GGO. Artículos

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

2021-01-01T00:00:00Z

[EN]The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.

Late Neogene evolution of the Peruvian margin and its ecosystems: a synthesis from the Sacaco record

2021-03-09T00:00:00Z

[EN]The highly productive waters of the Humboldt Current System (HCS) host a particular temperate ecosystem within the tropics, whose history is still largely unknown. The Pisco Formation, deposited during Mio-Pliocene times in the Peruvian continental margin has yielded an outstanding collection of coastal-marine fossils, providing an opportunity to understand the genesis of the HCS ecosystem. We present a comprehensive review, completed with new results, that integrates geological and paleontological data from the last 10 My, especially focusing on the southern East Pisco Basin (Sacaco area). We discuss the depositional settings of the Pisco Formation and integrate new U/Pb radiometric ages into the chronostratigraphic framework of the Sacaco sub-basin. The last preserved Pisco sediments at Sacaco were deposited ~ 4.5 Ma, while the overlying Caracoles Formation accumulated from ~ 2.7 Ma onwards. We identified a Pliocene angular unconformity encompassing 1.7 My between these formations, associated with a regional phase of uplift. Local and regional paleoenvironmental indicators suggest that shallow settings influenced by the offshore upwelling of ventilated and warm waters prevailed until the early Pliocene. We present an extensive synthesis of the late Miocene–Pleistocene vertebrate fossil record, which allows for an ecological characterization of the coastal-marine communities, an assessment of biodiversity trends, and changes in coastal-marine lineages in relation to modern HCS faunas. Our synthesis shows that: (i) typical endemic coastal Pisco vertebrates persisted up to ~ 4.5 Ma, (ii) first modern HCS toothed cetaceans appear at ~ 7–6 Ma, coinciding with a decline in genus diversity, and (iii) a vertebrate community closer to the current HCS was only reached after 2.7 Ma. The genesis of the Peruvian coastal ecosystem seems to be driven by a combination of stepwise transformations of the coastal geomorphology related to local tectonic pulses and by a global cooling trend leading to the modern oceanic circulation system.

Origin and implications of orbital-induced sedimentary cyclicity in Pliocene well-logs of the Western Mediterranean

2018-01-01T00:00:00Z

[EN]The climatic origin of astronomically induced sedimentary cycles in the Mediterranean and adjacent areas during the late Neogene and Quaternary remains puzzling; as cycles have been linked to concomitant but seasonally opposite changes in African summer monsoon precipitation (Eastern Mediterranean sapropels) and Atlantic regulated winter-precipitation (carbonate cycles on the Atlantic side of the Mediterranean). Particularly, little is known about the cyclic sedimentation on orbital time scales in the Western Mediterranean, with the prime exception of the Messinian sapropels from the Sorbas basin (southern Spain). Here we show that regular alternations in Pliocene downhole logs from the industrial drill-site Muchamiel-1, located along the Balearic Promontory in the Western Mediterranean, are related to eccentricity (bundles) and to obliquity and precession cycles (basic meter-scale alternations). We establish an astronomically based age model for the interval between 5.33 and 2.8 Ma, by rst correlating cycle bundles to eccentricity and then the basic dominantly precession-related cycles to the 65°N summer insolation of La2004. The striking bed-to-bed similarities between the Muchamiel-1 well-logs and other records from both the Atlantic margin and the Central Mediterranean suggest that the same climatic forcing was responsible for the formation of carbonate cycles across the Western Mediterranean and adjacent Atlantic. We conclude that formation of alternating carbonate-rich/carbonate-poor beds was controlled by Western Mediterranean cyclogenetic mechanisms as well as by peri-Mediterranean precipitation associated with changes in the North Atlantic System (NAS). These ndings highlight the importance of peri-Mediterranean precipitation on the sedimentary cyclicity by dictating terrigenous (clay) supply and potentially on the hydrology of the basin by providing additional freshwater required for sapropel formation. Consequently, cyclic sedimentation in the Mediterranean results from the combined efect of precipitation changes driven by (i) the North African monsoon, (ii) the Atlantic system, and (iii) intrabasinal Mediterranean atmospheric dynamics.

