Evaluation of reconstructed sea surface temperatures based on U37k′ from sediment surface samples of the North Pacific

The alkenone unsaturation index (Uk'37) as proxy for sea surface temperature (SST) is an important tool in paleoclimatology for reconstructing past ocean temperature variability. Typically, Uk'37 recorded in marine surface sediments shows a linear correlation with modern mean annual SST. However, in high-latitude oceanic regions, such as the subpolar Pacific, Uk'37-based SSTs do overestimate the mean annual temperature by up to 6 °C, potentially leading to obscured paleoclimatic information drawn from stratigraphic Uk'37-records. The reason for this “warm bias” is still not well understood. Here, we present a compilation of 97 sediment surface samples from Multicores collected in the Bering Sea, the Okhotsk Sea and the North Pacific to evaluate the alkenone-temperature proxy against observational data from the North Pacific. Sediment surface samples were analysed for alkenones and the derived Uk'37-indices converted to water temperatures using different calibration equations established in the literature. Uk'37-based SSTs were then compared to instrumental SST data, as well as modern alkenone flux data from sediment traps in the North Pacific. Our results confirm that most Uk'37-based SSTs from the subpolar Pacific are 2–6 °C too warm compared to instrumental mean annual SSTs for calibrations applied. However, with an uncertainty at the level of ±1.5 °C or less reconstructed SSTs fit quite well to modern autumn temperatures north of the Subarctic Front (SAF), when maximum export flux of alkenones to the seafloor is indicated by sediment trap data. South of the SAF, reconstructed SSTs largely mimic the modern mean annual SST signal with an uncertainty of ±1.5 °C or less, which is likely due to the attenuation of seasonality and longer growth season of coccolithophorids according to sediment trap data. Our study further demonstrates that Uk'37, when seasonality in alkenone production and export are known and considered, is able to provide reasonable estimates of SSTs in modern high-latitude ocean settings. We conduct a case study using available alkenone time-series derived from a sediment core collected from the south-western Okhotsk Sea to better understand the potential effect of seasonality in alkenone production on stratigraphic Uk'37-record in the subpolar Pacific. The case study from the Okhotsk Sea indicates that even a small shift in seasonality may lead to strongly biased SSTs with broader regional implications for paleoclimate reconstructions in high-latitude ocean settings

Testing the mantle plume hypothesis: An IODP effort to drill into the Kamchatka-Okhotsk Sea system

The great mantle plume debate (GPD) has been going on for ∼15 years (Foulger and Natland, 2003; Anderson, 2004; Niu, 2005; Davies, 2005; Foulger, 2005; Campbell, 2005; Campbell and Davies, 2006), centered on whether mantle plumes exist as a result of Earth’s cooling or whether their existence is purely required for convenience in explaining certain Earth phenomena (Niu, 2005). Despite the mounting evidence that many of the so-called plumes may be localized melting anomalies, the debate is likely to continue. We recognize that the slow progress of the debate results from communication difficulties. Many debaters may not truly appreciate (1) what the mantle plume hypothesis actually is, and (2) none of the petrological, geochemical and geophysical methods widely used can actually provide smoking-gun evidence for or against mantle plume hypothesis. In this short paper, we clarify these issues, and elaborate a geologically effective approach to test the hypothesis. According to the mantle plume hypothesis, a thermal mantle plume must originate from the thermal boundary layer at the core-mantle boundary (CMB), and a large mantle plume head is required to carry the material from the deep mantle to the surface. The plume head product in ocean basins is the oceanic plateau, which is a lithospheric terrane that is large (1000’s km across), thick (>200 km), shallow (2–4 km high above the surrounding seafloors), buoyant (∼1% less dense than the surrounding lithosphere), and thus must be preserved in the surface geology (Niu et al., 2003). The Hawaiian volcanism has been considered as the surface expression of a type mantle plume, but it does not seem to have a (known) plume head product. If this is true, the Hawaiian mantle plume in particular and the mantle plume hypothesis in general must be questioned. Therefore, whether there is an oceanic plateau-like product for the Hawaiian volcanism is key to testing the mantle plume hypothesis, and the Kamchatka-Okhotsk Sea basement is the best candidate to find out if it is indeed the Hawaiian mantle plume head product or not (Niu et al., 2003; Niu, 2004)

Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Thirteen Short sediment cores (30-50 cm) were collected from Bohai Sea, Yellow Sea and Changjiang Estuary in China, and the early diagenesis of several redox sensitive metals (Fe, Mn, Mo, U and V, referring to as RSMs) in sediment were studied. The recycling process of Mo and Mn was closely correlated with each other, generating benthic fluxes diffusing upward from sediment to overlying water column, and the flux rates are related to the organic carbon oxidation rates. The recycling of U and V were more tightly coupled with Fe oxides, generating benthic fluxes going downward into the sediment in most cores. Significant authigenic accumulation of U, in contrary to little to no accumulation of Mo and V, were found in the study region, even in Changjiang Estuary where hypoxic condition was often found during summer. Benthic diffusive fluxes were compared with authigenic mass accumulate rates (MAR), which indicated that, besides the benthic diffusion process, there are other processes controlling the authigenic accumulation of the RSMs. The close relationships between authigenic accumulation of RSMs with OCburial and OCburial with Sburial, indicating the authigenic accumulation of RSMs is a consequence of redox environment in shelf sediment, which directly influencing the organic carbon degradation process. Compared with other continental margin, moderate enrichment of U was found in China continental sediment. The authigenic U accumulation in BS and NYS sediments accounted for 20 - 68% of the Yellow River input, whilst in SYS sediments accounted for ~ 64% of the Yellow River and Changjiang River input, which acting as important U sinks that cannot be ignored

Provenance Discrimination of Siliciclastic Sediments in the Western Sea of Japan over the Past 30 kyr: Evidence from Major, Trace Elements, and Pb Isotopes

AbstractThe Sea of Japan (JS), a unique marginal sea without any large river influxes in the western Pacific, provides ample information about the evolution of sea level, East Asian monsoons (EAM), sea ice activity, and ocean currents in geological time. However, insufficient investigation in the western JS limits our knowledge of East Asian climate change. This study utilizes major and trace elements and Pb isotopes of fine siliciclastic components (<63 μm) of core LV53-18-2 and determines the provenances using statistical methods and discrimination diagrams. The results show that the terrigenous debris of LV53-18-2 was mainly composed of aeolian dust from northeast China, ice-rafted debris (IRD), and volcanic materials from the Far East coast over the last 30 kyr. During the late last glacial period, sea ice activity carried weakly weathered IRD to the study area. Meanwhile, the strengthened East Asian winter monsoon (EAWM) brought dust from northeast China to the study site owing to the cold climate and enlarged sandy land. During the late last deglacial period to early Holocene (15-8 kyr), ascending boreal summer insolation drove the intense melting of sea ice. This led to the deposition of large amounts of weakly weathered IRD and remarkably influenced the chemical composition of the core. After 8 kyr, the global sea level rose to -15 m below the modern sea level and opened the Tatar Strait. Consequently, freshwater supplied by the Amur River entered the JS and gave birth to the Liman Cold Current (LCC), which transported more mafic materials from the Kema terrane upstream

