Thibault de Garidel-Thoron

Stable sea surface temperatures in the western Pacific warm pool over the past 1.75 million years

About 850,000 years ago, the period of the glacial cycles changed from 41,000 to 100,000 years. This mid-Pleistocene climate transition has been attributed to global cooling, possibly caused by a decrease in atmospheric carbon dioxide concentrations. However, evidence for such cooling is currently restricted to the cool upwelling regions in the eastern equatorial oceans, although the tropical warm pools on the western side of the ocean basins are particularly sensitive to changes in radiative forcing. Here we present high-resolution records of sea surface temperatures spanning the past 1.75 million years, obtained from oxygen isotopes and Mg/Ca ratios in planktonic foraminifera from the western Pacific warm pool. In contrast with the eastern equatorial regions, sea surface temperatures in the western Pacific warm pool are relatively stable throughout the Pleistocene epoch, implying little long-term change in the tropical net radiation budget. Our results challenge the hypothesis of a gradual decrease in atmospheric carbon dioxide concentrations as a dominant trigger of the longer glacial cycles since 850,000 years ago. Instead, we infer that the temperature contrast across the equatorial Pacific Ocean increased, which might have had a significant influence on the mid-Pleistocene climate transition.

Millennial‐scale dynamics of the east Asian winter monsoon during the last 200,000 years

The primary productivity dynamics of the last 200,000 years in the Sulu Sea was reconstructed using the abundance of the coccolithophore Florisphaera profunda in the IMAGES MD97-2141 core. We find that primary productivity was enhanced during glacial periods, which we suggest is due to a stronger East Asian winter monsoon. During the last 80 kyr, eight significant increases in primary productivity (PP) in the Sulu Sea are similar to East Asian winter monsoon changes recorded in Chinese loess. The PP maxima are not linked with Heinrich events (HE) in the North Atlantic, although four PP peaks are synchronous with HE. The PP oscillations have frequencies near those of the Dansgaard-Oeschger cycles in Northern Hemisphere ice records and indicate a teleconnection of the East Asian winter monsoon with Greenland climate. In this Sulu Sea record the East Asian winter monsoon oscillates with periodicities of6, 4.2-3.4, 2.3, and 1.5 kyr. In particular, the 1.5 kyr cycle exhibits a strong and pervasive signal from stage 6 to the Holocene without any ice volume modulation. This stationarity suggests that the 1.5 kyr cycle is not driven by some high-latitude forcing.

Low-latitude hydrological cycle and rapid climate changes during the last deglaciation

Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Nino–Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.

A multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr

foraminiferal species assemblages, and Uk37 0 (1� –2� C). These estimates are consistent with estimates from other WPWP cores, thereby suggesting that the discrepancy is due to proxy compatibility rather than differences in cores qualitity. Postdepositional dissolution above the lysocline might have altered the Mg/Ca-based temperature estimates in our site, but this effect is insufficient to resolve discrepancies between Mg/Ca in G. ruber and the other proxies. We suggest that the lower estimates obtained from Mg/Ca in G. sacculifer, faunal transfer functions, and Uk37 0 might reflect subsurface temperature changes rather than strict surface estimates. Accounting for potential artefacts, including dissolution and bioturbation, we suggest that the glacial WPWP SST was about 2.5� ±0 .7 � C cooler than during the Holocene, whereas the subsurface/upper thermocline temperature change was only about 1.8� ±0 .7� C. Interpreting variations in d 18 OSW in terms of salinity changes suggests a possibly slight decrease in surface salinity at the site of MD97-2138 during the LGM. Though LGM freshening in MD97-2138 is not robust to postdeposition dissolution effects, this inferred freshening appears to be a general feature of the western equatorial Pacific.