Floristic implications of two contemporaneous inland upper Neogene sites in the eastern US: Pipe Creek Sinkhole, Indiana, and the Gray Fossil Site, Tennessee (USA)

2016-04-28T00:00:00Z

[EN]Late Neogene floras of North America are mainly represented by sites located along the east coast Piedmont and the Great Plains. To date, there are only two upper Neogene inland localities in the eastern half of North America, the Pipe Creek Sinkhole (Indiana) and the Gray Fossil Site (Tennessee). At both sites, a lacustrine environment was formed from sinkholes that preserve fossil assemblages including invertebrates, vertebrates, and plant remains. We reviewed the floristic record (micro and macrofossils) of each site to determine the late Neogene composition of the flora, its vertical stratification, and to provide insight into the changes associated with warmer and drier conditions in the eastern deciduous forest. The Pipe Creek Sinkhole flora when compared to extant pollen floras of the eastern US is nested within the Beach-Maple-Basswood forest type of Dyer; however, the Miocene forest is dominated by a pine-hickory or pine-hickory-cottonwood association. The Gray Fossil Site flora when compared to extant pollen floras of the eastern US is nested within the Mesophytic Forest and is dominated by an oak-hickory-pine assemblage. The Pipe Creek Flora and the Gray Fossil Site Flora lack the common sub story and understory taxa and are dominated by specific woody taxa and an array of herbaceous taxa that have high light requirements and indicate some disturbance. The lack of vertical forest structure indicates that at both sites, woodland to woodland savanna to prairie-like habitats may have prevailed. The common occurrence of charcoals in the lacustrine sediments indicates that fire was a frequent disturbance factor, which may have suppressed the development of a structured forest with a closed canopy at both localities. This was further exacerbated by the impact of large herbivores. Both fossil floras, despite changes in the vegetation composition and vertical stratification when compared to the extant indigenous deciduous forest, include the major elements from their respective extant mesophytic C3 floristic associations that currently exist at these locations, i.e. the Beech-Maple-Basswood (Pipe Creek Sinkhole) and the Hickory-Oak-Pine Mesophytic Forest (Gray Fossil Site). The Miocene-Pliocene climate, in comparison to the extant climate, affected the vegetation in eastern North America by the selective elimination of those taxa that could not tolerate the warmer and drier conditions; the concomitant increase in fire frequency and the impact of large herbivores further contributed to maintaining woodland/savanna communities at both localities.

Changes in rocky intertidal communities after the 2015 and 2017 El Niño events along the Peruvian coast

2021-04-01T00:00:00Z

[EN]The Peruvian coast experiences the largest interannual variability of sea surface temperature in the world due to the combined influence of the coastal upwelling and El Niño Southern Oscillation (ENSO). Although biological impacts of El Niño events have been widely reported, their effects on rocky intertidal communities remains largely unknown in Peru. Herein, we analyze the results of two biological surveys of rocky intertidal communities, conducted along 1400 km of the Peruvian coast. The first survey was conducted in 2015 yielding a snapshot of the distribution of rocky intertidal communities after 17 years of stable La Ni˜ na-like conditions. The second survey was carried in October 2017, after the 2015-16 and the 2017 El Niño events, which produced warm sea surface temperature (SST) anomalies up to 6 ◦C on the Peruvian coast. We find no changes throughout the latitudinal transect in taxonomic richness but an important turnover of species, especially marked in the transition zone (~4–8◦S) between the Panamic and the Humboldt provinces. Temperature-related southward migration of species was evidenced in a few sites but did not explain the large-scale change in communities observed from 2015 to 2017, primarily driven by large changes in the distribution of a few ecologically important species across the Panamic and Humboldt provinces. A primary environmental impact on some ecologically key species likely triggered a chain of secondary impacts through ecological relationships which lead to a complex change independent from SST gradients. Further studies are needed to better characterize and disentangle the seasonal and interannual variability of the rocky intertidal communities and their drivers. When this interannual variability is integrated, the nine study sites show a highly persistent community structure determined by the latitudinal SST gradient along the Peruvian coast.