Pemendapan sedimen dan proses 210Pb di Lembangan Melayu dan Selat Melaka

Sebanyak dua teras sedimen diperoleh dari Lembangan Melayu dan Selat Melaka bagi mengenal pasti pemendapan sedimen serta proses 210Pb di Lembangan Melayu dan Selat Melaka. Di kawasan kajian, purata aktiviti 210Pb adalah sebanyak 111.47 ± 15.01 Bq/kg dan 126.38 ± 16.8 Bq/kg di stesen M17 dan M43 dengan perbezaan antara stesen adalah sebanyak 10%. Kedua-dua stesen menerima input yang berlainan melalui saluran air ke arah kawasan persampelan, menyebabkan peningkatan kandungan 210Pb di dalam sedimen. Proses hidrologi yang dikawal melalui perbezaan temporal turut mempengaruhi proses fizikal dan secara tidak langsung, mempengaruhi proses sedimentasi. Proses hidrologi melalui perbezaan temporal yang mempengaruhi proses fizikal dan kemudiannya mempengaruhi kadar proses pemendapan. Memandangkan sistem monsun mempengaruhi daya kitaran hidrologi, mekanisme timbal-balik kesan saiz butiran terhadap 210Pb telah berubah berikutan pengaruh arus monsun bersama dengan lempung sebagai pembawa utama yang menonjol untuk teras sedimen, sekali gus mempunyai kesan serta-merta pada pemendapan sedimen. Di samping itu, inventori dan fluks Lembangan Melayu dan Selat Melaka menunjukkan kesan monsun juga menyumbang kepada perubahan inventori dan fluks 210Pb di Lembangan Melayu dan Selat Melaka apabila daya kitaran hidrologi mengubah keupayaan kesan penggarutan 210Pb di sekitar perantaraan sedimen dan air laut

Changing sources and burial of organic carbon in the Chukchi Sea sediments with retreating sea ice over recent centuries

Decreasing sea ice extent in summer caused by climate change is affecting the carbon cycle of the Arctic Ocean. In this study, surface sediments across the western Arctic Ocean are investigated to characterize sources of sedimentary organic carbon (OC). Bulk organic parameters (total organic carbon, total nitrogen, &delta;13Corg and &delta;15N) combined with molecular organic biomarkers (e.g., sterols and highly branched isoprenoids (HBIs)) are applied to distinguish between sympagic, pelagic, and terrestrial OC. Furthermore, downcore profiles of these parameters were also generated from the Chukchi Sea R1 core (74&deg; N) to evaluate changes in the relative contribution of these three components of sedimentary OC over the last 200 years with decreasing sea ice. Our data evidence that from 1820s to 1930s, prevailing high and variable sea ice cover inhibited in situ primary production resulting in prominent land-derived material stored in sediments. From 1930s to 1980s, with the gradual decline of sea ice, primary production increased progressively. The ratio of sympagic and pelagic OC began to rise to account for a larger portion of sedimentary OC. Since 1980s, accelerated sea ice loss led to enhanced primary production, stabilizing over the last decades due to freshwater induced surface ocean stratification in summer.</p

Climate-Driven Changes in High-Intensity Wildfire on Orbital Timescales in Eurasia since 320 ka

AbstractWildfire is an integral part of the Earth’s climate system and plays an important role in shaping terrestrial ecosystems and biodiversity, atmospheric chemistry, regional climate, and the carbon cycle in the Earth’s history. However, the lack of high-resolution records of long wildfires limits our understanding of the natural variability, long-term trends of wildfire activity, and the reasons behind the changes in wildfire on orbital timescales. Here, a 320 ka long high-resolution wildfire record from the subarctic North Pacific is reconstructed with black carbon (BC), including its two subtypes char and soot. A 7-day-long back trajectory simulation analysis reveals the higher frequency of trajectories comes from Siberia. Our data show that continuous incidence of wildfire on a continental scale over the last 320 ka was higher during glacial periods than during the interglacial periods. The increase in wildfire frequency during glacial periods is ascribed to less precipitation. Contrasting patterns of wildfire incidence between marine isotope stages 2 and 6 may be ascribed to different fuel availability, which is related to contrasting configurations of the Northern Hemisphere ice sheet between glacial periods. A significant periodicity of 23 ka of our wildfire record suggests the precession of the Earth’s orbit pace wildfire development. The tight coupling of intensified wildfire and enhanced nutrient utilization efficiency suggests a nontrivial role of fire in the climate system

