Lower Photic Zone Research Articles

Correction to: Calcification rates in the lower photic zone and their ecological implications

Correction to: Calcification rates in the lower photic zone and their ecological implications

Calcification rates in the lower photic zone and their ecological implications

Calcification rates in the lower photic zone and their ecological implications

Response of coccolithophore communities to oceanographic and atmospheric processes across the North- and Equatorial Atlantic

Changes in coccolithophore productivity in response to climate-driven ocean warming are likely to have cascading biogeochemical effects that feed back to the changing climate. This paper investigates the role (and interplay) of large-scale oceanographic and atmospheric processes across the North- and Equatorial Atlantic, including Saharan dust deposition, on the distribution of coccolithophore communities. The study is based on biological and hydrological data collected across the photic zone of the ocean, and aerosol data collected from the lower atmosphere, across 50°N–1°S during the Atlantic Meridional Transect in boreal Autumn of 2018 (AMT28), in synergy with Earth Observations. Results confirm existing understanding of the distribution of coccolithophore communities which are related to major meridional hydrological gradients across the North Atlantic. Dynamic, oxygenated and microphytoplankton-enriched waters at higher-latitudes were characterized by less diverse coccolithophore populations, dominated by placolith-bearing r-selected coccolithophores. In contrast, the heavily stratified and picoplankton-enriched waters of the subtropical gyre revealed more diverse populations, dominated by umbelliform coccolithophores and holococcolithophores at the surface, and by floriform taxa in the lower photic zone. Mean concentrations of 14.4×103cells/L present in the North Atlantic Tropical Gyre Province (30–12°N), only slightly lower compared to 17.7×103cells/L produced in the North Atlantic Drift province (50–40°N), provide a snapshot perspective on the importance of coccolithophore production in heavily stratified gyre conditions. Higher concentrations of 19’-Hexanoyloxyfucoxanthin (HexFuco) in regions of enhanced production of r-selected placolith-bearing species suggest that this pigment should not be generalized as a proxy for the entire coccolithophore community. Enhanced abundances of fast-bloomingEmiliania huxleyiandGephyrocapsa oceanica, and of cyanobacteria (including both picoplankton and N2-fixingTrichodesmiumspp.) at the surface of the region of more persistent Saharan dust deposition (at ~12-10°N) appeared to result from dust-born nutrient input. Underneath this stratified surface layer, enhanced productivity in the deep chlorophyll maximum (DCM) appeared decoupled from that on the surface, fueled by geostrophic eastward shoaling of the nutricline across the tropical North Atlantic. As this was the region of highest macronutrient concentrations measured along and below the nutricline, our data suggest that the NE tropical Atlantic may act as a permanent dust-born nutrient depocenter as previously hypothesized.

Relationship between thermohaline and biochemical patterns in the levantine upper and intermediate water masses, Southeastern Mediterranean Sea (2013–2021)

The relationships between the interannual variations of the Levantine intermediate water (LIW) core properties and the corresponding biochemical variations in the euphotic zone were systematically studied in the Southeastern Mediterranean during 2013–2021 and since 2002 based on a previous study. Salinity and temperature interannual fluctuations in the LIW continue to follow the Adriatic–Ionian Bimodal Oscillating System (BiOS) mechanism, with salinity and temperature peaks in the years 2008–2010, 2014–2015, and 2018–2019 coinciding with periods of anticyclonic circulation of the North Ionian Gyre (NIG). During these anticyclonic periods, the transport of Atlantic Water into the Levant is reduced together with the transport of LIW out of the basin. These interannual fluctuations are superimposed on a long-term warming trend clearly evident from previous studies, showing a maximal temperature in 2018–2019, higher than the previously mentioned temperature peaks by ~0.7°C and ~0.4°C. The enhanced warming in 2018–2019 has caused a decrease in density (sigma) values of the LIW core, which gave way to the shallowest record of this water mass (~110-m depth), bringing it well within the lower photic zone. We suggest that a higher level of nutrients became available, supporting the observed long-term rise of the intergraded chlorophyll a (Chl.a) (0.89 mg m−2 year−1), with a maximum recorded during 2018–2019. The long-term record of the mixed layer depths shows no significant change; thus, the uplift of nutrients during winter mixing cannot support the trend and variations of the integrated Chl.a. Additional biological parameters of specific pico-phytoplankton populations and integrated bacterial production and abundance were measured in 2013–2021, but the measurements were too sparse to follow a clear interannual dynamics. Yet significantly higher average levels for integrated primary production and bacterial abundances were observed during the anticyclonic period (as for Chl.a). The combined impacts of the BiOS mechanism and global warming, and hence the increase in LIW residence time and buoyancy, may impact the primary producers’ biomass at the photic zone. This latter feedback may slightly counter the enhanced oligotrophication due to enhanced stratification.

