Surface Water Stratification Research Articles

Phosphorus availability, phytoplankton community dynamics, and taxon‐specific phosphorus status in the Gulf of Aqaba, Red Sea

The relationships among phytoplankton taxon‐specific phosphorus‐status, phytoplankton community composition, and nutrient levels were assessed over three seasons in the Gulf of Aqaba, Red Sea. During summer and fall, stratified surface waters were depleted of nutrients, and picophytoplankton populations comprised the majority of cells (80% and 88%, respectively). In winter, surface nutrient concentrations were higher and larger phytoplankton were more abundant (63%). Cell‐specific alkaline phosphatase activity (APA) derived from enzyme‐labeled fluorescence was consistently low (<5%) in the picophytoplankton population throughout the year, whereas larger cells expressed increased APA (up to 68% labeling in some taxa) during the summer and fall but less in the winter. A nutrient addition bioassay during the fall showed that after addition of orthophosphate along with a nitrogen source, APA in larger cells was reduced by half relative to the control, whereas the APA of picophytoplankton groups remained low (<1%) across all treatments. These results indicate that the most abundant phytoplankton in the gulf are not limited by orthophosphate, and only some subpopulations (particularly of larger cells) exhibit orthophosphate limitation throughout the year, and more so in the summer and fall. Our results indicate that orthophosphate availability influences phytoplankton ecology, correlating with shifts in phytoplankton community structure and the nutrient status of individual cells. The role of dissolved organic phosphorus as an important phosphorus source for marine phytoplankton in oligotrophic settings and the need for evaluating nutrient limitation at the taxa and/or single cell level (rather than inferring it from nutrient concentrations and ratios or bulk enzyme activity measurements) are highlighted.

Vertical niche separation control of diversity and size disparity in planktonic foraminifera

Species distribution patterns in planktonic foraminiferal assemblages are fundamental to the understanding of the determinants of their ecology. Until now, data used to identify such distribution patterns was mainly acquired using the standard > 150 μm sieve size. However, given that assemblage shell size-range in planktonic foraminifera is not constant, this data acquisition practice could introduce artefacts in the distributional data. Here, we investigated the link between assemblage shell size-range and diversity in Recent planktonic foraminifera by analysing multiple sieve-size fractions in 12 samples spanning all bioprovinces of the Atlantic Ocean. Using five diversity indices covering various aspects of community structure, we found that counts from the > 63 μm fraction in polar oceans and the > 125 μm elsewhere sufficiently approximate maximum diversity in all Recent assemblages. Diversity values based on counts from the > 150 μm fraction significantly underestimate maximum diversity in the polar — and surprisingly also in the tropical provinces. Although the new methodology changes the shape of the diversity/sea-surface temperature (SST) relationship, its strength appears unaffected. Our analysis reveals that increasing diversity in planktonic foraminiferal assemblages is coupled with a progressive addition of larger species that have distinct, offset shell-size distributions. Thus, the previously documented increase in overall assemblage shell size-range towards lower latitudes is linked to an expanding shell-size disparity between species from the same locality. This observation supports the idea that diversity and shell size-range disparity in foraminiferal assemblages are the result of niche separation. Increasing SST leads to enhanced surface water stratification and results in vertical niche separation, which permits ecological specialisation. Specific deviations from the overall diversity and shell-size disparity latitudinal pattern are seen in regions of surface-water instability, indicating that coupled shell-size and diversity measurements could be used to reconstruct water column structures of past oceans.

Millennial-scale fluctuations in seasonal sea-ice and deep-water formation in the Japan Sea during the late Quaternary