Messinian Salinity Crisis deposits widespread over the Balearic Promontory: Insights from new high-resolution seismic data

2015-01-01T00:00:00Z

[EN]The current interpretation of the Messinian Salinity Crisis (MSC) involves the deposition of peripheral or marginal evaporites in onshore basins as well as the erosion of the margin and the deposition of thick evaporites in deep basins. The so-called intermediate basins are formed in domains between the onland outcrops and the deep basins. The Balearic Promontory is a bathymetric high located between the deep Algerian and Liguro-Provençal basins and the onland Spanish basin. The SIMBAD project aims to investigate the spatial variability of the MSC-related deposits and to assess the extent of post-MSC reactivation over the Balearic Promontory. We present here the first results of the SIMBAD highresolution seismic survey (January 2013) which imaged for the first time a thin MSC-related unit widely distributed in small sub-basins over the Balearic Promontory. Borehole analyses have shown that this unit could be correlated with primary gypsum formations linked to the peripheral evaporites. Locally, in the Central Depression between Mallorca and Ibiza islands, a thicker MSC unit is observed whose lowermost transparent part could correspond to a salt layer. Geometrical relationships suggest that the MSC in the Central Depression could postdate the primary gypsum. The occurrence of a halite layer in the Central Depression, at depths of 1000 to 1500 m, favours the hypothesis that the evaporites precipitated passively in closed or partially closed perched sub-basins, possibly as a result of evaporative drawdown at different depths and possibly diachronously, at least with respect to the deep-basin evaporites.

Synchronous onset of the Messinian evaporite precipitation: First Mediterranean offshore evidence

2015-10-01T00:00:00Z

[EN]The Messinian Salinity Crisis (MSC) was a major ecological crisis affecting shallow and deep-water settings over the entire Mediterranean basin. However, the evolution of the MSC and its ecological impacts have mainly been explained on the basis of sediments from onshore outcrops. Lack of complete and physically connected records from onshore and offshore settings has inhibited comprehensive understanding of basin behaviour during the MSC. Herein we present a continuous record from an intermediate-depth basin on the Balearic Promontory that comprises late Tortonian–Messinian marls and evaporitic beds from the first MSC phase (i.e., Primary Lower Gypsum-PLG stage). Well-log and biostratigraphic data allow us establishing a large-scale calibration to the astronomical solutions, and to correlate pre-MSC sediments with classical rhythmic successions outcropping onshore. Thickness and characteristic sedimentary patterns observed in the offshore evaporitic records resemble those from marginal PLG sequences. Furthermore, seismic reflectors from a Bedded Unit (BU), which corresponds to an evaporitic interval according to well-to-seismic ties, are correlated with the onshore PLG sequences. This correlation constitutes the first attempt to link well-known marginal sequences with intermediate-depth offshore settings, which have previously only been studied through seismic imaging. Our time-calibration provides direct evidence supporting a synchronous onset of the PLG phase between onshore and offshore settings along the southwestern Balearic Promontory margin. Those BU reflectors, which were positively correlated to the PLG, were likely precipitated offshore the continental shelf at Messinian times. These results suggest that gypsum precipitation and/or preservation was not always limited to 200 m water-depths and could occur in non-silled basins. Finally, we only found a major erosion at the top of the PLG sequences, implying that the MSC drawdown occurred after the precipitation of the onshore lower evaporites. Studied sequences provide new insights into the PLG precipitation/preservation settings, as well as into the land-sea correlations of MSC units, and thus could potentially help refine current MSC models.

Palynology of Neogene sediments at the Gray Fossil Site, Tennessee, USA: Floristic implications

2012-03-01T00:00:00Z

[EN]The Gray Fossil Site, northeastern Tennessee, is formed by multiple karst sub-basins filled with lacustrine sediments. The oldest sediments found were recently dated as Paleo-Eocene by palynological means, whereas the youngest sediments are considered Mio-Pliocene based on their faunal assemblage. In this study, we examined the Mio-Pliocene lacustrine sediments from the Gray Fossil Site to determine the Late Neogene floral characteristics of a site within the southern Appalachian Mountains. The Mio-Pliocene lacustrine sinkhole fill preserves a unique fossil assemblage, which includes invertebrate, vertebrate, and floral remains. Floral remains are represented by wood, seeds, leaves, and pollen grains. Forty-seven palynological samples from six different test-pits were analyzed. All pits exhibit a low pollen yield, a result of basic pH levels, drought, and fire events that occurred during deposition. The palynofloral assemblage has a low to moderate diversity and is largely dominated by a Quercus–Carya–Pinus assemblage (~ 90% of the palynoflora). The presence of Pterocarya grains supports a Late Neogene age for these lacustrine sediments. Comparison with modern pollen-based floras from North America suggests that: (1) examined pits can be discriminated into two separate groups based on their palynofloral signatures, (2) the Mio-Pliocene vegetation at the site ranged between a closed to open woodland setting, depending on the intensity and frequency of drought and fire events, and (3) the fossil palynofloral assemblage is comparable to what would be expected in the modern North American Mesophytic Forest region.

