Biogenic Barium Research Articles

A 70 kyr Record of Monsoon-induced Marine Productivity Changes, Terrigenous Flux and Weathering Variations in the Southern Bay of Bengal

Abstract The southwest (SW) and northeast (NE) monsoon winds significantly affect the oceanographic and climatic changes over the Bay of Bengal (BoB). We present here detailed geochemical and isotopic records from two sediment cores (SK157 14; 5°11′ N: 90°5′ E and SK157-16; 8°46′ N: 90°18′ E), collected from the southern BoB. In this study, we mainly focus on variations in paleoproductivity and terrigenous fluxes introduced into the BoB since last ~70 kyr: We have reconstructed biogenic barium (Babio), calcium carbonate (CaCO3) content, and total organic content (TOC) in both sediment cores to infer paleoproductivity during the period of study. The distributions of productivity proxies in both these cores indicate that the productivity may have been significantly high during MIS 1 and MIS 3 (warmer interglacial periods), when SW monsoon was intensified. The terrigenous proxies (Al%, Ti%, d(0.9)) show that the southern BoB may have received higher terrigenous input during the MIS 2 and MIS 4 (glacial periods) coinciding with the intensification of NE monsoon. The weathering proxy (K/Al), in contrast, shows no significant changes during MIS 2, 3 and 4. However; a significant shift is seen during glacial to Holocene transition.

Geochemical records provide evidences for the evolution of the marine redox environment in the southwestern Sumatra waters over the past 35,000 years

The marine redox environment (MRE) can affect marine productivity and nutrient cycling, as well as governs material and energy transport. However, its evolution process and driving mechanism in different historical periods revealed significant regional characteristics. Southwestern Sumatra waters, located at the Indo-Pacific Ocean convergence, are an ideal region for studying the evolution of MRE due to significant upwelling and variable productivity levels resulting from the seasonal monsoonal activity. In this study, based on the sediment core SO184–10043, systematic trace element geochemical analysis was conducted for reconstructing the MRE over the past 35,000 years using redox proxies, namely, Ni/Co, U/Th, and biogenic Barium (Babio). Ni/Co and U/Th varied in a range of 1–4 and 0.2–1, indicating general oxic conditions with matching productivity levels indicated by Babio (ranging from 300 × 10–6 to 580 × 10–6). A comparison with other reconstructed archives indicated that the MRE records of the sediment core in this study were in consistence in responding to the high-latitude climate records with strong semi-precession and millennial fluctuations. Suggestively, these fluctuations correspond to the natural summertime insolation, triggering the latitudinal motions of the Intertropical Convergence Zone (ITCZ), as well as leading to corresponding changes in the seasonal Indian monsoon system. Freshwater lids modulate the mixing effect by acting as barriers and influencing the upwelling development, thereby leading to strong oxic conditions in the MRE. Therefore, robust evidence on the millennial-scale variability in MRE driven by the ITCZ movement and regional monsoon changes in the tropical Indian Ocean is provided by the trace element archives employed in the present study, thereby providing novel insights into the evolution of MRE in response to global and local climate changes.

High- and low-latitude forcing on the subarctic Pacific environment and productivity over the past 230 kyr

The subarctic Pacific Ocean, which is linked to both polar and tropical climates, is an area of high marine productivity and is sensitive to ocean-atmosphere CO2 flux. However, the mechanisms underlying the drivers of its productivity dynamics or patterns remain elusive. Using biogenic barium, opal, and terrigenous input in a sediment core for the past 230 kyr, we present a continuous high-resolution nutrient availability dynamic for the subarctic Pacific Ocean and assess possible changes to sea ice and glacial North Pacific Intermediate Water on nutrient upwelling in the region. We suggest that export production in this region is controlled by the freshwater discharge associated with moisture and heat from low-latitude and the iceberg discharge from Kamchatka glaciations under a weak Aleutian Low. These findings indicate that both high- and low-latitude climate systems forced the evolution of the marine environment and primary productivity in the high-latitude North Pacific.

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

Abstract. In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record in the form of strong variance in the precession band (19–23 kyr), because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale monsoon variability in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (<1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost part of the Bay of Bengal spanning the late Miocene (9 to 5 million years ago). Underpinned by a new orbitally tuned benthic isotope stratigraphy, we use X-ray fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Results show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma in the equatorial sector of the Indian Ocean, with no apparent intensification over the latest Miocene.