Mineralogical study of surface sediments in the western Arctic Ocean and their implications for material sources

Mineralogical analysis was performed on bulk sediments of 79 surface samples using X-ray diffraction. The analytical results, combined with data on ocean currents and the regional geological background, were used to investigate the mineral sources. Mineral assemblages in sediments and their distribution in the study area indicate that the material sources are complex. (1) Feldspar is abundant in the sediments of the middle Chukchi Sea near the Bering Strait, originating from sediments in the Anadyr River carried by the Anadyr Current. Sediments deposited on the western side of the Chukchi Sea are rich in feldspar. Compared with other areas, sediments in this region are rich in hornblende transported from volcanic and sedimentary rocks in Siberia by the Anadyr Stream and the Siberian Coastal Current. Sediments in the eastern Chukchi Sea are rich in quartz sourced from sediments of the Yukon and Kuskokwim rivers carried by the Alaska Coastal Current. Sediments in the northern Chukchi Sea are rich in quartz and carbonates from the Mackenzie River sediments. (2) Sediments of the southern and central Canada Basin contain little calcite and dolomite, mainly due to the small impact of the Beaufort Gyre carrying carbonates from the Canadian Arctic Islands. Compared with other areas, the mica content in the region is high, implying that the Laptev Sea is the main sediment source for the southern and central Canada Basin. In the other deep sea areas, calcite and dolomite levels are high caused by the input of large amounts of sediment carried by the Beaufort Gyre from the Canadian Arctic Islands (Banks and Victoria). The Siberian Laptev Sea also provides small amounts of sediment for this region. Furthermore, the Atlantic mid-water contributes some fi ne-grained material to the entire deep western Arctic Ocean

Millennial-scale variations in sedimentary oxygenation in the western subtropical North Pacific and its links to North Atlantic climate

The deep-ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. An increase in the respired CO2 concentration in the glacial deep-ocean due to biological pump generally corresponds to deoxygenation in the ocean interior. This link thus offers a chance to study oceanic ventilation and coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here, we investigate a suite of geochemical proxies in a sediment core from the Okinawa Trough to understand sedimentary oxygenation variations in the subtropical North Pacific over the last 50 000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals and after 8.5 ka, while oxygenation decreased during the Bölling-Alleröd (B/A) and Preboreal. The enhanced oxygenation during cold spells is linked to the North Pacific IntermediateWater (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast tradewinds over the tropics. The enhanced formation of the NPIW during Heinrich Stadial 1 (HS1) was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in the high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A due to a decreased NPIW formation and enhanced export production, indicates an expansion of the oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters, along with the termination of atmospheric CO2 concentration increase. We attribute the millennial-scale changes to an intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation

Human impact overwhelms long-term climate control of fire in the Yangtze River Basin since 3.0 ka BP

A high-resolution fire history in the Yangtze River Basin over the past 7.0 ka BP is reconstructed based on the proxy of black carbon of sediment core ECMZ on the continental shelf of the East China Sea in order to reveal the interactions among fire, climate, vegetation and human activity on a regional scale. A comparison of fire activity with climatic and vegetation proxies suggests that changes in fire activity prior to 3.0 ka BP on both millennial- and centennial-timescales were closely related to variations in temperature and precipitation, with more fire during warm and humid periods, suggesting climatic control on regional fire activities. In contrast, the significant decoupling between fire and climate on multi-timescales since similar to 3.0 ka BP implies increasing anthropogenic impact on regional fire activity. There is also a distinct response of fire activity to human disturbance at different time scales. Long-term reduction in regional fire activity since similar to 3.0 ka BP was caused by a general decrease in forest cover with increasing human activity while short-term (centennial-timescale) enhancement in biomass burning usually coincides with periods characterized by increasing human activity associated with population migration or technological advances. (C) 2020 Elsevier Ltd. All rights reserved