Carbonate fluxes by coccolithophore species between NW Africa and the Caribbean: Implications for the biological carbon pump

Coccolithophores are among the most important calcifying pelagic organisms. To assess how coccolithophore species with different coccolith‐carbonate mass and distinct ecological resilience to ocean warming will influence the “rain ratio” and the “biological carbon pump”, 1 yr of species‐specific coccolith‐carbonate export fluxes were quantified using sediment traps moored at four sites between NW Africa and the Caribbean (i.e., CB‐20°N/21°W, at 1214 m; M1‐12°N/23°W, at 1150 m; M2‐14°N/37°W, at 1235 m; M4‐12°N/49°W, at 1130 m). Highest coccolith‐CaCO3 fluxes at the westernmost site M4, where the nutricline is deepest along the tropical North Atlantic, were dominated by deep‐dwelling small‐sized coccolith species Florisphaera profunda and Gladiolithus flabellatus. Total coccolith‐CaCO3 fluxes of 371 mg m−2 yr−1 at M4 were followed by 165 mg m−2 yr−1 at the north‐easternmost CB, 130 mg m−2 yr−1 at M1, and 114 mg m−2 yr−1 at M2 in between. Coccoliths accounted for nearly half of the total carbonate flux at M4 (45%), much higher compared to 23% at M2 and 15% at M1 and CB. At site M4, highest ratios of coccolith‐CaCO3 to particulate organic carbon fluxes and weak correlations between the carbonate of deep‐dwelling species and particulate organic carbon suggest that increasing productivity in the lower photic zone in response to ocean warming might enhance the rain ratio and reduce the coccolith‐ballasting efficiency. The resulting weakened biological carbon pump could, however, be counterbalanced by increasing frequency of Saharan dust outbreaks across the tropical Atlantic, providing mineral ballast as well as nutrients to fuel fast‐blooming and ballast‐efficient coccolithophore species.

Moderate zooxanthellate coral growth rates in the lower photic zone

The ecology of phototrophic corals in the lower photic zone remains poorly understood. Studies to date indicate that growth rates generally decrease as available light attenuates with depth and are very slow at depths > 40 m. Here, we provide detailed evidence for moderate growth for obligate zooxanthellate corals at extreme depths. Using reliable U–Th dating techniques, Leptoseris spp. from 70 to 110 m in Hawaii were determined to grow (linear extension) from 8.0 ± 3.0 to 24.6 ± 2.7 mm yr−1. Given the modest growth rates of other agariciids in well-lit shallow waters, these deep-water growth rates are unexpectedly high, suggest specialized physiology and help alter the traditional paradigm on photo-ecology in mesophotic coral ecosystems. The thin plate-like colonies of Leptoseris spp., which dominate the coral community in the lower photic zone throughout the Indo-Pacific, primarily grow radially and do not appear to appreciably thicken over time. In the lower photic zone, this growth strategy maximizes surface area with minimal calcification, thereby enabling Leptoseris to expand planar area more quickly than thickly calcified species. Photosynthetic and growth efficiencies may also be facilitated by the optical geometry of their skeletons, increased access to inorganic nutrients and cooler temperatures at depth. These robust growth rates have important implications on the recovery potential of MCE habitat in the event of disturbance.