Paleoproductivity variations and implications in the subarctic northwestern Pacific since MIS 7: Geochemical evidence

Knowledge of changes in marine primary productivity and export production in high-nutrient, low-chlorophyll regions of the subarctic NW Pacific Ocean is vital for understanding glacial-interglacial pCO2 variations induced by the biological pump mechanism. We conducted a multi-proxy of geochemical and biological productivity study of sedimentary core LV63-4-2, from the subarctic NW Pacific Ocean, spanning the last ~230 kyr (since ~MIS 7). The measurements included total organic carbon (TOC), opal, biogenic barium, and CaCO3 content as indicators of biological productivity; and the stable carbon isotope composition of TOC and trace elemental abundances to indicate sources of organic matter and redox conditions. The results reveal cyclic changes in paleoproductivity, with higher export production corresponding to periods of low Earth orbital obliquity and high winter insolation. Opal accumulation in core LV63-4-2 was higher during MIS 7, MIS 5d, and MIS 4, which differs from records from the Bering Sea, Okhotsk Sea, and the region to the south of the site LV63-4-2, but it is consistent with records from the Gulf of Alaska. We suggest that export production in the region is controlled by the combined effects of regional vertical convection associated with the formation of North Pacific Intermediate Water (NPIW), and the local iron supply sourced from the Gulf of Alaska. High export production is promoted by the coupling of weakened NPIW formation and an increased iron supply caused by enhanced upwelling in the Gulf of Alaska due to a strengthened Aleutian Low during warmer winters. By contrast, stronger NPIW formation dominated reduced export production during MIS 6 and MIS 2, despite an increased iron supply. Our results highlight the heterogeneity of export production in the subarctic North Pacific and the significant role of the local environment on productivity.

Variations in export production, lithogenic sediment transport and iron fertilization in the Pacific sector of the Drake Passage over the past 400 kyr

Abstract. Changes in Southern Ocean export production have broad biogeochemical and climatic implications. Specifically, iron fertilization likely increased subantarctic nutrient utilization and enhanced the efficiency of the biological pump during glacials. However, past export production in the subantarctic southeastern Pacific is poorly documented, and its connection to Fe fertilization, potentially related to Patagonian Ice Sheet dynamics, is unknown. We report biological productivity changes over the past 400 kyr, based on a combination of 230Thxs-normalized and stratigraphy-based mass accumulation rates of biogenic barium, organic carbon, biogenic opal and calcium carbonate as indicators of paleo-export production in a sediment core upstream of the Drake Passage (57.5∘ S, 70.3∘ W). In addition, we use fluxes of iron and lithogenic material as proxies for terrigenous input, and thus potential micronutrient supply. Stratigraphy-based mass accumulation rates are strongly influenced by bottom-current dynamics, which result in variable sediment focussing or winnowing at our site. Carbonate is virtually absent in the core, except during peak interglacial intervals of the Holocene, and Marine Isotope Stages (MIS) 5 and 11, likely caused by transient decreases in carbonate dissolution. All other proxies suggest that export production increased during most glacial periods, coinciding with high iron fluxes. Such augmented glacial iron fluxes at the core site were most likely derived from glaciogenic input from the Patagonian Ice Sheet promoting the growth of phytoplankton. Additionally, glacial export production peaks are also consistent with northward shifts of the Subantarctic and Polar Fronts, which positioned our site south of the Subantarctic Front and closer to silicic acid-rich waters of the Polar Frontal Zone. However, glacial export production near the Drake Passage was lower than in the Atlantic and Indian sectors of the Southern Ocean, which may relate to complete consumption of silicic acid in the study area. Our results underline the importance of micro-nutrient fertilization through lateral terrigenous input from South America rather than eolian transport and exemplify the role of frontal shifts and nutrient limitation for past productivity changes in the Pacific entrance to the Drake Passage.

