the glacial period. The mass accumulation rates of alkenones, CaCO3, and biogenic opal indicate that the dominant phytoplankton species during deglaciation was the coccolithophorid, Emiliania huxleyi, which was replaced by diatoms in the late Holocene. Such a phytoplankton succession was probably caused by an increase in silicate supply to the euphotic layer, possibly associated with a change in surface hydrography and/or linked to enhanced upwelling of North Pacific Deep Water. INDEX TERMS: 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; 4267 Oceanography: General: Paleoceanography; KEYWORDS: Okhotsk Sea, paleoproductivity, sediment Citation: Seki, O., M. Ikehara, K. Kawamura, T. Nakatsuka, K. Ohnishi, M. Wakatsuchi, H. Narita, and T. Sakamoto (2004), Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr, Paleoceanography, 19, PA1016,

Sediment core profiles of long‐chain n‐alkanes in the Sea of Okhotsk: Enhanced transport of terrestrial organic matter from the last deglaciation to the early Holocene

[1] Terrestrial plant-derived n-alkanes (C25–C35 )w ere measured in three piston cores (PC1, PC2 and PC4) in the Sea of Okhotsk covering the last 30 kyrs. Down core profiles of the n-alkane concentrations and mass accumulation rates (MAR) were characterized by deglacial maxima. In particular, cores PC2 and PC4, which were collected from the central and western Sea of Okhotsk, respectively, show a two-step increase around the Meltwater Pulse events (MWP) 1A (14.5–13.5 kyr BP) and 1B (about 10 kyr BP). This finding was interpreted by the outflow of terrestrial organic matter from the submerged land shelf to the Sea of Okhotsk through the East Sakhalin Current. This study demonstrated that the sea level rise forced by global warming in the deglaciation period may have caused the enhanced transport of terrestrial organic matter in marginal seas. INDEX TERMS: 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 4267 Oceanography: General: Paleoceanography; 1055 Geochemistry: Organic geochemistry. Citation: Seki, O., K. Kawamura, T. Nakatsuka, K. Ohnishi, M. Ikehara, and M. Wakatsuchi, Sediment core profiles of long-chain n-alkanes in the Sea of Okhotsk: Enhanced transport of terrestrial organic matter from the last deglaciation to the early Holocene, Geophys. Res. Lett. , 30 (1), 1001, doi:10.1029/ 2001GL014464, 2003.

Large changes in seasonal sea ice distribution and productivity in the Sea of Okhotsk during the deglaciations

Changes in the surface environment of the central Sea of Okhotsk were reconstructed using TEX86 paleothermometry, facilitated by the BIT index, in order to investigate paleoenvironmental changes during the glacial-interglacial cycles. The core top TEX86-derived temperature is the same as the present-day summer sea surface temperature (SST), suggesting that TEX86 records seasonal rather than annually averaged SSTs in this region. The TEX86 record reveals ∼3°C lower summer SST during glacial maxima than during interglacial periods and significant warming during the deglaciations (15–10 ka and 136–130 ka). This is consistent with previous inferences of more expanded and persistent seasonal sea ice during the glacial periods than the present-day and its substantial retreat during the deglaciation. Timing of the deglacial warming also coincided with a distinct increase in coccolithophorid productivity, suggesting a causal relationship between sea surface warming and coccolithophorid blooms during the deglaciations. The relationship could reflect an increase in the supply of fresh and warm water from the Amur River during deglaciation, which significantly impacted hydrology by facilitating sea surface stratification, which in turn promoted production of coccolithophids.

Plant-wax hydrogen isotopic evidence for postglacial variations in delivery of precipitation in the monsoon domain of China

The Asian monsoon is a key component of the Earths climate system that directly affects the livelihood of 60% of the world population. Reliable reconstruction of changes in monsoon precipitation during the Holocene is required to better understand the present climate conditions and to predict future climate processes in the Asian monsoon domain. Reconstruction of changes in Holocene monsoon precipitation has been done in various regions of this domain using a variety of paleoclimate proxies. However, different proxies have yielded different paleoclimate records of long-term monsoon rainfall variability. We apply compound-specific stable hydrogen isotope compositions of the plant wax n -alkanes isolated from the Hongyuan peat sequence in eastern Tibet to reevaluate the regional carbonate oxygen isotopic and lake-level records. The D/H ratios of the n -alkanes resolve the apparent discrepancy among the different paleoclimate proxies. Our result indicates that lake-level fluctuation is a reliable recorder of changes in the amount of summer monsoonal precipitation while long-term isotope records largely reflect large-scale changes in the source of water vapor rather than a local precipitation signal. Combining lake-level, biomarker, and carbonate isotopic records, we conclude that the early Holocene monsoon precipitation in southwest China has been greatly influenced by the Indian monsoon.

Pliocene cooling enhanced by flow of low-salinity Bering Sea water to the Arctic Ocean

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 Water formation and enhancing sea ice formation. However, the timing of Arctic freshening has not been defined. Here we present neodymium and lead isotope records of detrital sediment from the Bering Sea for the past 4.3 million years. Isotopic data suggest the presence of Alaskan glaciers as far back as 4.2 Myr ago, while diatom and C37:4 alkenone records show a long-term trend towards colder and fresher water in the Bering Sea beginning with the M2 glaciation (3.3 Myr ago). We argue that the introduction of low-salinity Bering Sea water to the Arctic Ocean by 3.3 Myr ago preconditioned the climate system for global cooling.

Ice-core records of biomass burning

We review the approaches for estimating biomass burning from ice-cores and consider the challenges and assumptions in their application. In particular, we consider the potential of biomarker proxies for biomass burning, hitherto not widely applied to glacial ice archives. We also review the available records of biomass burning in ice-cores and consider how variations in fire regimes have been related to atmospheric and land-use changes. Finally, we suggest that future developments in ice-core science should aim to combine multiple biomarkers with other records (black carbon, charcoal) and models to discern the types of material being burnt (C3 versus C4 plants, angiosperms, gymnosperms, peat fires, etc.) and to improve constraints on source areas of biomass burning. An ultimate goal is to compare the biomass burning record from ice-cores with hindcasts from models to project how future climate change will influence biomass burning and, inversely, how fire will affect climate.

Carbonaceous aerosol tracers in ice-cores record multi-decadal climate oscillations

Carbonaceous aerosols influence the climate via direct and indirect effects on radiative balance. However, the factors controlling the emissions, transport and role of carbonaceous aerosols in the climate system are highly uncertain. Here we investigate organic tracers in ice cores from Greenland and Kamchatka and find that, throughout the period covered by the records (1550 to 2000 CE), the concentrations and composition of biomass burning-, soil bacterial- and plant wax- tracers correspond to Arctic and regional temperatures as well as the warm season Arctic Oscillation (AO) over multi-decadal time-scales. Specifically, order of magnitude decreases (increases) in abundances of ice-core organic tracers, likely representing significant decreases (increases) in the atmospheric loading of carbonaceous aerosols, occur during colder (warmer) phases in the high latitudinal Northern Hemisphere. This raises questions about causality and possible carbonaceous aerosol feedback mechanisms. Our work opens new avenues for ice core research. Translating concentrations of organic tracers (μg/kg-ice or TOC) from ice-cores, into estimates of the atmospheric loading of carbonaceous aerosols (μg/m3) combined with new model constraints on the strength and sign of climate forcing by carbonaceous aerosols should be a priority for future research.

n-Alkanes in Fresh Snow in Hokkaido, Japan: Implications for Ice Core Studies

Abstract Plant waxes (e.g. long-chain n-alkanes) in ice cores are a promising paleovegetation proxy. However, much work needs to be done to assess how n-alkanes are transported from source areas to, and incorporated into, glacial archives. In this paper we present analyses of n-alkanes in seasonal snow and assess the information on source vegetation. n-Alkanes with carbon numbers C18 to C43 were extracted from snow samples collected at two sites in Hokkaido, northern Japan, during winter 2009–2010. Molecular distributions revealed that the majority of the n-alkanes originated from higher vegetation (ca. 65%), rather than anthropogenic sources. The distribution characteristics confirmed that the n-alkane signal had a wide regional origin, rather than a local source. We determined stable carbon and hydrogen isotopic compositions for the C27 n-alkane. The &dgr;13C of the C27 ( -28.2 to -33.0‰) was more representative of C3 than C4 vegetation, while the &dgr;D of the C27 (- 169.9 to -223.1‰) indicated growth latitudes more northerly than Hokkaido. The n-alkanes in the snow preserve information about the source vegetation type (photosynthetic group, growth site), confirming that if deposited with seasonal snows that firnify to form glacial ice, they have potential to record broad, regional vegetation changes over time.

A 60 Year Record of Atmospheric Aerosol Depositions Preserved in a High‐Accumulation Dome Ice Core, Southeast Greenland

The Southeastern Greenland Dome (SE-Dome) has both a high elevation and a high accumulation rate (1.01 m we yr ), which are suitable properties for reconstructing past environmental changes with a high time resolution. For this study, we measured the major ion fluxes in a 90 m ice core drilled from the SE-Dome region in 2015 and present the records of annual ion fluxes from 1957 to 2014. From 1970 to 2010, the trend of nonsea-salt (nss) SO4 2 flux decreases, whereas that for NH4 + increases, tracking well with the anthropogenic SOx and NH3 emissions mainly from North America. The result suggests that these fluxes reflect histories of the anthropogenic SOx and NH3 emissions. In contrast, the decadal trend of NO3 flux differs from the decreasing trend of anthropogenic NOx emissions. Although the cause of this discrepancy remains unclear, it may be related to changes in particle formation processes and chemical scavenging rates caused by an increase in sea salt and dust and/or a decrease in nssSO4 2 . We also find a high average NO3 flux (1.13 mmol m 2 yr ) in the ice core, which suggests a negligible effect from postdepositional NO3 loss. Thus, the SE-Dome region is an excellent location for reconstructing nitrate fluxes. Over a decadal time scale, our NO3 flux record is similar to those from other ice cores in Greenland high-elevation sites, suggesting that NO3 concentration records from these ice cores are reliable.

Water Chemistry of the Middle Amur River

This chapter examines water chemistry in terms of iron, organic matter, and nutrients in river, drainage, and groundwater from the middle reaches of the Amur River, to ascertain the watershed environment of the Amur River Basin during 2005–2007. Nitrate-N concentration was 0.46 ± 0.38 mg/L for river water and 1.9 ± 5.0 mg/L for groundwater, corresponding respectively to 87 ± 11 % and 39 ± 42 % of total inorganic nitrogen. Dissolved iron concentrations were 0.10–1.08 mg/L for river water and 0.39 ± 0.26 mg/L on average. A positive correlation was found between dissolved organic carbon (DOC) and dissolved iron concentrations in river water. However, the groundwater had average iron concentrations of 4.9 ± 3.1 mg/L and 4.3 ± 9.9 mg/L of DOC concentration, and there was no correlation between DOC and dissolved iron concentrations. The results indicate that wetland and forest areas are important in the export of dissolved iron and organic matter in the middle reaches of the Amur River. In the groundwater system, sources of dissolved iron and organic matter are independent for each underground environment, because of differences in geologic media, groundwater flow, and redox conditions.