Matthieu Carré

Holocene history of ENSO variance and asymmetry in the eastern tropical Pacific

El Niño shifted between the center and the East El Niño has changed quite a bit over the past 10,000 years. During some periods it was less variable than now, and during others it shifted from its current locale toward the central Pacific. Carré et al. analyzed the shells of mollusks from Peru to construct a record of the El Niño–Southern Oscillation (ENSO) in the eastern Pacific over the Holocene period. They compared this record with other records from the rest of the Pacific to reveal how much the strength and frequency of El Niños changed and how their positions varied. Science, this issue p. 1045 El Niño–Southern Oscillation warming of the sea surface was strongly skewed toward the central Pacific in the Mid-Holocene. Understanding the response of the El Niño–Southern Oscillation (ENSO) to global warming requires quantitative data on ENSO under different climate regimes. Here, we present a reconstruction of ENSO in the eastern tropical Pacific spanning the past 10,000 years derived from oxygen isotopes in fossil mollusk shells from Peru. We found that ENSO variance was close to the modern level in the early Holocene and severely damped ~4000 to 5000 years ago. In addition, ENSO variability was skewed toward cold events along coastal Peru 6700 to 7500 years ago owing to a shift of warm anomalies toward the Central Pacific. The modern ENSO regime was established ~3000 to 4500 years ago. We conclude that ENSO was sensitive to changes in climate boundary conditions during the Holocene, including but not limited to insolation.

Radiocarbon reservoir age variations in the south Peruvian upwelling during the Holocene.

In Quebrada de los Burros in coastal southern Peru (Tacna department), human settlements containing shells and charcoal deposits have been excavated since 1995. The sea surface (super 14) C reservoir ages, estimated by calculating the difference of (super 14) C age between marine shells and terrestrial organic materials, exhibit high values during the lower and middle Holocene and decrease abruptly after 4000 cal BP. The increase of reservoir age at around 7000-8000 cal BP suggests an enhancement of Peruvian coastal upwelling intensity and changes in ocean circulation at intermediate depth.

Controls on Gut Phosphatisation: The Trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)

Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagerstätten. However, the reasons for their fossilisation and their biological implications remain to be thoroughly explored. This is particularly true with arthropods – typically the most diverse fossilised organisms in Cambrian ecosystems – where digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on their biology. Here we describe the phosphatised digestive structures of three trilobite species from the Cambrian Weeks Formation Lagerstätte (Utah). Their exquisite, three-dimensional preservation reveals unique details on trilobite internal anatomy, such as the position of the mouth and the absence of a differentiated crop. In addition, the presence of paired pygidial organs of an unknown function is reported for the first time. This exceptional material enables exploration of the relationships between gut phosphatisation and the biology of organisms. Indeed, soft-tissue preservation is unusual in these fossils as it is restricted to the digestive structures, which indicates that the gut played a central role in its own phosphatisation. We hypothesize that the gut provided a microenvironment where special conditions could develop and harboured a source of phosphorus. The fact that gut phosphatization has almost exclusively been observed in arthropods could be explained by their uncommon ability to store ions (including phosphorous) in their digestive tissues. However, in some specimens from the Weeks Formation, the phosphatisation extends to the entire digestive system, suggesting that trilobites might have had some biological particularities not observed in modern arthropods. We speculate that one of them might have been an increased capacity for ion storage in the gut tissues, related to the moulting of their heavily-mineralised carapace.

Quebrada de los burros: Los primeros pescadores del litoral pacífico en el extremo sur peruano

Resumen es: Las excavaciones en la Quebrada de los Burros, en el litoral de Tacna (Peru), han descubierto un campamento de pescadores y recolectores de moluscos (QlB...

Temperature range shifts for three European tree species over the last 10,000 years

We quantified the degree to which the relationship between the geographic distribution of three major European tree species, Abies alba, Fagus sylvatica and Picea abies and January temperature (Tjan) has remained stable over the past 10,000 years. We used an extended data-set of fossil pollen records over Europe to reconstruct spatial variation in Tjan values for each 1000-year time slice between 10,000 and 3000 years BP (before present). We evaluated the relationships between the occurrences of the three species at each time slice and the spatially interpolated Tjan values, and compared these to their modern temperature ranges. Our results reveal that F. sylvatica and P. abies experienced Tjan ranges during the Holocene that differ from those of the present, while A. alba occurred over a Tjan range that is comparable to its modern one. Our data suggest the need for re-evaluation of the assumption of stable climate tolerances at a scale of several thousand years. The temperature range instability in our observed data independently validates similar results based exclusively on modeled Holocene temperatures. Our study complements previous studies that used modeled data by identifying variation in frequencies of occurrence of populations within the limits of suitable climate. However, substantial changes that were observed in the realized thermal niches over the Holocene tend to suggest that predicting future species distributions should not solely be based on modern realized niches, and needs to account for the past variation in the climate variables that drive species ranges.