Late Quaternary sedimentary evolution of the distal Galician continental margin (north‐west Iberian Peninsula) based on palaeoclimatic, palaeoproductivity and diagenetic evidence

AbstractDeep‐sea sediments record palaeoenvironmental and oceanographic changes over time through variations in their magnetochemical and sedimentological properties. This study presents the Late Quaternary sedimentary reconstruction of three sediment cores extracted from a source‐to‐sink sedimentary system located in a high marginal platform of the north‐west Iberian Margin. The aim of this reconstruction is to explore the palaeoclimatic, palaeoproductivity and diagenetic changes over the last 172 cal ka bp attending to geochemical, magnetic and sedimentological variations. The record is comprised of four sedimentary units consisting of pelagic, hemipelagic and detrital facies, the latter associated with ice‐rafted debris events at Heinrich Stadials. These findings allowed the reconstruction of the lithostratigraphy attending to climatic changes, covering the last eleven Heinrich Stadials. These units and facies successions are controlled by the superposition of the Milankovitch and Dansgaard–Oeschger cycles. Low sedimentation rates have been reported, especially during cold periods. Palaeoproductivity was assessed using total organic carbon and calcium carbonate content, together with biogenic barium (Babio) and organic phosphorus (Porg), both calculated via total digestion analysis using standard calculations. Mass accumulation rates for total organic carbon, Babio and Porg were calculated. Furthermore, a sequential extraction was performed to explore the different phases of the barium within sediments: biogenic barite (Babar), barium associated with iron and manganese oxides (Baox) and Ba associated with aluminosilicates (Badet). Results display a limitation in the use of the biogenic barium and phosphorus and mass accumulation rates, suggesting detrital Ba (Badetr) contribution during Heinrich Stadials, which could mask the calculations. In contrast, only calcium carbonate fluctuates attending to glacial/interglacial cycles. Regarding some diagenetic proxies (i.e. Mn, Mn/Fe and Mn/Ti), they reveal oxidation fronts of different generations, suggesting that the early diagenesis has not reached steady‐state, because the redoxcline fluctuates. This fact suggests an erosional activity that causes the continuous oxygenation of the sediment and non‐steady‐state conditions.

On the barium–oxygen consumption relationship in the Mediterranean Sea: implications for mesopelagic marine snow remineralization

Abstract. In the ocean, remineralization rate associated with sinking particles is a crucial variable. Since the 1990s, particulate biogenic barium (Baxs) has been used as an indicator of carbon remineralization by applying a transfer function relating Baxs to O2 consumption (Dehairs's transfer function, Southern Ocean-based). Here, we tested its validity in the Mediterranean Sea (ANTARES/EMSO-LO) for the first time by investigating connections between Baxs, prokaryotic heterotrophic production (PHP) and oxygen consumption (JO2-Opt; optodes measurement). We show that (1) higher Baxs (409 pM; 100–500 m) occurs in situations where integrated PHP (PHP100/500=0.90) is located deeper, (2) higher Baxs occurs with increasing JO2-Opt, and (3) there is similar magnitude between JO2-Opt (3.14 mmol m−2 d−1; 175–450 m) and JO2-Ba (4.59 mmol m−2 d−1; transfer function). Overall, Baxs, PHP and JO2 relationships follow trends observed earlier in the Southern Ocean. We conclude that such a transfer function could apply in the Mediterranean Sea.

New insights from multi-proxy data from the West Antarctic continental rise: Implications for dating and interpreting Late Quaternary palaeoenvironmental records

The Antarctic Peninsula’s Pacific margin is one of the best studied sectors of the Antarctic continental margin. Since the 1990s, several research cruises have targeted the continental rise with geophysical surveys, conventional coring and deep-sea drilling. The previous studies highlighted the potential of large sediment drifts on the rise as high-resolution palaeoenvironmental archives. However, these studies also suffered from chronological difficulties arising from the lack of calcareous microfossils, with initial results from geomagnetic relative palaeointensity (RPI) dating promising a possible solution.This paper presents data from new sediment cores recovered on cruise JR298 from seven continental rise sites west of the Antarctic Peninsula and in the Bellingshausen Sea with the objectives to (i) seek calcareous foraminifera, especially at shallow drift sites, to constrain RPI-based age models, and (ii) investigate the depositional history at these locations. We present the results of chronological and multi-proxy analyses on these cores and two cores previously collected from the study area. We establish new age models for the JR298 records and compare them with published RPI-based age models. In addition, we evaluate the reliability of different palaeoproductivity proxies and infer depositional processes.Planktic foraminifera are present in various core intervals. Although their stable oxygen isotope (δ18O) ratios, tephrochronological constraints and glacial-interglacial changes in sediment composition provide age models largely consistent with the RPI chronologies, we also observe distinct differences, predominantly in the Bellingshausen Sea cores. Enrichments of solid-phase manganese together with evidence for “burn-down” of organic carbon in late glacial and peak interglacial sediments document non-steady-state diagenesis that may have altered magnetic mineralogy and, thus, RPI proxies. This process may explain discrepancies between RPI-based age models and those derived from δ18O data combined with tephrochronology. The data also indicate that organic carbon is a much less reliable productivity proxy than biogenic barium or organically-associated bromine in the investigated sediments.In agreement with previous studies, sediment facies indicate a strong control of deposition on the rise by bottom currents that interacted with detritus supplied by meltwater plumes, gravitational down-slope transport processes and pelagic settling of iceberg-rafted debris (IRD) and planktic microfossils. Bottom-current velocities underwent only minor changes over glacial-interglacial cycles at the drift crests, with down-slope deposition only rarely affecting these shallow locations. Maximum concentrations of coarse IRD at the seafloor surfaces of the shallow sites result predominantly from upward pumping caused by extensive bioturbation. This process has to be taken into account when past changes in IRD deposition are inferred from quantifying clasts >1 mm in size.

