Highly branched isoprenoid

Highly branched isoprenoids are a type of lipid produced by marine diatoms. Highly branched isoprenoids are biomarkers, and their presence or absence in sedimentary and ice records can be used as a direct proxy for the presence of sea ice. Generally, the highly branched isoprenoids that are used as sea ice proxies are 25-carbon molecules containing one to three double bonds. The longest carbon chains in the C25 highly branched isoprenoids used for sea ice reconstructions are 15 carbons, but these molecules are highly branched and have shorter carbon chains attached to the primary carbon chain. There are 3 C25 highly branched isoprenoids used as ice proxies: a C25 monoene (HBI I), a C25 diene (HBI II), and a C25 triene (HBI III). Highly branched isoprenoid I and II are unique in that they are primarily produced by sympagic diatoms. Sympagic diatoms live in channels at the base of sea ice, making them a highly accurate proxy for sea ice. During the spring, highly branched isoprenoids are produced by diatoms in the sea ice. In the summer, the ice melts, releasing the highly branched isoprenoids into the water column, where they sink and are then deposited in the sediments. Highly branched isoprenoid III can be used as a biomarker for seasonal sea ice in the open ocean. Following this initial identification, highly branched isoprenoids were identified in a variety of marine environments, such as in the Puget Sound, Antarctica, Spain, and Peru. Highly branched isoprenoid I }} }} }} Highly branched isoprenoid (HBI I), a C25 monoene, is also known as IP25 (ice proxy with 25 carbon atoms). IP25 serves as a biomarker for ice conditions in the Arctic. The IPSO25 proxy is a less-developed biomarker than IP25, and its Arctic sources are unclear. IPSO25 has also been identified in the diatom Haslea ostrearia and in sediments in non-polar locations, indicating that more work is needed to fully understand and develop IPSO25 as a paleo ice proxy. Highly branched isoprenoid III }} }} }} Highly branched isoprenoid III (HBI III), a C25 triene, is a biomarker useful for the analysis of the marginal ice zone (MIZ), a zone between the open ocean and sea ice. Highly branched isoprenoid III is primarily produced by pelagic algae of the genus Rhizosolenia, particularly Rhizsolenia setigera, Rhizosolenia herbetata f. semispina, and Rhizosolenia polydactyla. Its source was determined by Simon Belt in 2017, who isolated highly branched isoprenoid III from phytoplankton samples from western Svalbard and the South Atlantic. The production of highly branched isoprenoid III appears to be enhanced in the MIZ, however why this occurs is currently not well-understood. The absence of highly branched isoprenoid III in sediments is typically attributed to sea ice cover in the region, given that ice cover would not allow for pelagic algae production. == Preservation ==

Many biomarkers possess functional groups or are unsaturated, causing them to undergo diagenesis when emplaced in sediments. Highly branched isoprenoids are observed to be well-preserved in the sedimentary record. C25 trienes are more likely to undergo degradation compared to C25 monenes and dienes, and degradation is enhanced by increasing temperature and exposure to light, which results in oxidation. More work is necessary to determine whether highly branched isoprenoids can be preserved beyond 12 million years, but it is likely that their time to degradation is dependent on the local sediment conditions. == Measurement techniques ==

GC/MS Highly branched isoprenoids and other organic materials can be extracted from sediments for analysis. Coupled gas chromatography and mass spectrometry can be utilized to analyze the organic materials present in the sediments. Peaks can be identified using a gas chromatograph and a mass spectrum, which provide information about the retention time and mass-to-charge ratio of the organic compounds present in the molecule. This allows for the identification of highly branched isoprenoids within a material. To identify highly branched isoprenoids in sediments, typically selective ion monitoring (SIM) is utilized. Isotope ratios Carbon isotopic measurements can be utilized to confirm the sea ice origin of highly branched isoprenoids. These carbon isotope measurements are obtained using an Isotope Ratio Mass Spectrometer. Highly branched isoprenoids with a sea ice origin are enriched in δ13C in both sea ice and in sediments. This enrichment has been hypothesized to be in part a result of the fact that the marine diatoms that produce highly branched isoprenoid I live under CO2-limited conditions. This value is indicative of an enrichment in 13C. This enrichment has also been observed for highly branched isoprenoid II in sediments and in waters near melting sea ice. This enrichment likely results from the CO2-limited conditions under which the diatom producers of highly branched isoprenoid II grow. This distinctive isotopic composition provides strong evidence for the sea ice origin of highly branched isoprenoid II, making it a good proxy for sea ice around Antarctica. Highly branched isoprenoid III Highly branched isoprenoid III is depleted in δ13C, with values ranging from −35 to −40‰. Highly branched isoprenoid III is depleted in δ13C because it is not produced under CO2-limited conditions, so the depletion is a result of biological fractionation. == Case study: Use of IP25 to reconstruct ice records ==

Currently, more than 60 paleo sea ice records that have been constructed based on IP25 over the Holocene, the Mid-Pleistocene Transition, the Pliocene/Pleistocene boundary, and the late Miocene. The presence of IP25 in sediments is a direct proxy for the presence of seasonal sea ice cover. One study utilized the concentration of IP25 in sediments to reconstruct sea-ice records in the western North Pacific and Bering Sea over the past 18,000 years. Researchers from Germany obtained sediment samples from the North Pacific Ocean and Bering Sea during a Sonne cruise in 2009. IP25 was identified in the samples using GC/MS, and the sediments were dated using a chronostratigraphic approach, X-ray fluorescence, and radiocarbon dating of planktonic foraminifera. The researchers found that variations in the concentration of the sea-ice proxy IP25 were consistent with known temperature variations based on other evidence, such as δ18O values and biogenic opal data. Generally, during cold intervals, the concentration of IP25 in sediments was elevated, indicating more extensive sea ice cover. More specifically, between 18,000 and 15,000 years ago, IP25 concentrations were relatively high, but decreased between 14,700 and 12,900 years ago during the Bølling/Allerød warming period. At 12,500 years ago, a significant increase in IP25 concentration was detected in the sediments, consistent with the start of the Younger Dryas period, which marked a return to glacial conditions. IP25 concentrations decreased to approximately 0 11,500 years ago, marking the end of the Younger Dryas. For the entire Holocene, IP25 concentrations have remained low, which is accordant with the lack of extensive ice cover throughout this period. This reconstruction is consistent with other paleoclimate proxies and known climate variations, demonstrating the ability of the IP25 proxy to reconstruct paleo ice records. == References ==