Diatom oxygen isotopes and biogenic silica concentrations: an examination of their potential for reconstructing palaeoenvironmental change.

Diatoms represent between 25% and 45% of all marine primary productivity. As yet, however, the potential of using diatom isotopes, in particular oxygen (518Odiatom), in palaeoceanographic reconstructions has yet to be fully assessed. Work here within this thesis demonstrates a method for extracting pure, mono or near mono species specific, diatom samples from marine sediments. Over two time intervals, the potential of using 8l8Odiatom in palaeoceanographic reconstructions is then shown at ODP site 882 in the North West Pacific Ocean over the onset of major Northern Hemisphere Glaciation (2.73 Ma) and over the last 200 kyr BP. During both periods, major changes in the halocline/stratification of the water column, relating to significant freshwater input to the region, are reconstructed. These results indicate a significant role for the region in initiating glacier growth across the North American continent and in causing changes in atmospheric pC02. Strong evidence exists, however, that both an inter- and intra-species vital/species effect of up to 3.5l%o may be present in 518Odiatom. As such, this may limit future applications of 518Odiatom to samples which are dominated by only one taxa. Section two of the thesis investigates the potential for wet-alkaline measurements of BSi to provide insights into the global silicon cycle. A sequential Si/Al technique, which directly accounts for levels of clay/aluminosilicate digestion, is demonstrated to produce lower and more accurate measurement of BSi in samples for which a rapid non-BSi digestion phase occurs. However, comparisons between wet-alkaline BSi measurements and diatom biovolume measurements, which more accurately reflect the true amount of BSi within a sample, show a poor relationship between the two variables. This is particularly true in samples where diatom dissolution is high. Consequently, diatom biovolume measurements may be better suited for making quantitative palaeoenvironmental reconstructions

Glacial discharge into the subarctic Northeast Pacific Ocean during the last glacial

Understanding the response of the climate to abrupt changes in the Earth system represents a key objective in paleoclimatology. Heinrich events in the last glacial, during which significant amounts of glacial discharge entered the North Atlantic Ocean, triggered the development of colder conditions across much of the globe. Despite widespread documentation of these events, including their occurrence and global significance, the impact of Heinrich events on the North American Cordilleran Ice Sheet and subarctic North Pacific Ocean remains relatively unconstrained. Here, records of diatom oxygen isotopes are used to show that significant amounts of glacial discharge from the Cordilleran Ice Sheet were released into the open waters of the northeast Pacific Ocean throughout the last glacial. Based on the available age model, many of these episodes and calculated changes in sea surface salinity coincide with Heinrich events. If accurate, these findings would confirm that ocean-atmospheric teleconnections linked the North Atlantic and North Pacific Oceans during intervals of abrupt change in the last glacial, as well as indicating the wider susceptibility of regional ice-sheets to global alterations in the climate system

Photic zone changes in the north-west Pacific Ocean from MIS 4–5e

In comparison to other sectors of the marine system, the palaeoceanography of the subarctic North Pacific Ocean is poorly constrained. New diatom isotope records of δ13C, δ18O δ30Si (δ13Cdiatom, δ18Odiatom and δ30Sidiatom) are presented alongside existing geochemical and isotope records to document changes in photic zone conditions, including nutrient supply and the efficiency of the soft-tissue biological pump, between Marine Isotope Stage (MIS) 4 and MIS 5e. Peaks in opal productivity in MIS 5b/c and MIS 5e are both associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone. Whereas the MIS 5e peak is associated with low rates of nutrient utilisation, the MIS 5b/c peak is associated with significantly higher rates of nutrient utilisation. Both peaks, together with other smaller increases in productivity in MIS 4 and 5a, culminate with a significant increase in freshwater input which strengthens/re-establishes the halocline and limits further upwelling of sub-surface waters to the photic zone. Whilst δ30Sidiatom and previously published records of diatom δ15N (δ15Ndiatom) (Brunelle et al., 2007, 2010) show similar trends until the latter half of MIS 5a, the records become anti-correlated after this juncture and into MIS 4, suggesting a possible change in photic zone state such as may occur with a shift to iron or silicon limitation

Holocene carbon fluxes and palaeoproductivity in aquatic ecosystems: a multiproxy, palaeolimnological approach

Inland waters act as an important control on the global carbon cycle. Deep tectonic lakes may provide a key link between short‐term and long‐term carbon cycles as buried carbon is essentially locked away from the atmosphere over geological timescales. Here we investigate Holocene carbon dynamics in one of the world’s most important lake ecosystems, Lake Baikal, Siberia

A review of the stable istotope bio-geochemistry of the global silicon cycle and its associated trace elements

Silicon (Si) is the second most abundant element in the Earth's crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, δ13C, δ15N, δ18O, δ30Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies

Modelling silicon supply during the Last Interglacial (MIS 5e) at Lake Baikal

Limnological reconstructions of primary productivity have demonstrated its response over Quaternary timescales to drivers such as climate change, landscape evolution and lake ontogeny. In particular, sediments from Lake Baikal, Siberia, provide a valuable uninterrupted and continuous sequence of biogenic silica (BSi) records, which document orbital and sub-orbital frequencies of regional climate change. We here extend these records via the application of stable isotope analysis of silica in diatom opal (δ30Sidiatom) from sediments covering the Last Interglacial cycle (Marine Isotope Stage [MIS] 5e; c. 130 to 115 ka BP) as a means to test the hypothesis that it was more productive than the Holocene. δ30Sidiatom data for the Last Interglacial range between +1.29 to +1.78‰, with highest values between c. 127 to 124 ka BP (+1.57 to +1.78‰). Results show that diatom dissolved silicon (DSi) utilisation, was significantly higher (p=0.001) during MIS 5e than the current interglacial, which reflects increased diatom productivity over this time (concomitant with high diatom biovolume accumulation rates [BVAR] and warmer pollen-inferred vegetation reconstructions). Diatom BVAR are used, in tandem with δ30Sidiatom data, to model DSi supply to Lake Baikal surface waters, which shows that highest delivery was between c. 123 to 120 ka BP (reaching peak supply at c. 120 ka BP). When constrained by sedimentary mineralogical archives of catchment weathering indices (e.g. the Hydrolysis Index), data highlight the small degree of weathering intensity and therefore representation that catchment-weathering DSi sources had, over the duration of MIS 5e. Changes to DSi supply are therefore attributed to variations in within-lake conditions (e.g. turbulent mixing) over the period, where periods of both high productivity and modelled-DSi supply (e.g. strong convective mixing) account for the decreasing trend in δ30Sidiatom compositions (after c. 124 ka BP)

Pliocene diatom and sponge spicule oxygen isotope ratios from the Bering Sea: isotopic offsets and future directions

Oxygen isotope analyses of different size fractions of Pliocene diatoms (δ18Odiatom) from the Bering Sea show no evidence of an isotope offset and support the use of bulk diatom species samples for palaeoceanographic reconstructions. Additional samples containing concentrations of sponge spicules produce δ18O values several per mille lower than δ18Odiatom with a calculated mean offset of 3.6‰ ± 0.7. This difference is significantly greater than modern day variations in water δ18O through the regional water column. Despite the potential for oxygen isotope disequilibrium within δ18Osponge, there appears to be some similarity between δ18Osponge and a global stacked benthic δ18Oforam record. This highlights the potential for δ18Osponge in palaeoenvironmental research at sites where carbonates are not readily preserved

δ18O Records

Diatom silica is a form of biogenic opal (SiO2⋅nH2O) that contains oxygen isotopes that can be used in lacustrine and marine paleoenvironmental studies. Here we describe the various analytical considerations and methodologies to achieve good environmental signals. We describe diatom oxygen isotope systematics – in particular, the mineral-water temperature fractionation and silica maturation processes. Finally we give examples of studies both in lake and ocean environments. We conclude that diatom δ18O in sediments can be an extremely useful indicator of paleoenvironmental change and tends to be most successful in areas where there are likely to have been large changes in the isotope composition of the host water. In lakes this can be due to changes in the precipitation/evaporation balance, or source of precipitation, and in oceans it can be due to variations in meltwater flux. The change in δ18Odiatom due to these factors is normally greater than temperature and methodological errors

Sedimentary records of sewage pollution using faecal markers in contrasting peri-urban shallow lakes

Sewage contamination in shallow lake sediments is of concern because the pathogens, organic matter and nutrients contribute to the deterioration of the water-bodies' health and ecology. Sediment cores from three shallow lakes (Coneries, Church and Clifton Ponds) within Attenborough nature reserve located downstream of sewage treatment works were analysed for TOC, C/N, δ13C, δ15N, bacterial coliforms and faecal sterols. 210Pb and 137Cs activities were used to date the sediments. Elemental analysis suggests that the source of organic matter was algal and down profile changes in δ13C indicate a possible decrease in productivity with time which could be due to improvements in sewage treatment. δ15N for Coneries Pond are slightly higher than those observed in Church or Clifton and are consistent with a sewage-derived nitrate source which has been diluted by non-sewage sources of N. The similarity in δ15N values (+ 12‰ to + 10‰) indicates that the three ponds were not entirely hydrologically isolated. Analysis by gas chromatography/mass spectrometry (GC/MS) reveals that Coneries Pond had sterol concentrations in the range 20 to 30 μg/g (dry wt.), whereas, those from Clifton and Church Ponds were lower. The highest concentrations of the human-sourced sewage marker 5β-coprostanol were observed in the top 40 cm of Coneries Pond with values up to 2.2 μg/g. In contrast, Church and Clifton Pond sediments contain only trace amounts throughout. Down-profile comparison of 5β-coprostanol/cholesterol, 5β-coprostanol/(5β-coprostanol + 5α-cholestanol) and 5β-epicoprostanol/coprostanol as well as 5α-cholestanol/cholesterol suggests that Coneries Pond has received appreciable amounts of faecal contamination. Examination of 5β-stigmastanol (marker for herbivorous/ruminant animals) down core concentrations suggests a recent decrease in manure slurry input to Coneries Pond. The greater concentration of β-sitosterol in sediments from Church and Clifton Ponds as compared to Coneries is attributed in part to their greater diversity and extent of aquatic plants and avian faeces