An Assessment of xsBa Flux as a Paleoproductivity Indicator and Its Water‐Depth Dependence in the Easternmost Equatorial Pacific Ocean

AbstractWe present the first regional‐scale records of biogenic Barium (xsBa) fluxes in the Panama basin of the eastern‐equatorial margin of the Pacific Ocean in order to assess xsBa as a paleoproductivity proxy. Measurements of xsBa from 13 cores that range in water depths from about 700 to 3,000 m show an increase in 230Th‐normalized xsBa mass accumulation rates (MARs) with increasing water depth during both marine oxygen isotope stages (MIS) 1 and 2. The correlation of xsBa MARs with depth are strong despite differences in bulk sediment MARs and differing degrees of sediment redistribution. We interpret the increasing xsBa with water depth as likely due to the continued decomposition and remineralization of falling and/or resuspended biogenic particles. xsBa does not seem to be affected by diagenetic sulfate reduction in most of the cores. Calculated estimates of xsBa preservation in the sediment pile are high and fluctuate between 45% and 52% throughout the last 25 kyr. Although xsBa fluxes can be a robust indicator of paleoproductivity, caution is needed if (a) there is evidence of sulfate reduction in sediments being analyzed, and (b) one is trying to quantify differences in paleoproductivity among sites that are located at different depths in the water column.

Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon (TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze (Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio (biogenic molybdenum), Babio (biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nC18–nC25, and odd–even priority values are closed to 1 (0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation: (1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms; (2) high productivity provided the material foundation for organic generation; (3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic

Abstract. The remineralisation of sinking particles by prokaryotic heterotrophic activity is important for controlling oceanic carbon sequestration. Here, we report mesopelagic particulate organic carbon (POC) remineralisation fluxes in the North Atlantic along the GEOTRACES-GA01 section (GEOVIDE cruise; May–June 2014) using the particulate biogenic barium (excess barium; Baxs) proxy. Important mesopelagic (100–1000 m) Baxs differences were observed along the transect depending on the intensity of past blooms, the phytoplankton community structure, and the physical forcing, including downwelling. The subpolar province was characterized by the highest mesopelagic Baxs content (up to 727 pmol L−1), which was attributed to an intense bloom averaging 6 mg chl a m−3 between January and June 2014 and by an intense 1500 m deep convection in the central Labrador Sea during the winter preceding the sampling. This downwelling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. In comparison, the temperate province, characterized by the lowest Baxs content (391 pmol L−1), was sampled during the bloom period and phytoplankton appear to be dominated by small and calcifying species, such as coccolithophorids. The Baxs content, related to oxygen consumption, was converted into a remineralisation flux using an updated relationship, proposed for the first time in the North Atlantic. The estimated fluxes were of the same order of magnitude as other fluxes obtained using independent methods (moored sediment traps, incubations) in the North Atlantic. Interestingly, in the subpolar and subtropical provinces, mesopelagic POC remineralisation fluxes (up to 13 and 4.6 mmol C m−2 d−1, respectively) were equalling and occasionally even exceeding upper-ocean POC export fluxes, deduced using the 234Th method. These results highlight the important impact of the mesopelagic remineralisation on the biological carbon pump of the studied area with a near-zero, deep (> 1000 m) carbon sequestration efficiency in spring 2014.