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

A paleoreconstruction of the length and intensity of the rainy season over western Africa has been recently proposed, using analysis of fossil mollusk shells from the Saloum delta region, in western Senegal. In order to evaluate the significance of local long-term reconstructions of precipitations from paleoclimate proxies, and to better characterize the spatial homogeneity of rainfall distribution in northern Africa, we analyze here the spatial representativeness of rainfall in this region, from seasonal to decadal timescales. The spatial coherence of winter episodic rainfall events is relatively low and limited to surrounding countries. On the other hand, the summer rainfall, associated with the West African Monsoon, shows extended spatial coherence. At seasonal timescales, local rainfall over the Saloum is significantly correlated with rainfall in the whole western half of the Sahel. At interannual and longer timescales, the spatial coherence extends as far as the Red Sea, covering the full Sahel region. This spatial coherence is mainly associated to the zonal extension of the Inter Tropical Convergence Zone. Coherently, summer rainfalls appear to be driven by SST anomalies mainly in the Pacific, the Indian Ocean, the Mediterranean basin, and the North Pacific. A more detailed analysis shows that consistency of the spatial rainfall coherence is reduced during the onset season of the West African Monsoon.

The Las Salinas palaeo-lagoon in the Sechura Desert (Peru): Evolution during the last two millennia

The Sechura Desert provides a unique example of a vast palaeo-lagoon system on the Peruvian coast that was active during the first millennium AD. Reconstruction of coastal evolution is made possible by the good resolution of the sedimentary records of the Las Salinas Noroeste coastal plain. Evidence from morphostratigraphy and sedimentary facies indicates marked environmental diversity between the 3rd and the 8th centuries AD and a wide variability of sedimentary dynamics: lagoon foreshores received alternately fine distal marine sediments and coarser continental sediments in pro-deltaic sheets. Evaporation phases periodically occurred in these foreshores causing the formation of salt crusts. After a last high water level in the 8th century AD, the lagoon ultimately dried out and remains dry today. The malacofauna and sedimentary facies indicate that marine marshes bordered by vegetation, perhaps mangrove, developed in the higher parts of these lagoons. This palaeogeography is explained by the progressive build-up of a sand bar which started at least in the middle Holocene. From the 3rd to 8th centuries AD, the lagoon had limited connection to the sea in its northern end and hosted a warm-water and productive ecosystem that was exploited by pre-Hispanic populations. Wetter conditions in the Andes and occasional El Niño rainfalls maintained the lagoon during this period. The freshwater input likely stopped in the 8th century AD, which led to the closing of the shore bar under the influence of the longshore drift rapidly followed by the drying up of the lagoon, and the abandonment of the archaeological site.

Reconstructing ENSO in the Eastern Tropical Pacific from short-lived marine mollusks

The focus of much paleoclimate work on ENSO has been on records spanning multiple decades, such as those derived from corals. Such long records are, however, relatively rare, especially in the Eastern Tropical Pacific. This important limitation for studying past changes in the spatial pattern of ENSO activity can be compensated for by obtaining records from a larger number of shorterlived organisms. In this case, information about past climate is not available as a continuous record, but is compiled to extract climate statistics. Carré et al. (2013) recently presented a technique that responds to the critical need for quantitative estimates of tropical marine interannual variability. This technique uses the shells of marine mollusks that live for 1-4 years, and thus allow us to reconstruct the seasonal range of sea surface temperature (SST). These data can then be compared to coral records and GCM outputs. The technique of using marine mollusk shells shares similarities with the approach of coral studies in that it produces floating windows of climate record at a very high, often monthly, resolution. It also shares similarities with the approach of analyzing many foraminifera shells individually from the same sediment layer (Koutavas et al. 2006; Leduc et al. 2009) in that paleoclimate statistics are estimated from a random sample. Isotopic records in mollusks enable independent reconstructions of the seasonal cycle. This approach is valid for any coastal mollusk species that faithfully records at least one annual SST cycle, and therefore opens up new opportunities for direct, quantitative paleo-ENSO reconstructions in the Eastern Tropical Pacific, using either archeological shell middens or uplifted fossil shell banks from Peru.