The cryptic and the apparent reversed: lack of genetic differentiation within the morphologically diverse plexus of the planktonic foraminifer Globigerinoides sacculifer

Abstract Previous genetic studies of extant planktonic foraminifera have provided evidence that the traditional, strictly morphological definition of species in these organisms underestimates their biodiversity. Here, we report the first case where this pattern is reversed. The modern (sub)tropical species plexus Globigerinoides sacculifer is characterized by large morphological variability, which has led to the proliferation of taxonomic names attributed to morphological end-members within the plexus. In order to clarify the taxonomic status of its morphotypes and to investigate the genetic connectivity among its currently partly disjunct (sub)tropical populations, we carried out a global survey of two ribosomal RNA regions (SSU and ITS-1) in all recent morphotypes of the plexus collected throughout (sub)tropical surface waters of the global ocean. Unexpectedly, we find an extremely reduced genetic variation within the plexus and no correlation between genetic and morphological divergence, suggesting taxonomical overinterpretation. The genetic homogeneity within the morphospecies is unexpected, considering its partly disjunct range in the (sub)tropical Atlantic and Indo-Pacific and its old age (early Miocene). A sequence variant in the rapidly evolving ITS-1 region indicates the existence of an exclusively Atlantic haplotype, which suggests an episode of relatively recent (last glacial) isolation, followed by subsequent resumption of unidirectional gene flow from the Indo-Pacific into the Atlantic. This is the first example in planktonic foraminifera where the morphological variability in a morphospecies exceeds its rDNA genetic variability. Such evidence for inconsistent scaling of morphological and genetic diversity in planktonic foraminifera could complicate the interpretation of evolutionary patterns in their fossil record.

Worldwide Genotyping in the Planktonic Foraminifer Globoconella inflata: Implications for Life History and Paleoceanography

The planktonic foraminiferal morpho-species Globoconella inflata is widely used as a stratigraphic and paleoceanographic index. While G. inflata was until now regarded as a single species, we show that it rather constitutes a complex of two pseudo-cryptic species. Our study is based on SSU and ITS rDNA sequence analyses and genotyping of 497 individuals collected at 49 oceanic stations covering the worldwide range of the morpho-species. Phylogenetic analyses unveil the presence of two divergent genotypes. Type I inhabits transitional and subtropical waters of both hemispheres, while Type II is restricted to the Antarctic subpolar waters. The two genetic species exhibit a strictly allopatric distribution on each side of the Antarctic Subpolar Front. On the other hand, sediment data show that G. inflata was restricted to transitional and subtropical environments since the early Pliocene, and expanded its geographic range to southern subpolar waters ∼700 kyrs ago, during marine isotopic stage 17. This datum may correspond to a peripatric speciation event that led to the partition of an ancestral genotype into two distinct evolutionary units. Biometric measurements performed on individual G. inflata from plankton tows north and south of the Antarctic Subpolar Front indicate that Types I and II display slight but significant differences in shell morphology. These morphological differences may allow recognition of the G. inflata pseudo-cryptic species back into the fossil record, which in turn may contribute to monitor past movements of the Antarctic Subpolar Front during the middle and late Pleistocene.

SSU rDNA divergence in planktonic foraminifera: molecular taxonomy and biogeographic implications.

The use of planktonic foraminifera in paleoceanography requires taxonomic consistency and precise assessment of the species biogeography. Yet, ribosomal small subunit (SSUr) DNA analyses have revealed that most of the modern morpho-species of planktonic foraminifera are composed of a complex of several distinct genetic types that may correspond to cryptic or pseudo-cryptic species. These genetic types are usually delimitated using partial sequences located at the 3′end of the SSUrDNA, but typically based on empirical delimitation. Here, we first use patristic genetic distances calculated within and among genetic types of the most common morpho-species to show that intra-type and inter-type genetic distances within morpho-species may significantly overlap, suggesting that genetic types have been sometimes inconsistently defined. We further apply two quantitative and independent methods, ABGD (Automatic Barcode Gap Detection) and GMYC (General Mixed Yule Coalescent) to a dataset of published and newly obtained partial SSU rDNA for a more objective assessment of the species status of these genetic types. Results of these complementary approaches are highly congruent and lead to a molecular taxonomy that ranks 49 genetic types of planktonic foraminifera as genuine (pseudo)cryptic species. Our results advocate for a standardized sequencing procedure allowing homogenous delimitations of (pseudo)cryptic species. On the ground of this revised taxonomic framework, we finally provide an integrative taxonomy synthesizing geographic, ecological and morphological differentiations that can occur among the genuine (pseudo)cryptic species. Due to molecular, environmental or morphological data scarcities, many aspects of our proposed integrative taxonomy are not yet fully resolved. On the other hand, our study opens up the potential for a correct interpretation of environmental sequence datasets.