Untangling biogeochemical processes from the impact of ocean circulation: First insight on the Mediterranean dissolved barium dynamics

Based on an unprecedented dissolved barium (D_Ba) data set collected in the Mediterranean Sea during a zonal transect between the Lebanon coast and Gibraltar (M84/3 cruise, April 2011), we decompose the D_Ba distribution to isolate the contribution of biogeochemical processes from the impact of the oceanic circulation. We have built a simple parametric water mass analysis (Parametric Optimum Multiparameter analysis) to reconstruct the contribution of the different Mediterranean water masses to the thermohaline structure. These water mass fractions have then been used to successfully reconstruct the background vertical gradient of D_Ba reflecting the balance between the large‐scale oceanic circulation and the biological activity over long time scales. Superimposed on the background field, several D_Ba anomalies have been identified. Positive anomalies are associated with topographic obstacles and may be explained by the dissolution of particulate biogenic barium (P_Ba barite) of material resuspended by the local currents. The derived dissolution rates range from 0.06 to 0.21 μmol m−2 d−1. Negative anomalies are present in the mesopelagic region of the western and eastern basins (except in the easternmost Levantine basin) as well as in the abyssal western basin. This represents the first quantification of the nonconservative component of the D_Ba signal. These mesopelagic anomalies could reflect the subtraction of D_Ba during P_Ba barite formation occurring during organic carbon remineralization. The deep anomalies may potentially reflect the transport of material toward the deep sea during winter deep convection and the subsequent remineralization. The D_Ba subtraction fluxes range from −0.07 to −1.28 μmol m−2 d−1. D_Ba‐derived fluxes of P_Ba barite (up to 0.21 μmol m−2 d−1) and organic carbon (13 to 29 mmol C m−2 d−1) are in good agreement with other independent measurements suggesting that D_Ba can help constrain remineralization horizons. This study highlights the importance of quantifying the impact of the large‐scale oceanic circulation in order to better understand the biogeochemical cycling of elements and to build reliable geochemical proxies.

Bioevents and redox conditions around the Cenomanian–Turonian anoxic event in Central Mexico

The Xilitla section of central Mexico (western margin of the proto-North Atlantic) is characterized by pelagic sediments enriched in marine organic matter. Using biostratigraphic and radiometric data, it was dated at the latest Cenomanian–earliest Turonian transition. We identified an interval coeval with the faunal turnover associated with the Oceanic Anoxic Event 2 (OAE 2), recording the Heterohelix shift and the “filament event” for the first time in Mexico. An integral analysis of sedimentary facies, pyrite and geochemical proxies reveals vertically variable redox conditions, with prevailing anoxic to dysoxic bottom waters. Along with phosphorous and manganese depletion, the highest content of total organic carbon and of certain redox-sensitive trace elements (RSTEs) is found during part of the anoxic event, confirming more uniform and constant oxygen-depleted conditions. This interval is also characterized by a significant enrichment in biogenic barium and elevated TOC/NTOT ratios, suggesting a link between productivity and anoxia. Sulfur isotope fractionation has a maximum value within the anoxic event, favored by the increase in the flux of organic matter and intensified through sulfur recycling. Highly bioturbated beds representing short-lived episodes of oxic conditions are intermittent within the OAE 2 and become more frequent in the early Turonian. This study proposes a model similar to that of modern upwelling regions. High marine productivity controlled organic matter burial and oxygenation at the seafloor, varying between anoxic (laminated facies with small pyrite framboids) and dysoxic conditions (bedding-parallel bioturbated facies with inoceramid bivalves and large pyrite framboids), interrupted by short-term well-oxygenated episodes (thoroughly bioturbated facies with common benthic foraminifera). General low-oxygen conditions led to the formation of glauconite and pyrite (bacterially mediated); the enrichment of redox-sensitive trace elements in sediments (Cd, Zn, V and Cr scavenged by organic matter and Ni, Mo, Pb, Co and Re by pyrite) and resulted in Mn and P depletion.