Transatlantic gradients in calcifying phytoplankton (coccolithophore) fluxes

Tropical oceans provide a benchmark for future primary productivity in increasingly warmer, stratified and nutrient-depleted waters. In this context, we assess the export fluxes of calcifying phytoplankton (coccolithophores) across the tropical North Atlantic, from upwelling affected NW Africa, via three ocean sites along 12°N to the Caribbean. Sampling was undertaken by means of a spatial array of four time-series sediment traps collecting particle fluxes in two-week intervals, from October 2012 to February 2014, allowing to track temporal changes along the southern margin of the North Atlantic central gyre. Species composition and seasonal export fluxes of coccolithophores show steep gradients in two groups. Upper photic zone (UPZ) and placolith-bearing species dominated by Emiliania huxleyi and Gephyrocapsa oceanica are most abundant in the mesotrophic surface waters of the Cape Blanc upwelling system off NW Africa. They decline gradually towards the Caribbean, paralleled by increasing surface temperatures and decreasing surface chlorophyll-a. Meanwhile the abundance of lower photic zone (LPZ) species Florisphaera profunda and Gladiolithus flabellatus increase in the same direction, reaching fluxes up to 3–5 times higher in the western end of the transect compared to the UPZ flora in mesotrophic waters. Adapted to low light conditions, the LPZ species follow the geostrophic wind-forced deepening of the thermocline/nutricline westward in ever lower species diversity towards the Caribbean.Temporal changes were marked by weak seasonality in coccolith fluxes at all four sites, modulated by latitudinal migration of the Intertropical Convergence Zone (ITCZ) at the tropical sites M1, M2 and M4, and by spatiotemporal variation in wind-forced upwelling at site CB. A seasonal mismatch was observed between LPZ and UPZ-oligotrophic taxa (i.e. Umbellosphaera spp., Rhabdosphaera spp.) vs. UPZ-opportunistic species (E. huxleyi, G. oceanica) at the western site M4 contrasting with the more similar seasonal patterns amongst all species towards site CB. We interpret this as reflecting the entire photic zone becoming increasingly nutrient enriched towards east whenever wind-forced mixing occurs due to the eastward shoaling of the thermocline/nutricline. Our synoptic observations of seasonally resolved export fluxes at four sites contribute to the spatiotemporal understanding of coccolithophore fluxes across the entire tropical North Atlantic, urging for considering phytoplankton- and carbonate production across the entire photic zone when projecting the effects of ocean warming on future primary production.

Phaeocystis antarctica unusual summer bloom in stratified antarctic coastal waters (Terra Nova Bay, Ross Sea)

This study focuses on the potential explanations for a Phaeocystis antarctica summer bloom occurred in stratified waters of Terra Nova Bay (TNB) - which is part of the Antarctic Special Protected Area (n.161) in the Ross Sea - trough a multi-parameter correlative approach. Many previous studies have highlighted that water column stratification typically favors diatom dominance compared to the colonial haptophyte P. antarctica, in the Ross Sea, and this correlation has often been used to explain the historic dominance of diatoms in TNB.To explore the spatial and temporal progression of P. antarctica bloom in coastal waters, four stations were sampled three times each between December 31, 2009 and January 13, 2010.Taxonomic and pigment composition of phytoplankton communities, macro-nutrient concentrations and various different indices, all indicated the relative dominance of P. antarctica. Cell abundances revealed that P. antarctica contributed 79% of total cell counts in the upper 25 m and 93% in the lower photic zone. Similarly, a strong correlation was observed between Chl-a and the Hex:Fuco pigment ratio, corroborating the microscopic analyses. Recent studies have shown that iron can trigger colonial P. antarctica blooms. Based on the Hex:Chl-c3 proxy for iron limitation in P. antarctica, we hypothesize that anomalously higher iron fluxes were responsible for the unusual bloom of colonial P. antarctica observed in TNB.