Longitudinal differentiation among pelagic populations in a planktic foraminifer.

Evolutionary processes in marine plankton have been assumed to be dependent on the oceanic circulation system, which transports plankton between populations in marine surface waters. Gene flow facilitated by oceanic currents along longitudinal gradients may efficiently impede genetic differentiation of pelagic populations in the absence of confounding marine environmental effects. However, how responsible oceanic currents are for the geographic distribution and dispersal of plankton is poorly understood. We examined the phylogeography of the planktic foraminifer Pulleniatina obliquiloculata in the Indo-Pacific Warm Pool (IPWP) by using partial small subunit ribosomal DNA (SSU rDNA) sequences. We found longitudinal clines in the frequencies of three distinct genetic types in the IPWP area. These frequencies were correlated with environmental factors that are characteristic of three water masses in the IPWP. Noteworthy, populations inhabiting longitudinally distant water masses at the Pacific and Indian sides of the IPWP were genetically different, despite transportation of individuals via oceanic currents. These results demonstrate that populations of pelagic plankton have diverged genetically among different water masses within a single climate zone. Changes of the oceanic circulation system could have impacted the geographic patterns of dispersal and divergence of pelagic plankton.

Progressive shoaling of the equatorial Pacific thermocline over the last eight glacial periods

The depth of equatorial Pacific thermocline is diagnostic of the main modes of tropical climates. Past estimates of Pacific thermocline dynamics have been reconstructed either for the Last Glacial Maximum or on longer timescales at low resolution. Here we document a new high-resolution set of reconstructed past sea surface and subsurface waters temperatures from the southwestern subequatorial Pacific, core MD05-2930, in the Gulf of Papua, over the last 800 ka. We used two morphotypes of Globigerinoides ruber known to live at different water depths to reconstruct past stratification. We estimated calcification temperature of each morphotypes by Mg/Ca paleothermometry. Our subequatorial Pacific thermocline paleotemperature record indicates a response of the thermocline to both direct orbital forcing and glacial-interglacial changes. Our stratification record shows a systematic shallower glacial thermocline, whereas sea surface temperatures are characterized by precessional forcing. The record is indicative of a progressive long-term shoaling of the thermocline during the glacial stages during the late Pleistocene. The shoaling of the subequatorial Pacific thermocline is consistent with regional estimates. An enhanced South Pacific shallow overturning wind-driven circulation could have driven this progressive shoaling. We speculate that this late Pleistocene glacial shoaling of the thermocline could be related to an increase in the amplitude of the obliquity.

PFR2: a curated database of planktonic foraminifera 18S ribosomal DNA as a resource for studies of plankton ecology, biogeography and evolution

Planktonic foraminifera (Rhizaria) are ubiquitous marine pelagic protists producing calcareous shells with conspicuous morphology. They play an important role in the marine carbon cycle, and their exceptional fossil record serves as the basis for biochronostratigraphy and past climate reconstructions. A major worldwide sampling effort over the last two decades has resulted in the establishment of multiple large collections of cryopreserved individual planktonic foraminifera samples. Thousands of 18S rDNA partial sequences have been generated, representing all major known morphological taxa across their worldwide oceanic range. This comprehensive data coverage provides an opportunity to assess patterns of molecular ecology and evolution in a holistic way for an entire group of planktonic protists. We combined all available published and unpublished genetic data to build PFR2, the Planktonic foraminifera Ribosomal Reference database. The first version of the database includes 3322 reference 18S rDNA sequences belonging to 32 of the 47 known morphospecies of extant planktonic foraminifera, collected from 460 oceanic stations. All sequences have been rigorously taxonomically curated using a six‐rank annotation system fully resolved to the morphological species level and linked to a series of metadata. The PFR2 website, available at http://pfr2.sb-roscoff.fr, allows downloading the entire database or specific sections, as well as the identification of new planktonic foraminiferal sequences. Its novel, fully documented curation process integrates advances in morphological and molecular taxonomy. It allows for an increase in its taxonomic resolution and assures that integrity is maintained by including a complete contingency tracking of annotations and assuring that the annotations remain internally consistent.