Philippe Recourt

Carbonate Grain-Size Distribution in Hemipelagic Sediments from a Laser Particle Sizer

Laser grain-sizer instruments provide the opportunity to study the grain size distribution of sediments across a wide size range in a short time. Automatic measurements can therefore be made, on a routine basis, for a great number of samples. Oceanic studies have proved the utility of these methods in characterizing both climatic changes and changes in sediment provenances. In addition, carbonate content is estimated either directly by CaCO 3 measurement, by visual observations, or by proxies such as sediment color reflectance. Nevertheless, the grain size distribution of the carbonate fraction is still a matter of speculation, and only optical observations can distinguish the nature of each carbonate fraction. Here we present the improvements on a method to study rapidly, with a high resolution, the grain size distribution of the carbonate fraction by use of a laser grain-sizer. We describe the basic methodology and apply it to an example from the Pleistocene of the Northern Atlantic Ocean.

Carbonate Grain-Size Distribution in Hemipelagic Sediments from a Laser Particle Sizer: RESEARCH METHODS PAPERS

ABSTRACT Laser grain-sizer instruments provide the opportunity to study the grain size distribution of sediments across a wide size range in a short time. Automatic measurements can therefore be made, on a routine basis, for a great number of samples. Oceanic studies have proved the utility of these methods in characterizing both climatic changes and changes in sediment provenances. In addition, carbonate content is estimated either directly by CaCO3 measurement, by visual observations, or by proxies such as sediment color reflectance. Nevertheless, the grain size distribution of the carbonate fraction is still a matter of speculation, and only optical observations can distinguish the nature of each carbonate fraction. Here we present the improvements on a method to study rapidly, with a high resolution, the grain size distribution of the carbonate fraction by use of a laser grain-sizer. We describe the basic methodology and apply it to an example from the Pleistocene of the Northern Atlantic Ocean.

Tubular structures of northern Wairarapa (New Zealand) as possible examples of ancient fluid expulsion in an accretionary prism: evidence from field and petrographical observations

Abstract The Cape Turnagain area is located on the inboard portion of the Hikurangi subduction margin, on the northern Wairarapa coast of the North Island of New Zealand. A 4.5 km long coastal section of sea cliffs of Mio-Pliocene sediments contains numerous tubular carbonate-rich concretions. Their morphology and petrographical observations suggest they were possibly formed by fluid flows of carbonate-rich water through a silty sediment. These tubular concretions could be fossil fluid expulsion structures similar to dewatering chimneys described offshore in New Zealand. The external diameter of the concretions observed in situ reaches 60 cm and internal canal up to 4 cm. Up to four canals are encountered in a single concretion. A positive relationship is observed between the chimney size and the number of canals or cumulative diameter of canals, suggesting that the size of the concretion is a function of the fluid which flowed through the plumbing network. The increased number of tubular concretions in upper Miocene siltstones compared to overlying Pliocene strata could be linked to a compressive event that caused overpressuring and the expulsion of fluids through the sediment pile.

Multiproxy approach for Holocene paleoenvironmental reconstructions from microorganisms (testate amoebae and foraminifera) and sediment analyses: The infilling of the Loire Valley in Nantes (France)

Foraminifera and testate amoebae are jointly used with sediment characteristics (sediment size, calcium carbonate, C, H, N and S proportions, and clay mineralogy) to reconstruct the Holocene sediment infilling of the Loire estuary (Nantes, France). The results reveal the infilling of the Loire Valley during the Holocene. Deposited on the Hercynian substratum, the first deposits are made of gravels with rare associated fauna. This corresponds to the gravel lag usually present as basal deposits in the river course. The clay mineralogy indicates an erosion of the substratum. Then, from 8850 to 5850 cal. yr BP, estuarine fauna settled in a laminated sediment that alternates between clay and sand. The smectite associated with kaolinite and illite suggests increased input from the Sèvre Nantaise River. The fauna progressively shifts from estuarine to marine, indicating a rise in the sea level. From 5850 cal. yr BP, the pace of the sea level rise slowed and channels migrated, implying intense erosion. After 2100 cal. yr BP, the fauna was dominated by testate amoebae, indicating a continentalization of the environment. The top samples reflect recent human activity and urbanization.