A high resolution and quasi-zonal transect of dissolved Ba in the Mediterranean Sea

The dissolved barium (D_Ba) data set for the Mediterranean Sea is here expanded with data from a large-scale transect sampled in April 2011 (M84/3 cruise) at high resolution. A total of 833 seawater samples have been analyzed for D_Ba. Over the basin the D_Ba content ranges from 38 to 85nmolkg−1 with local deep D_Ba maxima reaching up to 172nmolkg−1. Deep D_Ba maxima are associated with near bottom waters influenced by benthic processes and brine waters. The water column is largely undersaturated with respect to barite (BaSO4, the main phase of particulate biogenic barium P_Ba), with water column barite saturation state ranging between 0.2 and 0.6 over the basin. This new D_Ba dataset shows that the general zonal distribution of D_Ba is impacted by the large-scale Mediterranean circulation, as evidenced by the Levantine Intermediate Water zonal and meridional progression as well as by the eastward flow of surface Atlantic Water. However biogeochemical processes are also at play, as suggested by an elevated D_Ba content of deep waters and by local lower D_Ba contents in intermediate waters. These features could be attributed to active cycling between the particulate and dissolved Ba phases. Since P_Ba barite has been recognized in previous studies as a proxy for particulate organic carbon remineralization at intermediate depths, the significance of local changes in the water column D_Ba patterns may be the key to better constrain the Ba and carbon dynamics in the Mediterranean Sea.

Shifting frontal regimes and its influence on bioproductivity variations during the Late Quaternary in the Indian sector of Southern Ocean

Reconstruction of palaeoproductivity from Southern Ocean is crucial for understanding the functioning of the Southern Ocean biological pump in the past. High resolution records of multi-proxy parameters (calcium carbonate, opal, total organic carbon biogenic barium and planktonic carbon isotope ratios (δ13C)) were investigated in two well-dated sediment cores (SK200/22a and SK200/27) from the Indian sector of Southern Ocean situated to the north and south of Antarctic Polar front (APF), respectively. The palaeoproductivity records extending ~95ka BP (SK200/22a) and 75ka BP (SK200/27) revealed inverse relationships between the calcite and opal productivity, indicating the influence of shifting nutrient regimes. At core SK200/22a, reduced calcite productivity during marine isotope stage (MIS) 2, 4, and part of MIS 3 suggest an equatorward migration of the frontal regimes during glacial intervals. Compared to this, the region south of the APF (core SK200/27) was characterized by the near absence of calcite content during the last glacial period and increased opal productivity during MIS 1 and MIS 3, supporting a southward migration of APF during warmer intervals. Ba(bio) records exhibit good correlation with opal records in both the cores and also correlate with that of calcite record at SK200/22a, indicating that Ba is influenced by the combined opal and calcite productivity. The enhanced opal productivity during the glacial periods north of the APF is attributed to the northward shifting of oceanic fronts and associated transfer of nutrients. Diatom productivity records of SK200/22a reveal significant similarities with the dust records from the Antarctic and Southern Ocean, but showed no significant relationships with the diatom record of SK200/27. It is proposed that the dust-derived Fe input had apparently influenced the palaeoproductivity north of the modern APF, but had a minor influence on opal productivity south of the APF. Comparison with the ice core climate records from Antarctica and Greenland revealed that bioproductivity peaks in the study region are nearly synchronous with the millennial Antarctic warming events. Remarkably, the calcite and opal productivity records at SK200/22a responded differently to the Antarctic warming events, with opal productivity lagging behind the calcite productivity peaks by ~1–2ka.

Using the natural spatial pattern of marine productivity in the Subarctic North Pacific to evaluate paleoproductivity proxies

AbstractSedimentary proxies used to reconstruct marine productivity suffer from variable preservation and are sensitive to factors other than productivity. Therefore, proxy calibration is warranted. Here we map the spatial patterns of two paleoproductivity proxies, biogenic opal and barium fluxes, from a set of core‐top sediments recovered in the Subarctic North Pacific. Comparisons of the proxy data with independent estimates of primary and export production, surface water macronutrient concentrations, and biological pCO2 drawdown indicate that neither proxy shows a significant correlation with primary or export productivity for the entire region. Biogenic opal fluxes, when corrected for preservation using 230Th‐normalized accumulation rates, show a good correlation with primary productivity along the volcanic arcs (τ = 0.71, p = 0.0024) and with export productivity throughout the western Subarctic North Pacific (τ = 0.71, p = 0.0107). Moderate and good correlations of biogenic barium flux with export production (τ = 0.57, p = 0.0022) and with surface water silicate concentrations (τ = 0.70, p = 0.0002) are observed for the central and eastern Subarctic North Pacific. For reasons unknown, however, no correlation is found in the western Subarctic North Pacific between biogenic barium flux and the reference data. Nonetheless, we show that barite saturation, uncertainty in the lithogenic barium corrections, and problems with the reference data sets are not responsible for the lack of a significant correlation between biogenic barium flux and the reference data. Further studies evaluating the factors controlling the variability of the biogenic constituents in the sediments are desirable in this region.