Life cycle association of the coccolithophore Syracosphaera gaarderae comb. nov. (ex Alveosphaera bimurata): Taxonomy, ecology and evolutionary implications

Abstract The coccolithophores are an important group of marine phytoplankton bearing external calcite plates called coccoliths, and displaying complex sexual life cycles, composed of two main cell phases (haploid and diploid) that in most species can produce distinct coccolith types. Typically, diploid cells produce heterococcoliths, whereas haploid cells produce holococcoliths. An important source of evidence for the ability of coccolithophores to undergo life cycle transitions comes from observations of combination coccospheres. These combination cells bearing both hetero- and holococcoliths are interpreted as cells undertaking sexual transitions and enable linking morphologically distinct life phases, thus expanding our knowledge of coccolithophore life cycle diversity, but more widely on coccolithophore ecology and evolution. Here we report on a newly discovered life cycle in the genus Syracosphaera, linking the heterococcolithophore previously named Alveosphaera bimurata Okada & McIntyre, erroneously placed within the Calciosoleniaceae, and the holococcolithophore Poricalyptra gaarderae Borsetti & Cati, resulting in a new combination, Syracosphaera gaarderae (Borsetti & Cati) comb. nov. This life cycle association is supported by several combination coccospheres collected in the Northern Red Sea and is the first involving both the genus Poricalyptra and a Syracosphaera belonging to the lamina type subgroup. Biennial survey indicates that cells of both life cycle phases are only found during summer stratification periods and display clear depth zonation with S. gaarderae appearing in the lower photic zone and P. gaarderae in the upper photic zone. This marked vertical separation between life phases was further confirmed across other oceanic basins through bibliographic revision, confirming the prevailing view that coccolithophore life phases occupy different ecological niches.

Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust

Abstract. Coccolithophores are calcifying phytoplankton and major contributors to both the organic and inorganic oceanic carbon pumps. Their export fluxes, species composition, and seasonal patterns were determined in two sediment trap moorings (M4 at 12° N, 49° W and M2 at 14° N, 37° W) collecting settling particles synchronously from October 2012 to November 2013 at 1200 m of water depth in the open equatorial North Atlantic. The two trap locations showed a similar seasonal pattern in total coccolith export fluxes and a predominantly tropical coccolithophore settling assemblage. Species fluxes were dominated throughout the year by lower photic zone (LPZ) taxa (Florisphaera profunda, Gladiolithus flabellatus) but also included upper photic zone (UPZ) taxa (Umbellosphaera spp., Rhabdosphaera spp., Umbilicosphaera spp., Helicosphaera spp.). The LPZ flora was most abundant during fall 2012, whereas the UPZ flora was more important during summer. In spite of these similarities, the western part of the study area produced persistently higher fluxes, averaging 241×107 ± 76×107 coccoliths m−2 d−1 at station M4 compared to only 66×107 ± 31×107 coccoliths m−2 d−1 at station M2. Higher fluxes at M4 were mainly produced by the LPZ species, favoured by the westward deepening of the thermocline and nutricline. Still, most UPZ species also contributed to higher fluxes, reflecting enhanced productivity in the western equatorial North Atlantic. Such was the case of two marked flux peaks of the more opportunistic species Gephyrocapsa muellerae and Emiliania huxleyi in January and April 2013 at M4, indicating a fast response to the nutrient enrichment of the UPZ, probably by wind-forced mixing. Later, increased fluxes of G. oceanica and E. huxleyi in October–November 2013 coincided with the occurrence of Amazon-River-affected surface waters. Since the spring and fall events of 2013 were also accompanied by two dust flux peaks, we propose a scenario in which atmospheric dust also provided fertilizing nutrients to this area. Enhanced surface buoyancy associated with the river plume indicates that the Amazon acted not only as a nutrient source, but also as a surface density retainer for nutrients supplied from the atmosphere. Nevertheless, lower total coccolith fluxes during these events compared to the maxima recorded in November 2012 and July 2013 indicate that transient productivity by opportunistic species was less important than background tropical productivity in the equatorial North Atlantic. This study illustrates how two apparently similar sites in the tropical open ocean actually differ greatly in ecological and oceanographic terms. The results presented here provide valuable insights into the processes governing the ecological dynamics and the downward export of coccolithophores in the tropical North Atlantic.

