Overview
Only a small fraction (<1%) of the organic matter produced photosynthetically in the ocean by microscopic plants (phytoplankton) escapes being recycled in surface waters. This small fraction largely packaged within biological aggregates (fecal pellets, marine snow) settles to the ocean bottom where it accumulates as a sediment deposit. The organic matter preserved in ocean sediments is comprised of many organic molecules some of which are uniquely traceable to the specific organisms that produced them. Such organic molecules are known as biomarkers. The following discussion will illustrate how systematic study of specific phytoplankton biomarkers known as long-chain alkenones in sediment cores can provide information about changes in primary productivity, sea-surface temperature and atmospheric carbon dioxide levels in the past.

Objective
Fossils preserved within marine sediment cores encode information about past biological, chemical and physical oceanographic conditions. Two types of fossils are found within cores: 1) those visible to the eye often assisted by a microscope; and 2) those visible only by analytical chemistry techniques. In the following discussion, you will be introduced to the second type, organic molecular fossils, and learn how various types of oceanographic information are potentially gleaned from systematic analysis of such substances.

Background

Microscopic plants called phytoplankton are the primary source of organic matter in the ocean (“primary production”). Phytoplankton come in a variety of taxonomic types (e.g., diatoms, dinoflagellates, coccolithophores). Nonetheless, all do one thing in common. Using energy from sunlight, they assemble organic matter from carbon dioxide and nutrients dissolved in seawater through a process called photosynthesis. Amino acids, carbohydrates and lipids are the major building blocks of the organic matter in phytoplankton, and, for that matter, in all organisms. Most molecules within these chemical classes are widely distributed in organisms. However, some molecules have unique structures, are restricted biologically, and represent biomarkers for specific types of organisms. Chlorophyll, common in all plants but absent in animals, provides an example of one such compound. In later discussion, we will look closely at another biomarker, one found only in a specific class of phytoplankton represented most notably by the coccolithophorid Emiliania huxleyi.

Organic matter produced in surface ocean waters illuminated by sunlight (the “euphotic zone”) provides the fuel for the food chain (zooplankton, fishes and mammals including humans). Very little of this primary production goes to waste in the ocean. The vast majority (~99%) is eaten and respired somewhere within the water column; the small remainder settles to the seafloor packaged within particles such as fecal pellets and marine snow. Continuation of this particle rain over time yields a sediment record. Under ideal conditions, the sediment record can be layered (“varved”), each varve representing a discrete time in the past. More often, however, the surfacemost part of the deposit is stirred biologically (bioturbation), smearing the time resolution of the sediment record.

Alkenones and Their Paleoceanographic Applications
Reconstruction of surface ocean history from analysis of specific phytoplankton biomarkers preserved in the sedimentary materials requires knowledge of three things:

E. huxleyi synthesizes a unique series of long-chain, unsaturated ketones (alkenones). The specific physiological function of the alkenones, which comprise 5-10% of total cellular carbon (!) in this unicellular plant, has until recently been ascribed to membrane architecture. Although this may yet prove to be so, new findings indicate the compounds are involved in some way with cellular energy storage. The explanation for physiological function must ultimately accomodate this observation.

The alkenones, visualized by gas chromatography, are not completely destroyed in the sedimentary process. A small fraction is preserved as part of the sedimentary record. Clever reading of sedimentary alkenone records provides insight about three different aspects of the ‘life and times’ of E. huxleyi in oceans of the past, present and future:

its productivity (from the absolute compound concentration) its growth temperature (from relative compound concentration) the carbon dioxide content of ocean surface waters in which it grew (from the carbon isotopic composition of these molecules).

Application #1: Estimate E. huxleyi Productivity
Primary productivity patterns are not uniform in the ocean in either space or time. For example, values in upwelling regions are much higher than those in the central ocean gyres. Furthermore, primary productivity at any one place is typically not constant throughout the year - it shows seasonality. Similar generalizations almost certainly apply to productivity patterns for E. huxleyi, a subset of total primary productivity in the ocean. This point is well illustrated by satellite images for massive, episodic blooms of E. huxleyi in surface waters throughout the ocean (see satellite images).

The total alkenone concentration measured in a given sedimentary sample is related to E. huxleyi productivity in overlying surface waters at some point in the past. Temporal variations in the alkenone flux measured in sediment trap time series provide an index for seasonal changes in organic carbon contribution from E. huxleyi blooms to the ocean floor (see Figure below). The stratigraphic record for alkenone concentration in dated sediment cores potentially provides information about annual, decadal and longer timeframe changes in E. huxleyi productivity.

Key Biological and Environmental Questions
From what euphotic depth does the alkenone flux to bottom sediment ('export' production) derive? What ecological/oceanographic factors control alkenone 'export' production? What factors at the seabed control preservation of the alkenone signal in the sediment record?

