The study of the modern and oceanic productivity and community
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structures plays a major role in understanding the global carbon cycle as well as its function in the earth’s past and modern climatic changes (Kohfeld et al., 2005; Cermeño et al., 2008).Upwelling intensity and primary production have been found to have evolved in phase with climatic oscillations leading to increasing local production, attributed to increasing nutrient delivered to the surface ocean (Martinez and Robinson, 2010;
Kienast et al., 2006; Etourneau et al., 2013). Numerous proxies have been used to determine paleo-productivity. Modern developments have centered on biogenic residues such as the organic carbon, carbonate, opal and lipid biomarkers.
Total organic carbon as paleoproductivity proxy
Total organic carbon (Corg) is a proxy for the amount of or ganic matter contained in the sediments; therefore, it is often used as a proxy for primary productivity. This is because of a relationship between productivity in surface waters and organic carbon accumulation underlying sediments. High Corg is regarded to be related to high paleo-productivity, while low Corg is thought to result from a low paleo-productivity. As a result of the biogeochemical process during the carbon cycle, the Corg in marine sediments is not only controlled by paleo-productivity but decomposition velocity and preservation of organic matter influx. For this reason, the mass accumulation rate (MARs) of the organic carbon is said to be a more useful tool to measure the delivery and preservation of organic matter than the percentage of organic matter. This helps in paleo-productivity study in the identification of changes in the organic matter delivery rates to the sediment.
Calcium carbonate and opal as paleoproductivity proxies
CaCO3 and biogenic opal in the marine sediments are mainly composed of hard parts of marine organisms (coccolithophores and diatoms). Both proxies are known to represent the changes in the export production of the organisms that precipitate these materials, leading to a carbonate-silicate geochemical cycle used to explore dynamic and climatic consequences of constraints on carbonate burial. Sediment traps have been shown to a have good correlation between the organic carbon fluxes and carbonate flux under certain condition (Ruhlemann et al., 1996). Lyle et al (1988), demonstrated the linkage between the mass accumulation rates of organic
matter, carbonate and opal and relates these proxies to biological productivity. Both proxies are affected by a variety of physical and chemical oceanographic processes and not only by productivity (Adina Paytan, 2009).
Opal preservation depends on global silica budget, the degree of silicification of the frustules, pore water dissolved silica concentration (Treuger et al., 1995; Hutchins and Bruland, 1998). As a result, the high accumulations of the proxy do not always represent an area of high biological productivity.
Dinosterol, brassicasterol, alkenone and diol as paleoproductivity proxies Recently, reconstruction based on phytoplankton lipid biomarker (alkenone, dinosterol and brassicasterol ) to evaluate the paleoproductivity and community structures of algae, have been successfully applied to estimate ecosystem changes (Schubert et al., 1998; Seki et al., 2004; He et al., 2008; Bolton et al., 2010a). Sterols commonly provide biomarkers for diatoms and dinoflagellates. Dinosterol and brassicasterol example of sterol compounds are commonly used as an unambiguous biomarker for organic matter derived from dinoflagellates and diatoms respectively. Alkenone are biosynthesized by some algal species belonging to the division Haptophyta, predominantly the cosmopolitan coccolithophore Emiliania huxleyi and Gephyrocapsa oceanica producing a series of compounds containing 37, 38 and 39 carbon atoms with either two or three double bonds (unsaturation, e.g. C37:2andC37:3)(Conte et al., 1994; Volkman et al., 1995). The relative concentration of these compounds varies in direct response to growth in temperature; as a result, it is used for the estimation of sea surface temperatures (SSTs) (Badejo et al., 2014). Sedimentary C37 alkenones have been proposed as an indicator of paleo-productivity marker as a result of the uniformity of the preservation conditions in the period of study (Sikes and Keigwin, 1994; Herbert et al., 1998; Schubert et al., 1998; Budziak et al., 2000). Proxies for global productivity, e.g foraminifera accumulation rates have shown good agreement with C37 alkenone productivity over a long time period(Sikes and Keigwin, 1994).
Earlier results have shown that profile distributions of these biomarkers are controlled by the level of nutrients and temperature. Coccolithophores tend to surpass diatoms and dinoflagellates under a low nutrient and high temperature while diatoms and dinoflagellates bloom in high nutrient and
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cold environment (Chen et al., 2007). Long chain alkyl diols consist of an alkyl chain containing an alcohol group at C1 and at a mid-chain position, with chain lengths varying from C24 to C36, and mid-chain alcohol positions ranging from C-11 to C-19 (Versteegh et al., 1997). The distribution of long chain diols is basically controlled by the temperature, salinity, and nutrient concentrations in water environments (Versteegh et al., 2001;
Rampen et al., 2008, 2009, 2012). C28 and C301, 14-diols have been identified in Proboscia diatoms as well as in the marine alga Apedinellaradians (Sinninghe Damsté et al., 2003; Rampen et al., 2007, 2011).
Volkman et al. (1999) and Méjanelle et al. (2003) reported that C28 and C301, 13-diols and C30 and C321, 15-diols are produced by eustigmatophyte algae. A series of studies have shown the potential application of long chain diols in climate reconstruction studies (Versteegh et al., 1997, 2000;
Rampen et al., 2008, 2009, 2012). Versteegh et al. (2000) used the following diol index as a tool in tracing the past sea surface.
water masses:
Willmott et al. (2010) applied the following modified diol index using 1,13 diol as a upwelling/nutrient index in the polar area: