Simon T. Belt

Highly branched isoprenoids (HBIs): identification of the most common and abundant sedimentary isomers

Abstract Tri- and tetraunsaturated highly branched isoprenoid (HBI) alkenes are widespread sedimentary geochemicals but few have been isolated from sediments in sufficient quantities for rigorous identification. However, two C25 trienes, four C25 tetraenes and two C25 pentaenes have now been isolated from the diatom Pleurosigma intermedium following bulk scale culture, and these have been purified by column chromatography and fully characterised by NMR spectroscopy and mass spectrometry. The compounds have been used to identify the previously unknown, but common and abundant HBIs found previously in many studies of sediments, particles and biota from around the world. These HBIs are structurally different to those reported from other diatoms. For example, unlike HBIs from the diatoms Haslea ostrearia and Rhizosolenia setigera, the alkenes in P. intermedium are unsaturated at the major branch point of the carbon skeleton and E/Z isomerism is observed for one of the trisubstituted double bonds. There is no evidence for the presence of configurational diastereoisomerism. The distributions of HBIs in P. intermedium (including the E/Z ratios) also show a dependence on the growth conditions within the five cultures studied. The positions of the double bonds in the HBIs of P. intermedium, and by inference, of the sediments, are consistent with the positions of sulphur incorporation in some of the HBI thiolanes and thiophenes which have been reported previously in some sediments and oils.

Source identification of the Arctic sea ice proxy IP25

Analysis of the organic geochemical biomarker IP25 in marine sediments is an established method for carrying out palaeo sea ice reconstructions for the Arctic. Such reconstructions cover timescales from decades back to the early Pleistocene, and are critical for understanding past climate conditions on Earth and for informing climate prediction models. Key attributes of IP25 include its strict association with Arctic sea ice together with its ubiquity and stability in underlying marine sediments; however, the sources of IP25 have remained undetermined. Here we report the identification of IP25 in three (or four) relatively minor (<5%) sea ice diatoms isolated from mixed assemblages collected from the Canadian Arctic. In contrast, IP25 was absent in the dominant taxa. Chemical and taxonomical investigations suggest that the IP25-containing taxa represent the majority of producers and are distributed pan-Arctic, thus establishing the widespread applicability of the IP25 proxy for palaeo Arctic sea ice reconstruction.

Structural characterisation of widespread polyunsaturated isoprenoid biomarkers: A C25 triene, tetraene and pentaene from the diatom Haslea ostrearia simonsen

Abstract Three highly branched isoprenoid (HBI) polyenes have been isolated from the diatom Haslea ostrearia and characterised by nmr spectroscopy

The emergence of modern sea ice cover in the Arctic Ocean

Arctic sea ice coverage is shrinking in response to global climate change and summer ice-free conditions in the Arctic Ocean are predicted by the end of the century. The validity of this prediction could potentially be tested through the reconstruction of the climate of the Pliocene epoch (5.33-2.58 million years ago), an analogue of a future warmer Earth. Here we show that, in the Eurasian sector of the Arctic Ocean, ice-free conditions prevailed in the early Pliocene until sea ice expanded from the central Arctic Ocean for the first time ca. 4 million years ago. Amplified by a rise in topography in several regions of the Arctic and enhanced freshening of the Arctic Ocean, sea ice expanded progressively in response to positive ice-albedo feedback mechanisms. Sea ice reached its modern winter maximum extension for the first time during the culmination of the Northern Hemisphere glaciation, ca. 2.6 million years ago.

A reproducible method for the extraction, identification and quantification of the Arctic sea ice proxy IP25 from marine sediments

IP25 is a highly branched isoprenoid lipid biomarker that is produced by some Arctic sea ice diatoms in the spring. The presence of IP25 in marine sediments has previously been used as a proxy measure of past spring sea ice occurrence in the Arctic. Here, a reliable analytical procedure for the reproducible extraction, identification and quantification of IP25 from marine sediments is presented for the first time. This protocol represents a Standard Operating Procedure (SOP) that should be straightforward to adopt by other researchers in the future. This paper also explains the significance of individual components and steps, including the internal standards used for quantification of IP25, the purification of sediment extracts to simplify the subsequent analysis and some key analytical considerations, especially when mass spectrometric methods are used for detection and quantification. The application of the SOP is illustrated with the analysis of IP25 in sediment material obtained from two nearby locations in the western region of the Canadian Arctic Archipelago (CAA). In the first case (Franklin Bay), sediment material was homogenised and aliquots used to demonstrate the reproducibility of the SOP and to provide a suitable reference material when studying other cores. In the second case (central Amundsen Gulf), IP25 concentrations were shown to be quite variable, consistent with observations reported previously for sediment cores from other regions of the CAA. Further experimental considerations are presented that permit the conversion of sedimentary IP25 concentrations into temporal fluxes that are probably more useful for palaeo sea ice studies.

Identification of a C25 highly branched isoprenoid (HBI) diene in Antarctic sediments, Antarctic sea-ice diatoms and cultured diatoms

Abstract A highly branched isoprenoid (HBI) diene found in Antarctic sea-ice diatoms and surface layer (0–2 cm) sediments from Antarctica is identified as 2,10,14-trimethyl-6-methylene-7-(3′-methylpent-1-enyl)pentadecane. The identification was made by comparison of GC–MS mass spectra and retention indices on both polar and apolar GC stationary phases with those of the diene authenticated by NMR after isolation from a laboratory culture of the diatom, Haslea ostrearia .

Sea ice and marine climate variability for NW Iceland/Denmark Strait over the last 2000 cal. yr BP

MD99-2263 is a 46 cm box core collected from Djupall, a trough that cuts across the NW Iceland Shelf and ends above Denmark Strait. We provide a multiproxy record that documents changes in the regional marine climate over the last ~1700 yr. The depth/age model is based on seven calibrated radiocarbon dates on mollusk shells and on 210Pb and 137Cs. Sediment accumulation rates were variable (0.2—0.8 mm/yr) but increased dramatically ~AD 1500. Grain-size, magnetic properties, quantitative mineral composition of the <2 mm sediment fraction, benthic foraminiferal composition, benthic and planktic Δ18O ratios, and abundances/fluxes of the sea ice biomarker IP25 were determined. To better compare the various proxies, 12 of the critical climate proxies were co-ordinated into 100-yr/sample time series, which were examined by Principal Component Analysis. The 1st axis explained 49% of the variance and the 2nd axis explained an additional 17%. The variables most strongly associated with the 1st axis were sediment properties (phi mean, clay%) and the sea ice biomarker. Mineralogical indicators of drift ice rafting, such as the presence of quartz and potassium- and sodium-feldspars, coincide with the IP25 biomarker data and show an increase after AD 1200, but high values of quartz and some feldspars also occurred between c. AD 300 and 900 with pronounced minima between AD 900 and 1100. Overall, our data suggest a simple two-fold division in climate conditions over the last 1700 yr, with the major change occurring c. AD 1200. In the last few decades, conditions have reverted towards those experienced prior to AD 1200.

Highly branched C25 isoprenoids in axenic cultures ofHaslea ostrearia

Abstract The hydrocarbon compositions of axenic cultures of the diatom Haslea ostrearia grown in the presence of penicillin, streptomycin and kanamycin were examined at lag,exponential and stationary growth phases. The production of highly branched isoprenoid (HBI)C 25 trienes to pentaenes with the 2,6,10,14-tetramethyl-7- (3-methylpentyl)pentadecane carbonskeleton was demonstrated at all three phases (2300–7000 fg cell −1 ). Of the ten HBI trienes tohexaenes reported previously from non-axenic cultures of H. ostrearia , four were present in theaxenic samples. In addition, two novel trienes and a pentaene were found. The most abundant ofthe new trienes was isolated from a larger, non-axenic batch culture and identified from 13 C- and 1 H-NMR data as 2,6,10,14-tetramethyl-7- (3-methylpent-4-enyl)pentadec-5,9-diene. Interestingdifferences in HBI isomer distributions were observed among the three growth phases. Forexample, the newly identified, non-methylenic triene above, only occurred in the exponentialgrowth phase. As a primary producer of these alkenes, several of which have demonstratedcytostatic activity, Haslea ostrearia , and perhaps related Haslea species, is clearly worthy offurther study.

A biomarker-based reconstruction of sea ice conditions for the Barents Sea in recent centuries

Variations in sea ice occurrence for the Barents Sea since c. AD 1700 have been determined by analysis of the abundance of the sea ice biomarker IP25 in three marine sediment box cores obtained from locations in the north, southeast and southwest parts of the region. Depth/age models for each core were established using excess 210Pb activity profiles. Comparisons between these depth/age models with those reported previously for the Barents Sea, suggest that the proxy sea ice record may, alternatively, extend back to c. AD 1500—1600. Sedimentation accumulation rates and bulk densities were combined with IP 25 concentrations, resulting in temporal changes to IP25 fluxes, which have been interpreted in terms of sea ice variability. The IP25 sea ice proxy data are also compared with other bulk organic geochemical parameters (total organic carbon and C/N ratios) and with sediment particle size distributions. The data indicate ice-free conditions for the southwest Barents Sea for the past c. 300 yr, consistent with previous reports based on historical sea ice records. In contrast, the combined proxy data from the southeast and north Barents Sea suggest variable spring sea ice occurrence on a c. 10—50 yr timescale, with reduced sea ice over the last c. 40—100 yr. In the early record, an enhanced sea ice occurrence is observed for the southeast Barents Sea during the mid—late nineteenth century, but as early as c. AD 1780 for the northern region. The outcomes of this study are broadly consistent with those obtained from ice edge position determinations derived previously from observational records. The study also demonstrates the potential of using IP25 as a sea ice proxy for longer-term palaeo sea ice determinations (e.g. the Holocene) for the Barents Sea.

Highly branched isoprenoids as proxies for variable sea ice conditions in the Southern Ocean

Abstract Concentrations of a highly branched isoprenoid (HBI) diene determined in over 200 sediment samples from the Arctic co-vary with those of an HBI monoene (IP25) shown previously to be a sedimentary sea ice proxy for the Arctic. The same diene, but not monoene IP25, occurred in nine sea ice samples collected from various locations around Antarctica. The diene has been reported previously in Antarctic sea ice diatoms and the 13C isotopic compositions of the diene determined in two Antarctic sea ice samples were also consistent with an origin from sea ice diatoms (δ13C -5.7 to -8.5‰). In contrast, HBIs found in two Antarctic phytoplankton samples did not include the diene but comprised a number of tri- to pentaenes. In sediment samples collected near Adélie Land, East Antarctica, both the diene and the tri- to pentaenes often co-occurred. 13C isotopic compositions of the tri- to pentaenes in three sediment samples ranged from -35 to -42‰ whereas that of the diene in a sediment sample was -18‰. We propose the presence of this isotopically 13C enriched HBI diene in Antarctic sediments to be a useful proxy indicator for contributions of organic matter derived from sea ice diatoms. A ratio of the concentrations of diene/trienes might reflect the relative contributions of sea ice to phytoplanktonic inputs of organic matter to Antarctic sediments.