Diatom Production Research Articles

Description of the biogeochemical features of the subtropical southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer of the International Polar Year

Abstract. Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (Chl a), phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively), four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC), and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of Chl a and phaeopigments never exceeded 0.5 µg L−1 and 0.25 µg L−1, respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isoclines. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area, Chl a level of 0.4–0.5 µg L−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 µM and 1.5 µM, respectively, indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a post-bloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 µM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid, suggesting a late stage of a relatively minor diatom bloom possibly driven by iceberg drifting releases of iron. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.

A Shift in the Biogenic Silica of Sediment in the Larsen B Continental Shelf, Off the Eastern Antarctic Peninsula, Resulting from Climate Change

In 2002, section B of the Larsen ice shelf, off of the Eastern Antarctic Peninsula, collapsed and created the opportunity to study whether the changes at the sea surface left evidence in the sedimentary record. Biogenic silica is major constituent of Antarctic marine sediment, and its presence in the sediment column is associated with diatom production in the euphotic zone. The abundance of diatom valves and the number of sponge spicules in the biogenic silica was analyzed to determine how the origin of the biogenic silica in the upper layers of the sediment column responded to recent environmental changes. Diatom valves were present only in the upper 2 cm of sediment, which roughly corresponds to the period after the collapse of the ice shelf. In contrast, sponge spicules, a more robust form of biogenic silica, were also found below the upper 2 cm layer of the sediment column. Our results indicate that in this region most of the biogenic silica in the sedimentary record originated from sponge spicules rather than diatoms during the time when the sea surface was covered by the Larsen ice shelf. Since the collapse of the ice shelf, the development of phytoplankton blooms and the consequent influx of diatom debris to the seabed have shifted the biogenic silica record to one dominated by diatom debris, as occurs in most of the Antarctic marine sediment. This shift provides further evidence of the anthropogenic changes to the benthic habitats of the Antarctic and will improve the interpretation of the sedimentary record in Polar Regions where these events occur.

Phytoplankton growth and microzooplankton grazing in the Homa Lagoon (İzmir Bay, Turkey)

Phytoplankton growth and microzooplankton grazing were investigated at one station in the Homa Lagoon from February to January in 2006-2007. Our results showed significant seasonal variations in phytoplankton dynamics. Microzooplankton was mainly composed of dinoflagellates and tintinnid ciliates and nauplii. Microzooplankton grazing increased with increasing of temperature. Grazing rate was maximum levels in spring and summer. Microphytoplankton, which dominated the total algal biomass and production, were characterized by the proliferation of several chain-forming diatoms. Small heterotrophic flagellates and aloricate ciliates were the main controllers of phytoplankton. Phytoplankton represented a significant for micrograzers, which grazing represented 20–120% of diatom and cynabacteria algal production during 2006-2007. Microzooplankton has, however, a relatively high impact on microphytoplankton, as > 45% of microalgal production was consumed throughout the year. These results suggest that the low grazing was one of the factors contributing to the development of the spring bloom. All of seasonal bases, the phytoplankton production were grazed by microzooplankton in summer, autumn and winter– spring seasons have been changing between 20%-120%. The seasonal variation in the microzooplankton grazing pressure seems to result from the dominant size class of the phytoplankton community of this lagoon diatom and cynabacteria.

Centennial- to millennial-scale climate shifts in continental interior Asia repeated between warm–dry and cool–wet conditions during the last three interglacial states: evidence from uranium and biogenic silica in the sediment of Lake Baikal, southeast Siberia