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Application #2: Estimate Sea-Surface Temperatures (SST)
E. huxleyi biosynthesizes alkenones as a series of compounds containing 37, 38 and 39 carbon atoms and either two or three double bonds (unsaturation) (e.g. C37:2 and C37:3). The relative concentration of compounds with two and three double bonds biosynthesized by the plant varies in direct response to growth temperature. That is, when growth temperatures get warmer, E. huxleyi biosynthesizes alkenones with fewer double bonds (e.g. C37:2) and visca versa. This type of biochemical response to growth temperature is well-known for fatty acids in phospholipids, the classical building blocks of cell membranes in bacteria, plants and animals.

A simple index UK'37 ([C37:2]/{[C37:2]+[C37:3]}) was formulated to quantify the degree of unsaturation in a given alkenone series. Experiments with laboratory cultures of E. huxleyi have shown the relationship between UK'37 and growth temperature is remarkably linear. Field experiments have also shown this relationship applies widely in the modern ocean. Consequently, paleoceanographers now routinely measure UK’37 stratigraphically in dated sedment cores to assess climatically driven changes in water temperature at the sea-surface for different parts of the world ocean. This application is performed on timescales ranging from interannual (El Nino) to millenial (Glacial/Interglacial).

	Muller and coworkers (Geochim. Cosmochim. Acta 62: 1757-1772, 1998) showed that values for the alkenone unsaturation index UK'37 measured in Modern sediments throughout the open World Ocean strongly correlate with annual mean sea-surface temperature (SST). Data for Modern sediments collected along the Chile-Peru margin and the NE Pacific margin (Herbert et al. Paleoceanogr. 13: 263-271, 1998) reveal this correlation applies to depositional environments not only in the open ocean but also along its continental boundaries (see Figure on left).
Our recent work with a single E. huxleyi strain grown isothermally (15oC) in batch culture (CCMP1742) has shown that UK'37 is quite sensitive to the physiological status of the cell. When nutrients are depleted but well-illuminated conditions prevail, cellular alkenone abundance increases dramatically and UK'37 decreases by -0.11 units. When exponentially dividing cells are shifted to prolonged darkness under otherwise nutrient replete conditions, cellular alkenone abundance decreases dramatically and UK'37 increases by +0.11 units. The response of cellular alkenone content to these changes in growth conditions suggests these biochemicals serve some yet to be determined energy storage function in E. huxleyi. Notably from a paleoceanographic perspective, the degree of shift in UK'37 values under these different growth conditions is equivalent to the magnitude of variability observed in the global calibration of UK'37 and mean annual SST. This finding suggests that caution must be taken when the environmental cause for downcore stratigraphic variability in UK'37 is interpreted. Factors besides just changes in growth temperature may have played a role in driving variability at various times in the past..
Key Biological and Environmental Questions
Why does such a strong statistical correlation between UK'37 and annual mean SST exist? How accurate is the UK'37-SST 'thermometer' when extrapolated back in time? What best explains observed differences that are often noted in stratigraphic records for UK'37 and other putative paleoproxies of sea-surface temperature?

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Application #3: Estimate Surface Water Carbon Dioxide Concentrations
Carbon atoms in the alkenones exists in two different isotopic forms - 12C and 13C. When E. huxleyi photosynthesizes, producing alkenones from dissolved carbon dioxide, carbon dioxide molecules containing 12C are taken up selectively to those containing 13C. This selective photosynthetic uptake of the ‘lighter’ carbon dioxide molecules is called isotopic fractionation. The magnitude of isotopic fractionation (i.e., the difference between the 13C/12C composition of the alkenones and dissolved carbon dioxide) is not constant but varies depending upon the dissolved carbon dioxide concentration of surface waters in which E. huxleyi grows. Variations in dissolved carbon dioxide concentration of surface waters can result from changes either in the partial pressure of carbon dioxide in the atmosphere or in upwelling of deep ocean waters enriched in carbon dioxide derived from remineralization of settling organic matter.

The quantitative relationship between the availability of dissolved carbon dioxide and isotopic fractionation in E. huxleyi has now been calibrated experimentally (Bidigare et al. Global. Biogeochem. Cycles 11: 279-292, 1997). And, estimates of isotopic fractionation in the past can also be made from combined measurement of 13C/12C in alkenones and in the calcareous shells of planktonic foraminifera preserved in dated sediment cores. Given such estimates, the dissolved carbon dioxide concentration of surface waters in which E. huxleyi grew can be calculated using the calibration relationship. Assuming these calculations are correct, the challenge remains to determine unequivocally whether dissolved carbon dioxide concentrations assessed for a given time at a specific location represent equilibrium values with the atmosphere or supersaturation values due to upwelling (see Figure below). Because this type of information is key to understanding what controls global climate change, this aspect of alkenone biomarker research is currently receiving considerable attention from the paleoceanographic community (see Laws et al. Funct. Plant Biol. 29: 323-333, 2002).

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