Diatoms are microscopic, unicellular, golden-brown algae belonging to the plant class Bacillariophyceae of the division Chromophyta (e.g., Hasle and Syvertsen, 1996). They are characterized by an external frustule (skeleton), typically 10-100 µm in length or diameter. This frustule is composed of amorphous silica (opal-A), which has the potential to be preserved within the sedimentary record.
Diatoms are ubiquitous to aquatic environments and as such are found in a wide range of habitats from terrestrial through brackish to fully marine. They are ecologically and environmentally sensitive and in the oceans constitute a major part of the marine phytoplankton.
Discrete samples were prepared for diatom analysis following a procedure modified from Battarbee (1986) for freshwater diatoms. Because of the high organic matter content, samples were left to stand in H2O2 overnight at room temperature before being gently heated in a water bath. Following the procedure of Battarbee and Kneen (1982), samples were spiked with a calibrated microsphere solution to enable quantitative data analysis. Diatom valves, including those of Chaetoceros resting spores, and microspheres were counted under a 400× phase objective lens with an Olympus BH-2 microscope. At least 300 valves were counted per slide.
All absolute abundance data were then converted to diatom accumulation rates (DARs) (e.g., Schuette and Schrader, 1979), to account for sedimentation rate (SR) and dry bulk density. Based upon the age model, the SR at Site 1083 over the investigated 250 k.y. ranged from 5.65 cm2/k.y. (at 2.468-2.464 Ma) to 12.45 cm2/k.y. (at 2.604-2.603 Ma), averaging at 9.45 cm2/k.y. Formulas are given in "Appendix".
Modern studies have shown oceanic productivity to be a function of nutrient availability, with nitrogen (principally in the form of nitrate [NO3-]) constituting one of the main limiting factors. Nitrogen limitation can have important implications for the amount of CO2 drawdown within a system, for reasons related to the Redfield ratio. This states that during photosynthesis carbon, nitrogen, and phosphorus are taken up in the ratio 106:16:1 (Brasier, 1995). If NO3- is limiting within the system, there will be a net reduction in CO2 drawdown, which creates a negative feedback within the system.
When NO3- is abundant, phytoplankton selectively discriminate against the isotopically heavier 15N during photosynthetic uptake, preferring the isotopically lighter isotope 14N. When NO3- is limited no discrimination is made (Ostrom et al., 1997). Organic matter should therefore yield a relatively lighter 15N (less positive) isotopic signature at times when NO3- is in ready supply, and a relatively heavier (more positive) signal at times when NO3- supplies are limiting.
Marine studies (e.g., Holmes et al., 1996, 1997; Farrell et al., 1995; Francoise et al., 1993) have also identified this signal in the surface sediments. A relationship exists between the sedimentary 14N/15N ratio and NO3- utilization in the surface waters. If this relationship is extended downcore, past changes in the nutrient supply demand, balance can be hindcasted from records of 15N in organic matter (Holmes et al., 1996; Farrell et al., 1995). This technique has been used successfully to reconstruct paleo-NO3- utilization in the Gulf of California (e.g., Pride et al., 1999), in the southwest Indian sector of subantarctic waters (e.g., Francoise et al., 1993), and in the late Quaternary sediments off Angola (e.g., Holmes et al., 1996, 1997, 1998).
In addition, the 15N signal is sensitive to NO3- source (i.e., marine or terrestrial in origin). Analysis of the 13C signature in sedimentary organic matter can help identify the source. Marine algae utilize dissolved bicarbonate (HCO3-) for photosynthesis, yielding stable organic carbon isotope ratios of an average of -21 (Meyers, 1992). Land plants derive their carbon from atmospheric CO2, which is isotopically lighter than marine HCO3-. Terrestrial organic matter typically has a 13Corg signature of about -28. Studies from DSDP Sites 362 and 532 suggest that the distinction between terrestrial and marine organic matter is well preserved in the sediments of the Walvis Ridge back to 14 Ma (Meyers, 1992).
Nitrogen isotope analysis was conducted on the raw sediment. For carbon isotope analysis, each sample was pretreated with HCl to remove the inorganic carbonate fraction. Analysis was conducted at the NERC Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) facility at Royal Holloway, University of London, United Kingdom. Nitrogen and carbon isotopes are expressed in the standard delta notation as parts per thousand (), relative to atmospheric nitrogen, and Peedee belemnite standards, respectively. Error ranges were derived through replication of laboratory standards.
Details of this technique and the standard methodology employed are given in Rosell-Melé et al. (submitted, this volume [N1]), and Durham et al. (Chap. 23, this volume) (see also Rosell-Melé et al., 1997; Harris et al., 1996). TOC data were converted into mass accumulation rates (MAR) to account for the sedimentation rate and to enable more direct comparison with the diatom data.