The East Asian winter monsoon is characterized by cold, dry air blowing from the Siberian high to the surrounding lows. This cold wind cools the surface water along the Russian coast of the Japan Sea. Modern oceanographic observations indicate that sea-ice formation along this coast during an extremely cold winter is an important process for deep-water formation. Thus, formation of both seasonal sea ice and deep-water masses in the Japan Sea are closely related to the East Asian winter monsoon. Two sediment cores were analyzed to reconstruct the history of sea-ice expansion and deep-water formation. Seasonal sea-ice history was reconstructed from the occurrence of ice-rafted debris (IRD) in a core from the northeastern Japan Sea, whereas the deep-water ventilation history was estimated on the basis of records of a deep-dwelling radiolarian species, Cycladophora davisiana, in a core from the southwestern Japan Sea. In the records of sea-ice expansion and deep-water ventilation for the last 160 ky, clear fluctuations on a millennial timescale were observed during marine oxygen isotope stages 3–5. Some of the peaks in sea-ice formation and C. davisiana abundance occurred contemporaneously, indicating strong surface cooling along the Russian coast by a strong winter monsoon. Decoupling of the two records might have occurred because of surface-water stratification caused by the freshening of surface water in relation to high precipitation in summer. Synchronicity of high IRD peaks with North Atlantic Heinrich events suggests that high-latitude climate changes related to the Bond cycle also occurred in the Asian monsoon region. On shorter timescales, IRD occurrence was not clearly related to Dansgaard–Oeschger cycles, which are revealed in the alternating dark and light layers in the late Quaternary Japan Sea sediments. Therefore, during the studied period, strong East Asian winter monsoons did not have a simple see-saw-like relationship with strong summer monsoons.

Bacterioplankton composition in the Scotia Sea, Antarctica, during the austral summer of 2003

Physical ocean processes (ice-melt, island run-off and upwelling of nutrients) were hypothesised to affect the bacterioplankton composition in the surface mixed layer of the Scotia Sea during the austral summer of 2003, and this was investigated using flow cytometry and catalysed reporter deposition fluorescence in situ hybridisation (CARD-FISH) techniques. The bacterio- plankton was composed predominantly of Alphaproteobacteria (PB), comprising SAR11, Roseobacter spp. and SAR116 groups, followed by Sphingobacteria/Flavobacteria and Gammaproteobacteria, including SAR86. Two distinct bacterioplankton communities were identified, largely based on bac- terioplankton abundance, which varied from 0.3 ± 0.06 × 10 6 cells ml -1 in the west to 0.8 ± 0.3 × 10 6 cells ml -1 in the east, and a corresponding difference in SAR11 percentages of 30 ± 15% in the west compared to 5 ± 5% in the east. The western community was present in waters that were largely in an over-wintered, pre-bloom condition. The eastern bacterioplankton community was associated with phytoplankton blooms developed within the eastern Scotia Sea nutrient upwelling zone, where the Antarctic Circumpolar Current (ACC) encounters the shallow bathymetry associated with the Scotia Arc, in combination with seasonal ice-melt and island effects that enabled surface water stratification.

Assessing the vulnerability of endemic diatom species in Lake Baikal to predicted future climate change: a multivariate approach

AbstractDiatoms in Lake Baikal exhibit significant spatial variation, related to prevailing climate, lake morphology and fluvial input into the lake. Here we have assessed the threats to endemic planktonic diatom species (through the development of empirical models), which form a major component of primary production within the lake. Multivariate techniques employed include redundancy analysis (RDA) and Huisman–Olff–Fresco (HOF) models. Our analyses suggest that eight environmental variables were significant in explaining diatom distribution across the lake, and in order of importance these are snow thickness on the ice, water depth, duration of days with white ice, suspended matter in the lake, days of total ice duration, temperature of the water surface in July, concentration of zooplankton and suspended organic matter. Impacts on dominant phytoplankton diatom species are highlighted using t‐value biplots. Predictions of future climate change on Lake Baikal are likely to result in shorter periods of ice cover, decreased snow cover across the lake in spring, increased fluvial input into the lake, and an increase in the intensification of surface water stratification during summer months. All these factors are likely to impact negatively on the slow‐growing, cold‐water endemics such as Aulacoseira baicalensis and Cyclotella minuta, which currently dominate diatom assemblages. Instead, taxa that are only intermittently abundant, at present, in offshore areas (e.g. Stephanodiscus meyerii) are likely to become more frequent. However, given the climatic gradient across the lake, the timing and extent of changes in community structure are likely to vary. Moreover, palaeolimnological records show that Lake Baikal diatom assemblages have been dynamic throughout the Holocene, with both endemic and cosmopolitan species exhibiting periods of dominance. Effects of climate change on the entire lake ecosystem may yet be profound as the structure of the pelagic food web may change from one based on endemic diatom taxa to one dominated by nondiatom picoplankton, and as limnological functioning (e.g. stratification and mixing) affects deepwater oxygen availability, nutrient cycling and trophic linkages.