Two new species of Vexillarius (Prymnesiophyceae) from the eastern Indian Ocean

:Seawater samples from the lower photic zone, obtained from shallow hydrocasts during an investigation of the eastern Indian Ocean, revealed the presence of two new species of the coccolithophorid genus Vexillarius: V. iaculifera sp. nov. and V. cereus sp. nov. Both possess dimorphic coccospheres: simple dishlike body coccoliths and coccoliths with an elongate, quadrangular central appendage. The main difference between these species and the type species, V. cancellifer, is the appendage shape: flaring (V. cancellifer), dart-shaped (V. iaculifer), or tapered (V. cereus). Furthermore, a recently described genus and species (Porsildia acerviphora) from the Arctic closely resembles V. cereus, but differs in having ‘irregularly piled elements’ in the central area that are not connected to the body coccolith wall. Since it possesses the characteristic features of Vexillarius, it is transferred to the genus as V. acerviphora comb. nov. It is believed that Vexillarius is closely related to Turrilithus and the Papposphaeraceae.

Calcareous nannofossil assemblage turnover in response to the Early Bajocian (Middle Jurassic) palaeoenvironmental changes in the Subbetic Basin

The calcareous nannofossil assemblages from three Lower Bajocian pelagic successions from the Subbetic Basin, well dated by ammonites, have been investigated. Biostratigraphically, they encompass the NJT9 to NJT10a nannofossil Tethyan zones. PCA factor analysis, performed on percentage abundances of selected taxa, suggests that the main factors controlling the Early Bajocian nannofossil assemblages of the Subbetic Basin were trophic level (nutrient availability) and nutricline depth. A gradual eutrophication is recorded in the calcareous nannofossil assemblages through an increase in the percentage abundance of Watznaueria spp., especially in the taxa best adapted to unstable and more eutrophic conditions (W. britannica gr. and W. manivitiae gr.). This eutrophication is concomitant with a trend of increasing carbonate δ13C values over the Early Bajocian, and a rise in biogenic silica production and deposits in the study sections and other basins of the Western Tethys. Around the middle part of the Laeviuscula Subzone, a decrease in the absolute and relative abundance of the deep-dwelling taxa (especially Crepidolithus crassus) suggests a deterioration of the environmental conditions in the lower photic zone. This has been interpreted as caused by a nutricline shallowing and/or greater turbidity that could be related, in turn, to surface-water stratification. Two different episodes have been differentiated in relation to nutricline dynamics: 1) from the latest Aalenian to earliest Early Bajocian, a weak and/or deep nutricline with nutrients rather uniformly spread throughout the photic zone, and concomitant with mesotrophic conditions and increasing productivity is suggested; 2) a shallowing of the nutricline and/or increase in surface-water turbidity, concomitant with more eutrophic conditions is suggested for the latest Early Bajocian. The observed decrease in the deep-dwelling taxa supports the hypothesis of a climatically driven fertilization of the surface waters instead of fertilization by upwelling for the Early Bajocian eutrophication episode.