We observed timescale-dependent changes in the correlations between biogenic silica (bioSi) and uranium concentrations of bulk sediment from the Buguldeika Saddle, Lake Baikal. There was a positive correlation for the glacial–interglacial (100 kyr) scales over the 180 kyr, with inverse correlations for centennial- to-millennial scales during the last three interglacial states (MIS 5, 3, and 1). The distinction of these correlations between the time scales suggests that the sedimentation processes of bioSi and U differ from each other. The Baikal bioSi concentration record is generally regarded as a paleotemperature proxy, reflecting the diatom production of the lake. On the other hand, we concluded that the temporal variation of the U concentration in the sediment reflected the weathering intensity in the south of Lake Baikal watershed associated with changes in the rainfall/moisture levels.Based on the paleoclimate proxies from Lake Baikal, climates in continental interior Asia are identified as having dry and wet conditions during the glacial and interglacial periods, respectively. This accounts for the rhythms in growth and retreat of ice sheets in Eurasia on glacial–interglacial scales. In the interglacial periods (MIS 5, 3, and 1), the continental interior is, contrary to the glacial–interglacial climate changes, characterized by alternating warm–dry and cool–wet climates with periods in tens of thousands years. In particular, such shifts in the climate stand out prominently at the beginning of MIS 5d. The detrended U record for the last 5.2 kyr shows wet events at 4.3–3.7, 3.2–2.3, 1.8–1.2, and 0.8–0.3 kyr. These events coincide with the timing of the North Atlantic ice-rafted debris events, which were possibly related to solar activity. All this evidence suggests that climate in the continental interior followed significantly different fluctuation patterns in the glacial–interglacial and shorter time scales.

Contribution of changes in opal productivity and nutrient distribution in the coastal upwelling systems to Late Pliocene/Early Pleistocene climate cooling

Abstract. The global Late Pliocene/Early Pleistocene cooling (~3.0–2.0 million years ago – Ma) concurred with extremely high diatom and biogenic opal production in most of the major coastal upwelling regions. This phenomenon was particularly pronounced in the Benguela upwelling system (BUS), off Namibia, where it is known as the Matuyama Diatom Maximum (MDM). Our study focuses on a new diatom silicon isotope (δ30Si) record covering the MDM in the BUS. Unexpectedly, the variations in δ30Si signal follow biogenic opal content, whereby the highest δ30Si values correspond to the highest biogenic opal content. We interpret the higher δ30Si values during the MDM as a result of a stronger degree of silicate utilisation in the surface waters caused by high productivity of mat-forming diatom species. This was most likely promoted by weak upwelling intensity dominating the BUS during the Late Pliocene/Early Pleistocene cooling combined with a large silicate supply derived from a strong Southern Ocean nutrient leakage responding to the expansion of Antarctic ice cover and the resulting stratification of the polar ocean 3.0–2.7 Ma ago. A similar scenario is hypothesized for other major coastal upwelling systems (e.g. off California) during this time interval, suggesting that the efficiency of the biological carbon pump was probably sufficiently enhanced in these regions during the MDM to have significantly increased the transport of atmospheric CO2 to the deep ocean. In addition, the coeval extension of the area of surface water stratification in both the Southern Ocean and the North Pacific, which decreased CO2 release to the atmosphere, led to further enhanced atmospheric CO2 drawn-down and thus contributed significantly to Late Pliocene/Early Pleistocene cooling.

Late Quaternary productivity changes from offshore Southeastern Australia: A biomarker approach

Reconstructions of primary productivity at low latitudes have been the focus of several studies to better understand how the export of nutrient-rich, intermediate Southern Ocean (SO) waters influences productivity at these latitudes. This was triggered by the general observation of minima in the planktonic foraminiferal δ13C values during deglaciations, which was interpreted as an isotopic signal of intermediate SO waters, together with a concomitant increase in diatom productivity at some equatorial sites. However, the impact of these SO waters on productivity at higher latitudes is not well constrained. Here, we compare a high-resolution planktonic foraminiferal δ13C record with total organic carbon and biomarker records for Proboscia diatoms and haptophytes in marine sediments from offshore Southeastern Australia. The planktonic foraminiferal δ13C record shows distinct minima during deglaciations and the Marine Isotope Stage 4/3 transition, tentatively suggesting that 13C-depleted SO waters reached the coast of Southeastern Australia. However, it did not result in increased productivity during these periods. Instead, the highest primary productivity period, as indicated by total organic carbon and alkenone accumulation rates, occurred during the Last Glacial Maximum while Proboscia diatoms mainly proliferated during interglacials and Marine Isotope Stage 3 matching periods of increased diatom productivity in some sites of the Eastern Equatorial Pacific. Our study suggests that increased primary productivity offshore Southeastern Australia was mainly due to stronger westerly winds during glacial periods while Proboscia diatom productivity was probably controlled by the transport of silicic acid to this area.