Production of giant marine diatoms and their export at oceanic frontal zones: Implications for Si and C flux from stratified oceans

From a synthesis of recent oceanic observations and paleo‐data it is evident that certain species of giant diatoms includingRhizosoleniaspp.Thalassiothrixspp. andEthmodiscus rexmay become concentrated at oceanic frontal zones and subsequently form episodes of mass flux to the sediment. Within the nutrient bearing waters advecting towards frontal boundaries, these species are generally not dominant, but they appear selectively segregated at fronts, and thus may dominate the export flux. AncientThalassiothrixdiatom mat deposits in the eastern equatorial Pacific and beneath the Polar Front in the Southern Ocean record the highest open ocean sedimentation rates ever documented and represent vast sinks of silica and carbon. Several of the species involved are adapted to a stratified water column and may thrive in Deep Chlorophyll Maxima. Thus in oceanic regions and/or at times prone to enhanced surface water stratification (e.g., during meltwater pulses) they provide a mechanism for generating substantial biomass at depth and its subsequent export with concomitant implications for Si export and C drawdown. This ecology has important implications for ocean biogeochemical models suggesting that more than one diatom “functional type” should be used. In spite of the importance of these giant diatoms for biogeochemical cycling, their large size coupled with the constraints of conventional oceanographic survey schemes and techniques means that they are undersampled. An improved insight into these key species will be an important prerequisite for enhancing our understanding of marine biogeochemical cycling and for assessing the impacts of climate change on ocean export production.

Late Neogene history of deepwater ventilation in the Southern Ocean

We compiled carbon isotope records from the North Atlantic, South Atlantic, and Pacific oceans to estimate changes in vertical and interbasinal δ13C gradients for the last 9 Myr. Benthic δ13C values of deep water in the South Atlantic decreased in a series of steps at ∼7, 2.75, and 1.55 Ma away from the North Atlantic and toward the Pacific. The benthic δ13C of intermediate water in the South Atlantic evolved differently from deep waters, resulting in an increase in the intermediate‐to‐deep δ13C gradient (Δ13CID). The Δ13CID increased in steps at ∼2.75 and 1.55 Ma, marking the development and intensification of a chemical divide in the Atlantic Ocean between well‐ventilated intermediate water and poorly ventilated deep water. We suggest these changes in interbasinal and vertical gradients resulted from decreasing Northern Component Water and reduced ventilation of Southern Component Water (SCW). The latter was caused by several interrelated processes in Antarctic sources areas, including increased sea ice cover, enhanced surface water stratification, decreased Ekman‐induced upwelling, and reduced vertical mixing across the thermocline. Because Antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric CO2 concentration, these processes may have acted as a positive feedback that contributed to the major cooling steps in late Neogene climate.

Phosphorus-limited growth dynamics in two Baltic Sea cyanobacteria, Nodularia sp. and Aphanizomenon sp.

Rates of carbon (C) specific growth and nitrogen (N2) fixation were monitored in cultures of Baltic Sea Nodularia and Aphanizomenon exposed to gradual limitation by inorganic phosphorus (P). Both cyanobacteria responded by decreased cellular P content followed by lowered rates of growth and N2 fixation. C-specific growth and cellular N content changed faster in Aphanizomenon both when inorganic P was lowered as well as during reintroduction of P. Aphanizomenon also showed a more rapid increase in N-specific N2 fixation associated with increased C-specific growth. When ambient concentrations of inorganic P declined, both cyanobacteria displayed higher rates of alkaline phosphatase (APase) activity. Lower substrate half-saturation constants (KM) and higher Vmax : KM ratio of the APase enzyme associated with Nodularia suggest a higher affinity for dissolved organic P (DOP) substrate than Aphanizomenon. Aphanizomenon, which appears more sensitive to changes in ambient dissolved inorganic P, may be adapted to environments with elevated concentrations of P or repeated intrusions of nutrient-rich water. Nodularia on the other hand, with its higher tolerance to increased P starvation may have an ecological advantage in stratified surface waters of the Baltic Sea during periods of low P availability.