Effects of rapid global warming at the Paleocene-Eocene boundary on neotropical vegetation.

2010-11-12T00:00:00Z

[EN]Temperatures in tropical regions are estimated to have increased by 3° to 5°C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.

Calcification and latitudinal distribution of extant coccolithophores across the Drake Passage during late austral summer 2016

2019-01-01T00:00:00Z

[EN]Coccolithophores are globally distributed microscopic marine algae that exert a major influence on the global carbon cycle through calcification and primary productivity. There is recent interest in coccolithophore polar communities; however field observations regarding their biogeographic distribution are scarce for the Southern Ocean (SO). This study documents the latitudinal, as well as in depth, variability in the coccolithophore assemblage composition and the coccolith mass variation in the ecologically dominant Emiliania huxleyi across the Drake Passage. Ninetysix water samples were taken between 10 and 150m water depth from 18 stations during POLARSTERN Expedition PS97 (February–April 2016). A minimum of 200 coccospheres per sample were identified in the scanning electron microscope, and coccolith mass was estimated with light microscopy. We find that coccolithophore abundance, diversity and maximum depth habitat decrease southwards, marking different oceanographic fronts as ecological boundaries. We characterize three zones: (1) the Chilean margin, where E. huxleyi type A (normal and overcalcified) and type R are present; (2) the Subantarctic Zone (SAZ), where E. huxleyi reaches maximum values of 212.5 × 103 cells L-1 and types B/C, C and O are dominant; and (3) the Polar Front Zone (PFZ), where E. huxleyi types B/C and C dominate. We link the decreasing trend in E. huxleyi coccolith mass to the poleward latitudinal succession from the type A to the type B group. Remarkably, we find that coccolith mass is strongly anticorrelated to total alkalinity, total CO2, the bicarbonate ion and pH. We speculate that low temperatures are a greater limiting factor than carbonate chemistry in the Southern Ocean. However, further in situ oceanographic data are needed to verify the proposed relationships.We hypothesize that assemblage composition and calcification modes of E. huxleyi in the Drake Passage will be strongly influenced by the ongoing climate change.

Coccolithophore paleoproductivity since the Last Glacial Maximum in the Atlantic Ocean: Relationship with calcification and preservation variability

2023-01-01T00:00:00Z

[EN]Paleoproductivity, calcification and preservation patterns are presented from calcareous nannoplankton paleoassemblages records covering the last ~25 kyr retrieved from Integrated Ocean Drilling Program Sites U1385 (37◦34.285′ N; 10◦7.562′W) and U1313 (41◦0.0′ N; 32◦57.4′ W), located respectively along the Western Iberian Margin and in the North Atlantic Ocean. The main aim of the study is to provide paleoceanographic and paleoclimatic analyses together with an investigation of changes in coccolithophore calcification processes occurring in response to climate variability from the Last Glacial Maximum up to the Holocene. Coccolithophore calcification was investigated by comparing variations in coccolith thickness and total coccolith calcite; at the same time dissolution/preservation indices were used to better understand the response of coccolithophores to changes in environmental parameters. In general, the increasing CO2 recorded during the last 25 kyr has a negative effect on calcification, resulting in a reduction of the different species size-normalized thicknesses at both sites. However, differences between the responses at the two latitudes studied are identified, in particular during the Heinrich Stadials; in contrast, during both Younger Dryas and Holocene intervals, the different size-normalized species thicknesses show consistent patterns at the two sites. During the Last Glacial Maximum the influence of subtropical waters due to Azores Current recirculation leads to a moderate productivity, initially at Site U1385 and later at Site U1313. At the same time, temperature influences the calcification at both sites. At Site U1385, the temperature is the main parameter affecting the paleoproductivity during Heinrich Stadial events; in addition, during Heinrich Stadial 1 alone, it also affects the calcification. At Site U1313 the calcification processes during Heinrich Stadial events are apparently hampered by an increasing water turbidity, caused by icebergs acting as ice rafted debris transporters, which affects light filtration in the water column. Light availability is an important calcification-influencing factor. At Site U1313 during Heinrich Stadial 1 and at the Last Glacial Maximum inception, dissolution, a consequence of increased CO2 solubility in the colder ice-age ocean, has a negative influence on calcification. Furthermore, at Site U1313, in contrast to Site U1385, the continually increasing CO2 concentration recorded during the Bølling-Allerød hampers both calcification and preservation.