Particulate barium flux and its relationship with export production on the continental shelf of Prydz Bay, east Antarctica

Particulate barium fluxes on the continental shelf of Prydz Bay in east Antarctica were measured by using time series sediment trap deployed at two stations with trap depth about 500m during 7/12/2009–15/2/2010 and 16/12/2010–16/12/2011. The annual fluxes were 12.3mgm−2yr−1 and 18.4mgm−2yr−1 for barium, and 8.9mgm−2yr−1 and 14.9mgm−2yr−1 for biogenic barium at two stations, respectively. More than 72% of the barium is from biogenic source. The temporal–spatial variations of biogenic barium fluxes and the ratio of biogenic barium to organic carbon are related to the variation of organic matter export in the upper water column. Biogenic barium flux and organic matter export are generally positive correlated. However, biogenic barium precipitation is less efficient under conditions of high organic matter exports during times of greater primary productivity, which leads to relatively less cycling of organic carbon and in turn produces less barite formation based on the hypothesis that barite forms through organic matter decomposing in microenvironment. More than 72% of the rained biogenic barium at the trap depth was preserved in the sediment. High preservation efficiency of biogenic barium and good relationship between biogenic barium and organic carbon suggested that biogenic barium could be a reliable proxy for the marine export production. The calculated export productions are 6.9gCm−2yr−1 and 14.9gCm−2yr−1 at two stations respectively using the modified prediction equation, which is more suitable for predicating export production for margin and shelf systems.

Concentrations and distribution of biogenic barium in surface sediments of Prydz Bay, Antarctica 

Concentrations of biogenic barium were investigated in surface sediments of Prydz Bay, Antarctica, during the 21st and 27th CHINARE cruises. Factors controlling the observed distribution are explored. Biogenic barium concentrations obtained from a sequential extraction procedure are compared with total concentrations obtained from the normative calculation based on a total digestion, and differences in the results are examined. Concentrations of biogenic barium, calculated by the normative calculation, were much higher than the concentrations obtained through sequential extraction; this discrepancy is the result of the occurrence of barium associated with Mn/Fe oxides, which represents an important component of total barium in these sediments. Concentrations of biogenic barium obtained from the sequential extraction range from 104 to 445 μg∙g -1 , and the average concentration was 227 μg∙g -1 . The highest concentrations of biogenic barium occur in the central area of the bay, where the seawater is more stable, while lower values occur in the bank and the ice shelf. Biogenic barium is significantly linearly correlated with biogenic barium and organic carbon, and similar in distribution of Chl a, which may indicate that primary productivity of phytoplankton in the sur- face water column is the main environmental factor regulating barium concentration and distribution.

Biogenic sedimentation in the equatorial Pacific: Carbon cycling and paleoproduction, 12–24 Ma

The equatorial Pacific is an important part of the global carbon cycle and has been affected by climate change through the Cenozoic (65 Ma to present). We present a Miocene (12–24 Ma) biogenic sediment record from Deep Sea Drilling Project (DSDP) Site 574 and show that a CaCO3 minimum at 17 Ma was caused by elevated CaCO3 dissolution. When Pacific Plate motion carried Site 574 under the equator at about 16.2 Ma, there is a minor increase in biogenic deposition associated with passing under the equatorial upwelling zone. The burial rates of the primary productivity proxies biogenic silica (bio‐SiO2) and biogenic barium (bio‐Ba) increase, but biogenic CaCO3 decreases. The carbonate minimum is at ∼17 Ma coincident with the beginning of the Miocene climate optimum; the transient lasts from 18 to 15 Ma. Bio‐SiO2 and bio‐Ba are positively correlated and increase as the equator was approached. Corg is poorly preserved, and is strongly affected by changing carbonate burial. Terrestrial 232Th deposition, a proxy for aeolian dust, increases only after the Site 574 equator crossing. Since surface production of bio‐SiO2, bio‐Ba, and CaCO3 correlate in the modern equatorial Pacific, the decreased CaCO3 burial rate during the Site 574 equator crossing is driven by elevated CaCO3 dissolution, representing elevated ocean carbon storage and elevated atmospheric CO2. The length of the 17 Ma CaCO3 dissolution transient requires interaction with a ‘slow’ part of the carbon cycle, perhaps elevated mantle degassing associated with the early stages of Columbia River Basalt emplacement.