Seasonal variation in skeletal silicification of Neodenticula seminae, a marine planktonic diatom: Sediment trap experiments in the NW Pacific Ocean (1997–2001)

Neodenticula seminae is an important component of pelagic diatom floras in the subarctic North Pacific and its adjacent seas. We conducted an SEM image analysis of four series of samples collected by sediment traps in the NW Pacific, in order to investigate the relations between its morphology, evolution, and environment. Seasonal variation in the skeletal morphology was common: lightly silicified morphs were dominant during spring blooms and in the summer (under oligotrophic conditions owing to surface water stratification and progressive nutrient limitation), whereas heavily silicified morphs became abundant in the fall and winter, subsequent to the onset of intensive vertical mixing. These observations suggest that the morphology of N. seminae is influenced by environmental factors including the nutrient availability as well as by its reproductive strategy.

Coccolith distribution patterns in South Atlantic and Southern Ocean surface sediments in relation to environmental gradients

In this study, the coccolith compositions of 213 surface sediment samples from the South Atlantic and Southern Ocean were analysed with respect to the environmental parameters of the overlying surface waters. From this data set, the abundance patterns of the main species and their ecological affinities were ascertained. In general, Emiliania huxleyi is the most abundant species of the recent coccolith assemblages in the study region. However, the lower photic zone taxa, composed of Florisphaera profunda and Gladiolithus flabellatus often dominate the assemblages between 20°N and 30°S. If E. huxleyi is excluded, Calcidiscus leptoporus and F. profunda become the most abundant species, each dominating discrete oceanographic regimes. While F. profunda is very abundant in the sediments underneath warmer, stratified surface waters with a deep nutricline, Calcidiscus leptoporus is encountered in high-productivity environments. Furthermore, the results of a canonical correspondence analysis reveal affinities of Gephyrocapsa spp., Helicosphaera spp. and Coccolithus pelagicus for intermediate to higher nutrient conditions in a well-mixed upper water column. In contrast, Gladiolithus flabellatus seems to be associated with high temperatures and salinities under low-nutrient conditions. Based on the relative abundances of Calcidiscus leptoporus, F. profunda, Gladiolithus flabellatus, Helicosphaera spp., Umbilicosphaera foliosa, Umbilicosphaera sibogae and a group of subordinate subtropical species, six surface sediment assemblages have been identified, which reflect the distribution and characteristics of the overlying surface waters. Their distribution appears to be mainly a function of the relative position of the nutricline and thermocline in the overlying photic zone.

Calcareous nannofossil changes during the late Callovian–early Oxfordian cooling phase

Calcareous nannofossil quantitative and biostratigraphic analyses integrated with carbon and oxygen stable isotopes were carried out on the core ANDRA (Agence Nationale pour la gestion des Déchets Radio-Actifs—FRANCE) HTM 102 across the Callovian/Oxfordian boundary drilled at Cirfontaines-en-Ornois, Départment de Haute-Marne, eastern France. Calcareous nannofossil assemblages at the Callovian–Oxfordian transition are dominated by the genus Watznaueria. An increase in abundance of Biscutum spp. and A-group, which consists of Axopodorhabdus spp. ( A. atavus, A. rahla, and A. cylindratus), Podorhabdus grassei, Octopodorhabdus decussatus, Hexapodorhabdus cuvillieri (family Axopodorhabdaceae), and Triscutum spp., correlates with a significant positive excursion in δ 18O suggesting that these groups were probably adapted to cooler surface waters. A positive increase in δ 13C values is coupled with high abundances of eutrophic taxa such as Zeughrabdotus erectus, Biscutum spp., and small-sized Watznaueria britannica, and a decrease in abundance of the big and oligotrophic taxa Schizosphaerella punctulata and Watznaueria manivitae. Climate cooling across the Callovian/Oxfordian boundary probably triggered a breakdown in stratification of surface waters leading to more intense nutrient recycling and higher primary productivity that favoured the shift in abundance of small-sized eutrophic taxa in the East Paris Basin.