Growth of the coccolithophore <i>Emiliania huxleyi</i> in light- and nutrient-limited batch reactors: relevance for the BIOSOPE deep ecological niche of coccolithophores

Abstract. Coccolithophores are unicellular calcifying marine algae that play an important role in the oceanic carbon cycle via their cellular processes of photosynthesis (a CO2 sink) and calcification (a CO2 source). In contrast to the well-studied, surface-water coccolithophore blooms visible from satellites, the lower photic zone is a poorly known but potentially important ecological niche for coccolithophores in terms of primary production and carbon export to the deep ocean. In this study, the physiological responses of an Emiliania huxleyi strain to conditions simulating the deep niche in the oligotrophic gyres along the BIOSOPE transect in the South Pacific Gyre were investigated. We carried out batch culture experiments with an E. huxleyi strain isolated from the BIOSOPE transect, reproducing the in situ conditions of light and nutrient (nitrate and phosphate) limitation. By simulating coccolithophore growth using an internal stores (Droop) model, we were able to constrain fundamental physiological parameters for this E. huxleyi strain. We show that simple batch experiments, in conjunction with physiological modelling, can provide reliable estimates of fundamental physiological parameters for E. huxleyi that are usually obtained experimentally in more time-consuming and costly chemostat experiments. The combination of culture experiments, physiological modelling and in situ data from the BIOSOPE cruise show that E. huxleyi growth in the deep BIOSOPE niche is limited by availability of light and nitrate. This study contributes more widely to the understanding of E. huxleyi physiology and behaviour in a low-light and oligotrophic environment of the ocean.

Observations on Syracosphaera rhombica sp. nov.

A spherical coccosphere and two collapsed coccospheres composed of monomorphic rhombic coccoliths were encountered in 2005 in the Java upwelling system of the SE Indian Ocean, while a further two specimens with elongate coccospheres were recently found in the Gulf of Mexico. All of the specimens were collected from the lower photic zone (75–160m). The coccoliths possess a proximal flange, a slightly flared wall with a serrated distal margin, and a relatively plain central area structure comprised only of overlapping laths. The taxon appears to be an undescribed species of the Syracosphaera nodosa group, so we describe it herein as Syracosphaera rhombica sp. nov.

Mercury content in Atlantic sediments as a new indicator of the enlargement and reduction of Northern Hemisphere ice sheets

ABSTRACTThe total mercury content of Quaternary sediments over the last 1.1 Ma from IODP Site U1308 drilled in the North Atlantic Ocean has changed, varying with periodicities of 100 000 and 41 000 years. The Hg content of the sediment increased during glacial periods, associated with increases in ice‐rafted debris (IRD), and correlates positively with the total organic carbon (TOC) content and inversely with the absolute abundance of surface‐dwelling nannoplankton species. TOC/total nitrogen ratios and δ13Corg values of TOC indicate that the TOC deposited during glacial periods was marine and associated with increased Hg deposition. These observations suggest that marine Hg is incorporated into the organic matter produced by deep‐dwelling phytoplankton in the lower photic zone. Mercury‐ and IRD‐bearing icebergs flowed out to this area during glacial periods melt, increasing the Hg content of photic water, and depositing IRD on the ocean floor, where increased marine Hg during glacial periods was consumed by deep‐dwelling phytoplankton. Therefore, the change in the sediment Hg content from the North Atlantic Ocean is controlled by climatic change and can be used as a chemical indicator of Northern Hemisphere ice sheet variability.

The composition and distribution of living coccolithophores in the Aegean Sea (NE Mediterranean)