Carbon cycling within an East African lake revealed by the carbon isotope composition of diatom silica: a 25-ka record from Lake Challa, Mt. Kilimanjaro

The carbon cycle of a lake is a balance between supply from the atmosphere and catchment, and the net demand exerted by primary producers, minus losses back to the atmosphere and to sediment storage. Evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi-proxy approach. One promising technique is to explore the carbon-isotope composition (δ13Cdiatom) of organic matter incorporated within the silica frustules of diatom algae. Here we present a 25,000-year record of δ13Cdiatom from the sediments of crater Lake Challa on the eastern flank of Mt. Kilimanjaro, and along with other proxy data we make inferences about the three major phases in the history of the lake's carbon cycle. From 25 ka to 15.8 ka years BP, δ13Cdiatom is positively correlated with the δ13C of bulk sediment organic matter (δ13Cbulk), indicating that high diatom productivity, as recorded by high % biogenic silica at this time, was preferentially removing 12C and enriching the δ13C of lake-water dissolved inorganic carbon. From 15.8 to 5.5 ka the correlation between δ13Cdiatom and δ13Cbulk breaks down, suggesting carbon supply to the lake satisfied or exceeded the demand from productivity. From 5.5 ka BP the positive correlation resumes, indicating an increase in the internal demand for carbon relative to external supply. Diatom frustule-bound carbon isotopes offer an original tool in examining long-term fluctuations in a lake's carbon budget and how the balance between supply and demand has changed through time.

High interglacial diatom paleoproductivity in the westernmost Indo‐Pacific Warm Pool during the past 130,000 years

A wealth of sedimentary records aimed at reconstructing late Quaternary changes in productivity and temperature have been devoted to understanding linkages between the Indo‐Pacific Warm Pool (IPWP) and other distant oceanic areas. Most of these reconstructions are based, however, on biogeochemical and sedimentological proxies, with comparatively less attention devoted to microfossils. A high‐resolution (<1 ka) study of diatom concentrations and the community at site GeoB10038‐4, recovered off southern Sumatra (ca. 6°S, 103°E), closely tracks the variations of diatom concentrations in the westernmost IPWP during the last glacial‐interglacial cycle. The diatom record provides evidence that diatom paleoproductivity was highest during interglacials, primarily due to the input of lithogenics and nutrients following the rise in sea level after full glacials. In addition, the co‐variation of total diatom concentration and Northern Hemisphere forcing for Marine Isotope Stage 5 suggests a direct response of diatom productivity and upwelling intensity to boreal summer insolation. Temporal shifts of the diverse diatom community at site GeoB10038‐4 correspond well with the present‐day seasonal monsoon pattern and the strengthening and weakening phases of upwelling along the southern coast of Sumatra. Resting spores ofChaetoceros, typical of nutrient‐rich waters, were dominant during periods of highest diatom paleoproductivity and responded to the strengthening of the SE monsoon, while diatoms of oligotrophic to mesotrophic waters characterized intermonsoon periods. The close correspondence between the dominance of upwelling diatoms and the boreal summer insolation resembles the present‐day dynamics of diatom production. The observed interglacial highs and glacial lows of diatom productivity at site GeoB10038‐4 is a unique pattern in the late Quaternary tropics.

Enhancement of coccolithophorid blooms in the Bering Sea by recent environmental changes

Since 1997, ocean color satellite images have revealed large‐scale blooms of the coccolithophorid Emiliania huxleyi in the eastern Bering Sea. The blooms are often sustained over several months and have caused ecosystem changes in the Arctic Ocean, as well as in the Bering Sea. We examined continental shelf sediment profiles of alkenone, a biomarker for E. huxleyi, covering the past ∼70 years. The alkenone records suggest that large E. huxleyi blooms are a novel feature in the Bering Sea as they have occurred only since the late 1970s. Recent changes in alkenone content were closely related to the 1976–77 climatic regime shift in the North Pacific, implying that warming and freshening of Bering Sea waters promoted E. huxleyi blooms. The production rate of diatoms (total valves in sediment samples), the dominant primary producers in the Bering Sea, also increased during the past several decades. However, the ratio of alkenone content to total diatom valves in the sediments increased as E. huxleyi production increased, suggesting that the increase in the E. huxleyi production rate frequently exceeded the increase in the diatom production rate. Overall, our results indicate a possible subarctic region ecosystem shift driven by climate change.