Distribution of age-classes of striped weakfish ( Cynoscion guatucupa) along an estuarine–marine gradient: Correlations with the environmental parameters

The age-class distribution of the striped weakfish ( Cynoscion guatucupa) and its relationship with environmental factors were evaluated along the coast between Argentina and Uruguay, including the Río de la Plata estuary (36° S, 56° W). One hundred and seven stations were sampled between 2001 and 2003 during autumn, winter, and spring. Multivariate analyses (Cluster and MDS) were used to define sampling station groups, and a BIO-ENV procedure used to estimate the association of stations with environmental factors (depth, stratification, temperature and salinity of surface and bottom waters). The BIO-ENV procedure indicated that the salinity during autumn–winter and the temperature in spring had a significant influence on the spatial age-class pattern. The proportion of the youngest age-classes decreased from the estuarine area to the marine coastal area, except during spring where it showed the reverse pattern. The older age-classes inhabited the marine coastal area (more saline) toward the deepest zone, while the youngest individuals inhabited the outer estuarine area. Examination of the coastal circulation associated with the spatial distribution, and of the age-class frequency in the estuarine and marine coastal area, suggested that spawning took place in inshore coastal waters at very shallow depths.

Geochemical and micropaleontological characterisation of a Mediterranean sapropel S5: A case study from core BAN89GC09 (south of Crete)

Several geochemical and micropaleontological proxies were studied in a well developed sapropel S5 from a core collected south of Crete to investigate the causes and the mechanisms of its deposition, to reconstruct the paleoenvironmental and paleoceanographical conditions at time of its formation and to detect the roles played by productivity and anoxia. The geochemical proxies are also used to unravel the sediment provenance and therefore to help tracing water circulation patterns and freshwater sources. The multiproxy approach reveals that the sapropel layer has high internal variability and five different depositional phases can be identified, four within the visible sapropel layer. An increase in temperature and the development of a Deep Chlorophyll Maximum characterise the onset of the sapropel (phase 1), with enhanced productivity being favoured by local riverine input, not clearly ascribable to Nile river freshwater discharge, and by the shoaling of the pycnocline/nutricline. Surface water stratification and productivity develop, together with strong seafloor dysoxia/anoxia. Phase 2 is characterised by lower stratification, slightly lower temperature and/or higher seasonal contrast, and still high productivity. Phase 3 records high productivity, high temperature and high stratification, probably more sustained by the Nile input. A sudden partial re-oxygenation takes place at the end of this phase, changing the main features of the sapropel. In the fourth phase, productivity starts to decrease, water stratification seems to be partially broken down and sediment oxygenation increases. A transitional phase (phase 5) occurs between the end of the visible sapropel and the normal pelagic post-sapropel sedimentation, where productivity is still high and oxygenation improves compared to the visible sapropel layer. The massive occurrence of Polysphaeridium zoharyi, cyst of a red tide forming dinoflagellate, also characterises this phase. The geochemical proxies indicate that pre-sapropel and post-sapropel sediments are quite different from each other, and the provenance shifts from a more mixed to a more markedly southern origin of sediments.

Photochemical Mineralization of Dissolved Organic Nitrogen to Ammonium in the Baltic Sea

Solar-radiation-induced photochemistry can be considered as a new source of nutrients when photochemical reactions release bioavailable nitrogen from biologically nonreactive dissolved organic nitrogen (DON). Pretreatments of Baltic Sea waters in the dark indicated that >72% of DON was recalcitrant to biological mineralization. When this DON (16-21.5 microM) was exposed to simulated solar radiation, the concentration of NH4+ increased 0.5-2.5 microM more in irradiated waters than in the dark controls. The photochemical production of NH4+ and the dose of absorbed photons were used to calculate the apparent quantum yield spectrum for photoammonification [mol NH4+ (mol photons)(-1) nm(-1)] at wavelengths (lambda) of 290-700 nm (phiNH4,lambda). The modeled mean rates of photoammonification based on phiNH4,lambda were 143 and 53 micromol NH4+ m(-2) d(-1) at the surface and in the whole water column, respectively, of Baltic Sea stations during summer. The results of this study indicate thatthe rate of photoammonification approximately equals and periodically exceeds the rate of atmospheric deposition of reactive inorganic nitrogen to the northern Baltic Sea. Forthese stratified surface waters beyond riverine input of labile nitrogen, photoammonification can periodically be the largest source of new bioavailable nitrogen.

Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian ‘Niveau Breistroffer’ black shales (Oceanic Anoxic Event 1d, SE France)

The Niveau Breistroffer black shale succession in the Vocontian Basin (SE France) is the regional equivalent of the widely distributed Late Albian Oceanic Anoxic Event 1d. The studied black shale-rich interval at the Col de Palluel section is 6.28 m thick and comprises four black shale units with up to 2.5 wt% total organic carbon (TOC) intercalated with marlstones. Calcareous nannofossil, palynomorph, planktic Foraminifera and stable isotopic data from the Niveau Breistroffer succession suggest that short-term climate changes influenced its deposition, with relatively warm and humid climate during black shale formation in comparison with relatively cool and dry climatic conditions during marlstone deposition. An increase in the terrigenous/marine ratio of palynomorphs indicates enhanced humidity and higher runoff during black shale formation. A nutrient index based on calcareous nannofossils and the abundance pattern of small (63–125 μm) hedbergellid Foraminifera show short-term changes in the productivity of the surface water. Surface-water productivity was reduced during black shale formation and increased during marlstone deposition. A calcareous nannofossil temperature index and bulk-rock oxygen isotope data indicate relative temperature changes, with warmer surface waters for black shale samples. At these times, warm–humid climate and reduced surface-water productivity were accompanied by greater abundances of ‘subsurface’-dwelling calcareous nannofossils (nannoconids) and planktic Foraminifera (rotaliporids). These taxa presumably indicate more stratified surface-water conditions. We suggest that the formation of the Niveau Breistroffer black shales occurred during orbitally induced increase in monsoonal activity that led to increasing humidity during periods of black shale formation. This, in turn, caused a decrease in low-latitude deep-water formation and probably an increase in surface-water stratification. The combination of these two mechanisms caused depleted O 2 concentrations in the bottom water that increased the preservation potential of organic matter.

Precessional forcing of coccolithophore assemblages in the northern Arabian Sea: Implications for monsoonal dynamics during the last 200,000 years

The paleoceanographic potential of coccolithophores was used to decipher the monsoon induced paleoproductivity changes in the northern Arabian Sea during the past 200,000 years. Core SO 90–94 KL was taken in the north-eastern part of the study area representing the primarily NE-monsoon-influenced domain. For comparison, core ODP117–723 A from the western side of the Arabian Sea off Oman under the dominating influence of the SW-monsoon was investigated. Samples were prepared using a quantitative dilution/filtering technique and investigated in the SEM. We focus here only on the most important contributors to the assemblages Gephyrocapsa oceanica, Gephyrocapsa ericsonii, Emiliania huxleyi, Florisphaera profunda contributing at least 63% up to 92.5% of the coccolith assemblages. Abundances of the two species E. huxleyi and G. ericsonii were summed up and depicted as a combined record (EhuxGeric). The 200,000 year record from the north-eastern Arabian Sea yielded a significant Precession derived signal in the abundances of EhuxGeric which could be related by cross spectral analysis to maximum low latitudinal insolation. In contrast, G. oceanica seemed to correspond to insolation minima. Relative abundances of F. profunda are generally rather low compared to other oceanographic regions and exhibit no distinct variations except for the thermal maxima of isotope stages II/I and V. A schematic model is presented to show the changes in surface water stratification in the Arabian Sea driven by changing monsoon intensities over periodicities of the precessional band of Milankovitch cycles. This model is based on the variations of the two taxa groups G. oceanica and EhuxGeric together with changes in the abundance of Cocco total and F. profunda. Probably the Arabian Sea remained eutrophic at least during one season of the year despite drastic changes in the summer monsoon strength. Lower productivities during the summer monsoon were probably compensated by higher phytoplankton growth due to stronger winter monsoons.