This study presents the species composition of living coccolithophore communities in the Aegean Sea (northeastern Mediterranean), investigating their spatial and temporal variations in various environmental conditions from mesotrophic to ultra- oligotrophic regions. Coccolithophores of the photic zone in the Aegean Sea are relatively diverse (65 heterococcolithophores and 34 holococcolithophores) and dominated mostly by Emiliania huxleyi, Syracosphaera spp., Rhabdosphaeraceae and holococcolithophores. Hierarchical classification using R-mode cluster analysis distinguished five coccolithophore groups: Group Ia (Emiliania huxleyi, Syracosphaera molischii and Syracosphaera ossa) prevails in the high cell density and low diversity assemblages during the winter and early spring, when low temperatures and high nutrient concentrations are prevailing. Particularly in the north Aegean, E. huxleyi is dominating the upper photic zone being affected by the Black Sea Water inflow and the associated control on the water column stratification. Group Ib (Florisphaera profunda, Algirosphaera robusta, Syracosphaera anthos and Syracosphaera lamina) becomes important in the lower photic zone, making up the typical deep assemblages, whereas Group Ic (mainly Helicosphaera carteri and Gephyrocapsa oceanica) implies an opportunistic behavior in distinctly polluted neritic regions. Group IIa (Rhabdosphaera clavigera, Syracosphaera protrudens, Syracosphaera halldalii and numerous holococcolithophores) dominates the late spring-early autumn low cell density and high diversity assemblages, mainly in the thermally-stratified south Aegean and/or shallow, coastal environments with normal/ oligotrophic conditions, while Group IIb (Umbellosphaera tenuis and Syracosphaera pulchra) dominates the coccolithophore assemblages mainly during the early autumn in the north Aegean, thus reflecting the influence of Levantine Intermediate Water masses in the middle-lower photic zone. Our results suggest that abundance and variability in Aegean Sea coccolithophore assemblages are primarily controlled by surface water circulation and the associated water column stratification,with the sea temperature gradient affecting species composition.

The relationship between environmental change and the extinction of the nannoplankton Discoaster in the early Pleistocene

AbstractThe long‐ranging nannoplankton genus Discoaster went extinct in the early Pleistocene, one of the most significant events in the Cenozoic nannofossil record. The causal factors of this extinction are poorly constrained, at least partially because the ecology of Discoaster is not well understood. We investigate the relationship between the extinction and environmental changes by comparing a suite of high‐resolution nannofossil assemblage data at four Ocean Drilling Program sites to published environmental proxy records. Through the use of multivariate analysis, we determine Discoaster shared environmental preferences with the extant species Florisphaera profunda which thrives in the lower photic zone near the nutricline in locations where the water column is stratified. We show that the last occurrence of Discoaster was globally diachronous, rather than synchronous as previously thought, and likely took place in the equatorial Atlantic Ocean. We propose that the demise of Discoaster was a two‐stage process. A decrease in sea surface temperatures and increase in climate variability led to a global decline in Discoaster abundance. The extinction event itself correlates to shoaling of the nutricline as signified by a decrease in the carbon isotopic gradient between surface and thermocline foraminifera. We hypothesize that the combination of global cooling and a shoaling of the nutricline greatly reduced Discoaster's deepwater niche and contributed to its extinction. This may suggest that ocean stratification and the depth of the nutricline are important contributors to overall nannoplankton diversity.

Coccolithophore assemblages during the Holocene Climatic Optimum in the NE Mediterranean (Aegean and northern Levantine Seas, Greece): Paleoceanographic and paleoclimatic implications