Middle to late Holocene environmental changes in western Ireland inferred from fluctuations in preservation of biological variables in lake sediment

Knowledge of natural variability in aquatic ecosystems is vital for assessing the nature and amplitude of human-induced change, and for predicting future anthropogenic impacts. Distinguishing between naturally and anthropogenically caused variability in lake sediment records can be problematic, however, because both drivers can produce similar ecological effects. Standard sediment-based approaches for reconstructing past environmental changes tend to focus on qualitative and quantitative variations in palaeoenvironmental indicators, with little significance attached to their complete absence. We used multiple variables in radiometrically dated sediment cores collected from two sites in Lough Mask, a lake in western Ireland. Results suggest that the Lough Mask sediment record has been a sensitive recorder of past climate variability, especially changing precipitation, since the middle Holocene. Variations in the presence of aquatic siliceous microfossils and calcareous macrofossils, and changing sediment lithology and geochemistry, indicate a quasi-cyclic response to oscillations in climate conditions that correspond generally with palaeoclimate findings from elsewhere in NW Europe, including other sites in Ireland. We conclude that during much of the middle to late Holocene, prolonged periods of relatively high rainfall in the catchment reduced nutrient inputs to the lake, particularly silica and calcite. Diatom productivity consequently declined, whereas dissolution of frustules was enhanced. During relatively dry climate periods, availability of these nutrients increased, diatom productivity was higher, and dissolution was reduced. Relatively early human impacts are evident in the sediment record beginning ca. 1,000 BP. The results highlight the aquatic and taphonomic effects of complex interactions among past variations in catchment conditions, climate and water chemistry. The complexity of these interactions and their effects, mediated through the characteristics of Lough Mask and its catchment, pose problems for conventional interpretation of palaeolimnological data and their use in computer-based simulations of future changes in stresses on aquatic ecosystems and their consequent impacts.

Climate forcing of diatom productivity in a lowland, eutrophic lake: White Lough revisited

Summary1. In cultural landscapes, lake response to climate can be masked by land‐use change and nutrient loss from their catchments. Palaeolimnological methods were used to reconstruct the ecological response of diatoms in a eutrophic lowland lake (White Lough, Co. Tyrone, Northern Ireland) to altered nutrient P loading and precipitation variability over c. 100 years.2. 210Pb‐dated sediment cores were analysed to determine diatom assemblage variability, biogenic silica concentration, geochemical phosphorus concentration and accumulation rate. Manure P and agricultural N surplus data were collated from documentary sources. Long‐term trends in annual temperature and precipitation were derived from the Armagh Observatory.3. Diatom community turnover from 1890 until c. 1960 was limited, and assemblages were dominated by Aulacoseira subarctica; after this date, changes primarily reflected a eutrophication sequence owing to increased diffuse nutrient inputs associated with intensification of land use (external P loading increased by a factor of three).4. Diatom and biogenic Si profiles were compared with North Atlantic Oscillation (NAO) records, an index of regional weather patterns. Biogenic Si exhibited a c. 7‐year cycle, which tracked a cycle of similar timescale in the Armagh climate record for dry summers. In turn, this cycle was related to the variation in the NAO.5. Monitoring data from 1971 to 2007 of nitrate exports from the Blackwater River showed that these too followed a roughly 7‐year cycle at least up to 2000, in which dry summers were followed by sharp increases in nitrate export. It is argued that diatom production in White Lough reflects the cyclic behaviour in nitrate loading and the constraints that nitrogen availability places on the spring diatom bloom in a lake that is dominated by cyanobacteria.

The effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed (Tana River, Kenya)

We present the first study of the Si isotopic composition of dry season river waters from the Tana River, Kenya. Data encompasses the entire river basin, with samples collected from headwaters to the estuary, thereby capturing a salinity gradient. In the headwaters, the isotopic signature is affected by climate, as a result of its control on soil drainage and weathering. The δ30Si signatures in the basin range from +0.69‰ up to +2.23‰. Signatures are clearly affected by dams: an increase in δ30Si ratios of 0.54‰ and a decrease in the dissolved Si (DSi) concentration by 41% were observed downstream of the Masinga dam, the largest of a succession of 5 hydroelectric dams. This reduction in Si load is most likely due to increased diatom productivity as the corresponding change in δ30Si signature is consistent with the known fractionation by these organisms. The δ30Si composition of waters entering the estuary is ca. +2‰ and DSi concentrations are 349μM. In the estuary, the DSi concentrations decrease linearly following the salinity, while the δ30Si ratio remains stable, indicating the absence of processes affecting the DSi pool for the studied range of salinity.