North Atlantic warming during global cooling at the end of the Cretaceous

Research Article| June 01, 2005 North Atlantic warming during global cooling at the end of the Cretaceous Kenneth G. MacLeod; Kenneth G. MacLeod 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211-1380, USA Search for other works by this author on: GSW Google Scholar Brian T. Huber; Brian T. Huber 2Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20013-7012, USA Search for other works by this author on: GSW Google Scholar Carolina Isaza-Londoño Carolina Isaza-Londoño 3Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211-1380, USA Search for other works by this author on: GSW Google Scholar Geology (2005) 33 (6): 437–440. https://doi.org/10.1130/G21466.1 Article history received: 14 Dec 2004 rev-recd: 08 Feb 2005 accepted: 10 Feb 2005 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Kenneth G. MacLeod, Brian T. Huber, Carolina Isaza-Londoño; North Atlantic warming during global cooling at the end of the Cretaceous. Geology 2005;; 33 (6): 437–440. doi: https://doi.org/10.1130/G21466.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Differences in regional responses to climate fluctuations are well documented on short time scales (e.g., El Niño–Southern Oscillation), but with the exception of latitudinal temperature gradients, regional patterns are seldom considered in discussions of ancient greenhouse climates. Contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years, this paper shows that the North Atlantic warmed by as much as 6 °C (1.5‰ decrease in δ18O values of planktic foraminifera) during the Maastrichtian global cooling interval. We suggest that warming was the result of the importation of heat from the South Atlantic. Decreasing North Atlantic δ18O values are also associated with increasing gradients in planktic δ13C values, suggesting increasing surface-water stratification and a correlated strengthening of the North Atlantic Polar Front. If correct, this conclusion predicts arctic cooling during the late Maastrichtian. Beyond implications for the Maastrichtian, these data demonstrate that climate does not behave as if there is a simple global thermostat, even on geologic time scales. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A global compilation of late Holocene planktonic foraminiferal [formula omitted]18O: relationship between surface water temperature and [formula omitted]18O

We review the different sources of uncertainty affecting the oxygen isotopic composition of planktonic foraminifera and present a global planktonic foraminifera oxygen isotope data set that has been assembled within the MARGO project for the Late Holocene time slice. The data set consists of over 2100 data from recent sediment with thorough age control, that have been checked for internal consistency. We further examine how the oxygen isotopic composition of fossil foraminifera is related to hydrological conditions, based on published results on living foraminifera from plankton tows and cultures. Oxygen isotopic values ( δ 18O) of MARGO recent fossil foraminifera are 0.2–0.8 ‰ higher than those of living foraminifera. Our results show that this discrepancy is related to the stratification of the upper water mass and generally increases at low latitudes. Therefore, as stratification of surface waters and seasonality depends on climatic conditions, the relationship between temperature and δ 18O established on fossil foraminifera from recent sediment must be used with caution in paleoceanographic studies. Before models predicting seasonal flux, abundance and δ 18O composition of a foraminiferal population in the sediment are available, we recommend studying relative changes in isotopic composition of fossil planktonic foraminifera. These changes primarily record variations in temperature and oxygen isotopic composition of sea water, although part of the changes might reflect modifications of planktonic foraminifera seasonality or depth habitat.

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l−1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 μS cm−1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.

Distribution of diatoms, coccolithophores and planktic foraminifers along a trophic gradient during SW monsoon in the Arabian Sea

The distribution of diatoms, coccolithophores and planktic foraminifers mirrored the hydrographic and trophic conditions of the surface ocean (0–100 m) across the upwelling area off the Oman coast to the central Arabian Sea during May/June 1997 and July/August 1995. The number of diatoms was increased in waters with local temperature minimum and enhanced nutrient concentration (nitrate, phosphate, silicate) caused by upwelling. Vegetative cells of Chaetoceros dominated the diatom assemblage in the coastal upwelling area. Towards the more nutrient depleted and stratified surface waters to the southeast, the number of diatoms decreased, coccolithophore and planktic foraminiferal numbers increased, and floral and faunal composition changed. In particular, the transition between the eutrophic upwelling region off Oman and the oligotrophic central Arabian Sea was marked by moderate nutrient concentration, and high coccolithophore and foraminifer numbers. Florisphaera profunda, previously often referred as a ‘lower-photic-zone-species’, was frequent in water depths as shallow as 20 m, and at high nutrient concentration up to 14 μmol NO 3 l −1 and 1.2 μmol PO 4 l −1. To the oligotrophic southeast of the divergence, cell densities of coccolithophores declined and Umbellosphaera irregularis prevailed throughout the water column down to 100 m depth. In general, total coccolithophore numbers were limited by nutrient threshold concentration, with low numbers (<10×10 3 cells l −1) at high [NO 3] and [PO 4], and high numbers (>70×10 3 cells l −1) at low [NO 3] and [PO 4]. The components of the complex microplankton succession, diatoms, coccoliths and planktic foraminifers (and possibly others), should ideally be used as a combined paleoceanographic proxy. Consequently, models on plankton ecology should be resolved at least for the seasonality, to account for the bias of paleoceanographic transfer calculations.