Recent paleoceanographic findings suggest that during the Holocene, NE Mediterranean climate was less stable than previously thought. Several recently generated Aegean paleoceanographic records of sapropel S1 suggest that during the Holocene short-term potentially global cooling episodes, reflecting the N. Atlantic climatic fluctuations and possibly related to solar variability, are superimposed on the underlying regional subtropical/tropical control on the hydrography and ecosystems. Detailed quantitative analyses of coccolithophores have been performed on the deep sea deposits of the NE Mediterranean Sea; four gravity core records from North Aegean Sea (SL- 152), SE Aegean (NS-14, NS-18) and South Cretan continental Margin/northern Levantine Sea (HCMR 2/22) respectively, with a focus on the regional expression of sapropel S1 deposition. In all sites, higher values of Florisphaera profunda, Helicosphaera carteri, Braarudosphaera bigelowii and the increase of stratification S-index within sapropel intervals can be interpreted as the result of elevated marine production in the deep photic zone and enhanced inflow of fresh water input during the S1 period. The stratification S-index is significantly higher in the North Aegean, in respect to SE Aegean and the South Cretan Margin (SCM), reflecting increased coccolithophore productivity at the lower photic zone, being associated with a remarkably stratified water column. The concomitant increased presence of B. bigelowii indicates the prominent riverine input in the North Aegean basins and possible influx of Black Sea waters after ∼9.0 ka BP. Consequently, the increased stratification of the water column is linked to the significant freshening of the North Aegean surface waters, particularly during the early stages of the Holocene Climatic Optimum (S1a depositional interval). A series of cooling events detected towards the end of sapropelic conditions (∼7.2 ka BP) mostly in the North Aegean site, may be linked to outbursts of cold northerly air masses and relevant pulses in the deep-intermediate water ventilation that caused the enhancement of surface marine productivity and the temporary cessation of S1 at ∼ 8.0 ka BP. In contrast, in the SE Aegean and the northern Levantine Sea, the water column remains still warm and stratified, influenced by moderate fresh water input, for at least 1 more ky.

PALEOENVIRONMENTAL DISTRIBUTION OF LARGER FORAMINIFERA IN UPPER CRETACEOUS SILICICLASTIC-CARBONATE DEPOSITS (AREN SANDSTONE FORMATION, SOUTH PYRENEES, NORTHEASTERN SPAIN)

ABSTRACT This study examines the distribution of larger foraminifera in an upper Campanian–lower Maastrichtian siliciclastic–carbonate depositional environment within the Aren Sandstone Formation. Three assemblages have been identified, which refer to prodelta, reef, and lagoon depositional facies. Prodelta deposits are characterized by Lepidorbitoides socialis, Lepidorbitoides aff. minor, Clypeorbis mammillata, and Siderolites cataluniensis. The reef deposit assemblage contains Siderolites calcitrapoides, Siderolites cataluniensis, Orbitoides gruenbachensis, “Orbitoides” aff. concavatus, Lepidorbitoides aff. minor, and Fallotia jacquoti. They are associated with the encrusting foraminifera Solenomeris and Haddonia. The lagoonal assemblage contains Fascispira colomi and Omphalocyclus macroporus. Among the siderolitids, the keeled saucer-shaped Siderolites cataluniensis colonized the deepest environments at the lower photic zone, whereas the canaliculate spine-bearing Siderolites calcitrapoides characteriz...

Paleobathymetry and paleoecology of colonial corals from the Oligocene–early Miocene (?) Qom Formation (Dizlu area, central Iran)

The scleractinian corals from the carbonate facies of the Oligocene–early Miocene (?) Qom Formation in Dizlu area (northern Isfahan), central Iran were described in three separate coral intervals. From the base to the top of the study carbonate sequence, the coral assemblages of the intervals include 1—Madracis, 2—Leptoseris–Stylophora and 3—Poritidae–Faviidae–Siderastreidae. The species identified within the carbonates are dominated by: Goniopora sp., Pachyseris sp., Madracis cf. ornate, Madracis sp., Montastraea parva, Solenastrea cf. desmoulinsi, Tarbellastraea reussiana, Stylophora sp., Siderastrea crenulata and Leptoseris sp. The identified colonial coral species were not colonizing through a strong carbonate reefal structure. Instead, they were distributed as sparse patches or building coral biostromal frameworks. The thin branching colonies of Madracis species and the second coral assemblage (Leptoseris–Stylophora) that is dominated by thin branching and thin platy colonial growth forms reflect deeper depths of a carbonate ramp with low light intensity within the lower photic zone in a depth range of 19–35 m and a maximum flooding surface. The coral associations of the Poritidae–Faviidae–Siderastreidae assemblage are mainly composed of massive colonies indicating the upper photic zone with depth of less than 20 m and a higher energy water regime. The three coral assemblages reflect marine paleotemperature of 19–20° C, respectively.