Southern Ocean and Southern African Monsoon Forcing of the Subtropical Indian Ocean Early Pliocene “Biogenic Bloom”

2024-12-01T00:00:00Z

[EN]The Pliocene epoch emerges as a pivotal juncture in Earth's climatic evolution, characterized by pronounced warmth and elevated atmospheric carbon dioxide compared to contemporary levels. While the broader climatic context of the Pliocene has garnered attention, there remains an outstanding gap in detailed paleoclimate reconstructions of the early Pliocene, a new potential target for data‐model intercomparison. Addressing this, we investigate the drivers of the early Pliocene “Biogenic Bloom” and implications for nutrient dynamics and climate. By analyzing high‐resolution biotic assemblage and geochemical records from the Agulhas Plateau, southwestern Indian Ocean (International Ocean Discovery Program, IODP Site U1475), we aim to elucidate the forcing and feedback mechanisms driving the early Pliocene marine ecosystems. We identify a distinct shift in coccolithophore assemblages at ∼4.6 million years ago, characterized by a notable change in dominance between the larger and smaller Reticulofenestra and high abundances of Noelaerhabdaceae < 5 μm. Our findings confirm the adaptive strategies of coccolithophore communities to prevailing environmental conditions, underscoring their evolutionary resilience by producing smaller coccoliths while increasing their abundances in a nutrient‐replete ocean. Surface water dynamics, particularly the subtropical front migration and expansion of Southern Ocean waters, in combination with the southern African monsoon variability, emerge as key drivers of phytoplankton productivity during the early Pliocene. We posit that a weakened biological carbon pump, due to increased phytoplankton production driven by intense ocean circulation and mixing during the early Pliocene, served as a potential precursor to the subsequent middle Pliocene abrupt climate extremes.

Variation in calcification of Reticulofenestra coccoliths over the Oligocene–Early Miocene

2022-10-01T00:00:00Z

Los cocolitóforos son fitoplancton marino calcificante cuyas placas de calcita producidas intracelularmente, los cocolitos, han sido la fuente dominante de carbonato de calcio en entornos de mar abierto desde el Cretácico. Una pregunta abierta es si su calcificación se ha visto afectada por condiciones ambientales cambiantes en escalas de tiempo geológicas, como las variaciones en el sistema de carbono del océano. Los métodos anteriores que utilizaban microscopía de luz polarizada circular permitían cuantificar solo el espesor de pequeños cocolitos de menos de 1,5 µm, pero antes del Plioceno, una fracción significativa de los cocolitos excedía este espesor y no han sido cuantificables. Aquí, implementamos un nuevo enfoque para la calibración de microscopía de luz polarizada circular que nos permite cuantificar cocolitos que presentan calcita de hasta 3 µm de espesor. Aplicamos esta técnica para evaluar la evolución de la calcificación en Reticulofenestra desde el Oligoceno temprano hasta el Mioceno temprano en sedimentos excepcionalmente bien conservados del margen de Terranova. A lo largo de este intervalo de tiempo, el espesor del cocolito y el factor de forma kse invariante de escala varían en aproximadamente un 20 % alrededor del espesor medio de 0,37 μm y un kse medio de 0,16. Factores de forma más bajos caracterizan muestras con una mayor abundancia relativa de nanolitos resistentes a la disolución, lo que sugiere que la disolución puede contribuir al adelgazamiento de los placolitos. Por lo tanto, definimos tendencias temporales en la calcificación solo en muestras en las que el conjunto sugiere una disolución mínima. El kse más bajo caracteriza el Oligoceno medio, y el kse más alto alrededor de 18 Ma se encuentra en el Mioceno temprano. Se han propuesto altas concentraciones de carbono inorgánico disuelto (DIC) en el océano para este período del Mioceno y pueden ser un factor que contribuya a los altos ks de cocolitos.

Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

2018-01-01T00:00:00Z

[EN]he Southern Ocean is experiencing rapid and relentless change in its physical and biogeochemical properties. The rate of warming of the Antarctic Circumpolar Current exceeds that of the global ocean, and the enhanced uptake of carbon dioxide is causing basin-wide ocean acidification. Observational data suggest that these changes are influencing the distribution and composition of pelagic plankton communities. Long-term and annual field observations on key environmental variables and organisms are a critical basis for predicting changes in Southern Ocean ecosystems. These observations are particularly needed, since high-latitude systems have been projected to experience the most severe impacts of ocean acidification and invasions of allochthonous species. Coccolithophores are the most prolific calcium-carbonate-producing phytoplankton group playing an important role in Southern Ocean biogeochemical cycles. Satellite imagery has revealed elevated particulate inorganic carbon concentrations near the major circumpolar fronts of the Southern Ocean that can be attributed to the coccolithophore Emiliania huxleyi. Recent studies have suggested changes during the last decades in the distribution and abundance of Southern Ocean coccolithophores. However, due to limited field observations, the distribution, diversity and state of coccolithophore populations in the Southern Ocean remain poorly characterised. We report here on seasonal variations in the abundance and composition of coccolithophore assemblages collected by two moored sediment traps deployed at the Antarctic zone south of Australia (2000 and 3700 m of depth) for 1 year in 2001–2002. Additionally, seasonal changes in coccolith weights of E. huxleyi populations were estimated using circularly polarised micrographs analysed with C-Calcita software. Our findings indicate that (1) coccolithophore sinking assemblages were nearly monospecific for E. huxleyi morphotype B/C in the Antarctic zone waters in 2001–2002; (2) coccoliths captured by the traps experienced weight and length reduction during summer (December–February); (3) the estimated annual coccolith weight of E. huxleyi at both sediment traps (2.11 ± 0.96 and 2.13 ± 0.91 pg at 2000 and 3700 m) was consistent with previous studies for morphotype B/C in other Southern Ocean settings (Scotia Sea and Patagonian shelf); and (4) coccolithophores accounted for approximately 2–5 % of the annual deep-ocean CaCO3 flux. Our results are the first annual record of coccolithophore abundance, composition and degree of calcification in the Antarctic zone. They provide a baseline against which to monitor coccolithophore responses to changes in the environmental conditions expected for this region in coming decades.

Coccolithophore biodiversity controls carbonate export in the Southern Ocean

2020-01-01T00:00:00Z

[EN]Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.

Full annual monitoring of Subantarctic Emiliania huxleyi populations reveals highly calcified morphotypes in high-CO2 winter conditions

2020-01-01T00:00:00Z

[EN]Ocean acidifcation is expected to have detrimental consequences for the most abundant calcifying phytoplankton species Emiliania huxleyi. However, this assumption is mainly based on laboratory manipulations that are unable to reproduce the complexity of natural ecosystems. Here, E. huxleyi coccolith assemblages collected over a year by an autonomous water sampler and sediment traps in the Subantarctic Zone were analysed. The combination of taxonomic and morphometric analyses together with in situ measurements of surface-water properties allowed us to monitor, with unprecedented detail, the seasonal cycle of E. huxleyi at two Subantarctic stations. E. huxleyi subantarctic assemblages were composed of a mixture of, at least, four diferent morphotypes. Heavier morphotypes exhibited their maximum relative abundances during winter, coinciding with peak annual TCO2 and nutrient concentrations, while lighter morphotypes dominated during summer, coinciding with lowest TCO2 and nutrients levels. The similar seasonality observed in both time-series suggests that it may be a circumpolar feature of the Subantarctic zone. Our results challenge the view that ocean acidifcation will necessarily lead to a replacement of heavily-calcifed coccolithophores by lightly-calcifed ones in subpolar ecosystems, and emphasize the need to consider the cumulative efect of multiple stressors on the probable succession of morphotypes.