Climate dependent diatom production is preserved in biogenic Si isotope signatures

Abstract. The aim of this study was to reconstruct diatom production in the subarctic northern tip of the Baltic Sea, Bothnian Bay, based on down-core analysis of Si isotopes in biogenic silica (BSi). Dating of the sediment showed that the samples covered the period 1820 to 2000. The sediment core record can be divided into two periods, an unperturbed period from 1820 to 1950 and a second period affected by human activities (from 1950 to 2000). This has been observed elsewhere in the Baltic Sea. The shift in the sediment core record after 1950 is likely caused by large scale damming of rivers. Diatom production was inferred from the Si isotope composition which ranged between δ30Si −0.18‰ and +0.58‰ in BSi, and assuming fractionation patterns due to the Raleigh distillation, the production was shown to be correlated with air and water temperature, which in turn were correlated with the mixed layer (ML) depth. The sedimentary record showed that the deeper ML depth observed in colder years resulted in less production of diatoms. Pelagic investigations in the 1990's have clearly shown that diatom production in the Baltic Sea is controlled by the ML depth. Especially after cold winters and deep water mixing, diatom production was limited and dissolved silicate (DSi) concentrations were not depleted in the water column after the spring bloom. Our method corroborates these findings and offers a new method to estimate diatom production over much longer periods of time in diatom dominated aquatic systems, i.e. a large part of the world's ocean and coastal seas.

A highly productive Subarctic Atlantic during the Last Interglacial and the role of diatoms

The Last Interglacial (LIG), corresponding to Marine Isotope Stage (MIS) 5e, provides a reference of interglacial climate variability in the absence of anthropogenic forcing. Using an expanded section of the LIG gained at Integrated Ocean Drilling Program Site U1304 in the Subarctic Atlantic, we demonstrate that the early MIS 5e was marked by oceanographic conditions conducive for high diatom production and accumulation. The appearance of diatom-dominated laminated oozes ∼3 k.y. after the beginning of MIS 5e at ca. 125 ka coincides with a shift to higher δ 30 Si diat values together with the dominance of Thalassiothrix longissima , indicative of increased nutrient availability and silicic acid utilization in surface waters. Though the Subarctic Front provided the physical conditions for high diatom production and deposition, these processes alone are insufficient to explain the high rates of siliceous productivity and the formation of diatomaceous sediments. Instead, the additional presence of an increased nutrient pool provided by Subantarctic Mode Water played the decisive role in initiating and sustaining diatom production. The high diatom productivity and the occurrence of diatomaceous sediments in the late Quaternary challenge the current hypothesis of a silica-depleted North Atlantic during the LIG.

Diatom abundance in Lake Aoki sediment as a proxy in demarcating the timing of the last Glacial-Holocene transition in central Japan

A 15 m long sediment core from Lake Aoki, central Japan was investigated for its diatom abundance to determine if the abundance change with time provides a reasonable basis to reconstruct the timing of the last Glacial-Holocene climate shift in central Japan. Silty clay dominated the sediment lithology, and the tephra and radiocarbon based age determinations provided an excellent sediment chronology extending back to the beginning of the lacustrine sedimentation ca. 43ka cal BP. The diatom abundance in the sediment showed a marked change at 13 ka cal BP, with a mean concentration of ca. 1.8x105 frustules mg-1 dry sediment before and ca. 10.3 x 105 frustules mg-1 dry sediment after that time. This abrupt increase in abundance after 13 ka cal BP is attributed to the increase in diatom productivity in the lake due to the climate shift from the last Glacial cold to the Holocene warm condition. The timing of rapid diatom abundance rise, 13 ka cal BP, in Lake Aoki sediment record appears to be the last Glacial-Holocene transition in central Japan.

Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases

Settling particles were sampled monthly for 1 year using an automated time-series sediment trap positioned at similar depths at two sites of high diatomaceous productivity in the North Pacific Ocean and Bering Sea. The particles were analyzed for rare earth elements (REEs) by inductively coupled plasma mass spectrometry (ICP–MS) with and without chemical treatment of the bulk samples to isolate siliceous fractions. The REE composition of the bulk samples is explained largely by the contribution of two distinct components: (i) carbonate with a higher REE concentration, a negative Ce anomaly and lighter REE (LREE) enrichment; (ii) opal with a lower REE concentration, a weaker negative Ce anomaly and heavier REE (HREE) enrichment. The siliceous fractions of settling particles are characterized by high Si/Al ratios (30–190), reflecting high diatom productivity at the studied sites. The La/Al ratio of the siliceous fraction is close to that of the upper crust, but the Lu/Al and Lu/La ratios are significantly higher than those of the upper crust or airborne particles, indicating the presence of excess HREEs in the siliceous fraction. Diatoms are believed to be important carriers of HREEs. The Ce anomaly, Eu anomaly, slope of the REE pattern, and ΣREE of the siliceous fraction vary exponentially with decreasing total mass flux. They can be well-reproduced according to the differential dissolution kinetics of elements in the order of Ce < lighter REEs (LREEs) < Eu = heavier REEs (HREEs) < Si from settling particles, where the dissolution rate is critically reduced through particle aggregation. This order is consistent with the vertical distribution of dissolved REEs and Si in oceans. The differential dissolution kinetics leads to HREE enrichment of the original diatoms and REE enrichment of dissolved diatoms. The Lu/Si ratio of the siliceous fraction of settling particles recovered from some of the highest diatom fluxes is identical to that of the two elements dissolved in deep seawater, providing further evidence for the dissolution of siliceous matter in deep water.

Phase transitions of wax esters adjust buoyancy in diapausing Calanoides acutus

We study the composition and biophysical properties of wax esters extracted from the Antarctic copepod, Calanoides acutus, to explore if these factors adjust buoyancy in diapausing copepods. Levels of wax ester in the copepods were correlated with depth, with deeper animals containing higher amounts. An unsaturation index was used to determine the proportions of polyunsaturated wax esters in the oil sac of the copepods. The proportion of polyunsaturated wax ester in copepods that had descended to depth was always close to 50% and attributable to high amounts of the diatom biomarker fatty acid 20:5(n‐3) contained in these storage lipids. High‐pressure differential scanning calorimetry indicated that wax esters with such high levels of unsaturation exhibited unusual properties, changing from a liquid to a solid phase at pressures and temperatures below 500 m in the ocean. The dense, solid wax esters reduce the overall buoyancy of the lipid‐rich copepods at depth and help facilitate neutral buoyancy. The composition of wax esters is a key factor in buoyancy control in these organisms during diapause.

Reconstruction of surface water conditions in the central region of the Okhotsk Sea during the last 180 kyrs

Core GC9A, a 6.7m long gravity core collected from the central region of the Okhotsk Sea during Cruise YK0712 on R/V Yokosuka (JAMSTEC), was used to reconstruct the changes in surface water conditions by measuring biogenic components (biogenic opal, CaCO3, total organic carbon and δ15N of sediment organic matter) of sediment samples. The age of Core GC9A was determined indirectly by graphic correlation comparing the b⁎ (psychometric yellow–blue chromaticness) values with those of well-dated Core MD01-2415, with complement to the tephra layer (K3; 50ka). The bottom age of Core GC9A was estimated to be about 180kyr; therefore it provides the history of surface water conditions from MIS 1 to MIS 6. The biogenic opal, CaCO3, and TOC contents were high during the interglacial periods as expected, indicating enhanced surface water production under warm climatic conditions. This condition resulted from sufficient nutrient supply to the surface waters by active vertical mixing, which was validated by low δ15N values of the sediment organic matter. In contrast, surface water productivity was depressed during the colder glacial periods, probably due to the expanded sea-ice distribution and limited nutrient supply. However, the glacial sediments had moderately high δ15N values, indicating enhanced nitrate utilization resulting from the limited nutrient supply caused by strong stratification of the surface water. High δ15N values were also observed during the deglaciation, which was attributed to the increased nitrate utilization during enhanced surface water productivity. However, the low δ15N values during the glacial and deglacial periods may be attributed to the increased supply of terrestrial organic matter. Diatom production was primarily responsible for surface water paleoproductivity during the interglacial periods rather than coccolithophores. However, the succession of glacial to early deglacial coccolithophore production and late deglacial to interglacial diatom production was remarkable, corresponding to the present-day seasonal phytoplankton succession. Such an advanced coccolithophore production relative to diatom production might be attributed to the degree of nutrient availability associated with surface water conditions on the basis of variations in the δ15N value. Finally, the opal and TOC contents decreased abruptly in conjunction with a gradual decrease in CaCO3 content from about 2ka, which seems to implicate a late Holocene sudden decrease in paleoproductivity in the central region of the Okhotsk Sea. According to the increase in δ15N values during this interval, nutrient availability appears to be poor, which is likely attributed to the resumed strong stratification that occurred due to the southward shift of the Aleutian Low atmospheric pressure system.

Millennial variability and long‐term changes of the diatom production in the eastern equatorial Pacific during the last glacial cycle

The modern eastern equatorial Pacific (EEP) is a major natural source for atmospheric carbon dioxide and is thought to be connected to high‐latitude ocean dynamics by oceanic teleconnections on glacial‐interglacial timescales. A wealth of sedimentary records aiming at reconstructing last Quaternary changes in primary productivity and nutrient utilization have been devoted to understanding those linkages between the EEP and other distant oceanic areas. Most of these records are, however, clustered in the pelagic EEP cold tongue, with comparatively little attention devoted to coastal areas. Here we present downcore measurements of the composition and concentration of the diatom assemblage together with opal (biogenic silica) concentration at site MD02‐2529 recovered in the coastal Panama Basin. Piston core MD02‐2529, collected in an area affected by a multitude of processes, provides evidence for strong variations in diatom production at the millennial timescale during the last glacial cycle. The maxima in total diatom concentration occurred during the early marine isotopic stage (MIS) 4 as well as during the MIS 4/3 transition and MIS 3. Rapid changes in diatom concentrations during the MIS 3 mimics Bond cycles as independently recorded by the sea surface salinity estimation derived from planktonic foraminifera from the same core. Such patterns indicate a clear linkage between diatom production in the coastal EEP and rapid climate changes in the high‐latitude North Atlantic. In parallel, the long‐term succession of the diatom community from coastal diatoms, predominantly thriving during MIS 5 and 4, toward pelagic diatoms, dominant during MIS 3 and 2, points to a long‐term change in the surface hydrology. During Heinrich events, diatoms strongly reduced their production, probably because of enhanced stratification in the upper water column. After the last glacial maximum, diatom production and valve preservation strongly decreased in response to the advection of nutrient‐depleted (H2SiO4), warmer water masses. Our high‐resolution record highlights how regional climatic processes can modulate rapid changes in siliceous primary production as triggered by wind‐induced local upwelling, indicating that millennial climatic variability can overtake other prominent hydrological processes such as those related to silicic acid leakage.

Diatom-inferred wind activity at Lac du Sommet, southern Québec, Canada: A multiproxy paleoclimate reconstruction based on diatoms, chironomids and pollen for the past 9500 years

Paleo surface wind for southern Quebec was inferred quantitatively for the past 9500 years from diatom assemblages archived in the sediment of the shallow mountain Lac du Sommet using biweekly sediment trap samples along an elevation gradient in the study area. The wind reconstruction was compared with diatom-inferred dissolved organic carbon concentration, chironomid-inferred mean August air temperature, pollen, grain size and loss-on-ignition. Increased lake circulation, interpreted as indicating stronger surface winds, was inferred from diatoms around 8700, 4500, 3000 to 2000 cal. yr BP and during the past 250 years. Increased lake stratification was diatom-inferred from 7500 to 5000 cal. yr BP and between 1500 and 500 cal. yr BP. Diatom-inferred paleowinds were congruent with the regional fire history. In general, diatom production was significantly correlated with diatom-inferred lake circulation. Diatom-inferred lake circulation and diatom production were not correlated with the pollen assemblage changes, diatom-inferred dissolved organic carbon and chironomid-inferred August air temperature, which were highly intercorrelated. After the disappearance of the meltwater in the St Lawrence River valley, the chironomids reflected a warming trend that lasted until about 5000 cal. yr BP, trees replaced shrubs and diatom-inferred dissolved organic carbon increased from 4 mg/l to 6 mg/l. Diatom-inferred lake circulation exhibited periodicities of 200 and 900 years, whereas chironomid-inferred August air temperatures indicated a distinct (significant) 200 year periodicity.