Diunsaturated Alkenones Research Articles

Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C37 alkenones expressed as UK'37 is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the UK'37 proxy for assessment of Cretaceous paleoclimates. Herein, we report a C40 alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C40 alkadienone (tetraconta-16E, 23E-dien-3-one; C40:2 Et) allowed calculation of the UK’40 unsaturation index, comparable to UK’37. Stratigraphic variations in the δ13C of C40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO2. The UK’40 profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO2 during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

Alkenone unsaturation during virus infection of Emiliania huxleyi

Alkenones are found in diverse prymnesiophytes including Emiliania huxleyi, which ranges throughout the world ocean. The number of double bonds in alkenones corresponds with growth temperature, and calibrations between sea surface temperature and alkenone unsaturation have been used to establish alkenone unsaturation as a temperature proxy in ancient sediments. Here we report virus infection causes a relative increase in the proportion of diunsaturated alkenones in E. huxleyi strain CCMP 374. As virus infection routinely terminates E. huxleyi blooms in the ocean, it is possible that this contributes to the variance in alkenone-based temperature estimates for core-top sediments.

Long-chain alkenones and related distinctive compounds in the late Miocene and Pliocene sediments from the Gulf of Cadiz, eastern North Atlantic

Long-chain alkenones and related compounds were analyzed in Miocene and Pliocene sediments from the Gulf of Cadiz (GoC) in the eastern North Atlantic, which were obtained by Integrated Ocean Drilling Project (IODP) Expedition 339. Both GC and GC–MS analyses using non- and mid-polar columns were performed on the sediments. Isomeric cis-C37 alkenones were identified along with common trans-C37–C39 alkenones. The use of a mid-polar column resulted in the complete separation of the trans-C37 tri-unsaturated alkenone and the cis-C37 di-unsaturated alkenone and thus contributed to a more precise estimation of the alkenone-based paleotemperature. The reconstructed SSTs range from 26 to 28°C during ca. 6–3Ma in the GoC. These variations are concordant with global climate change during the period, particularly the Pliocene Climatic Optimum and the onset of sustained ice sheet growth in the northern hemisphere. The concentrations of C37–C39 alkenones and the C37/C38 alkenone ratios are lower during the late Miocene to early Pliocene and increase after ca. 4.2Ma, which corresponds to the taxonomic change of the plausible alkenone producers from the genus Reticulofenestra to genera Gephyrocapsa and Reticulofenestra. Moreover, a C38 di-unsaturated alkene-2,36-dione was tentatively identified. Variations in the concentrations of a tentatively identified alkenedione are almost synchronous with those of other alkenones, and their relative abundances decrease after ca. 4.2Ma. These results suggest that the alkenedione originated from the ancient alkenone producers such as the genus Reticulofenestra.

Excluding the di-unsaturated alkenone in the UK37 index strengthens temperature correlation for the common lacustrine and brackish-water haptophytes

Alkenone unsaturation indices, represented by either UK37 or UK′37, are important tools for paleoclimate studies. The UK37 index is a reflection of the average number of double bonds from di-, tri-, tetra-unsaturated alkenones, but UK′37 excludes the C37:4 in the calculation. Extensive studies indicate UK′37 provides better regressions against in situ Sea Surface Temperatures (SST) than UK37 and is the most widely used SST proxy. However, recent studies have shown that including C37:4 alkenones for lacustrine or brackish water haptophytes such as Ruttnera (Chrysotila) lamellosa and Isochrysis galbana improves temperature correlations although there are still significant deviations at the extreme high and low temperatures. In this study, we use new culture-based calibration experiments alongside published culture data and in situ water column or surface sediment data, to demonstrate that a further improved temperature calibration for alkenones is, in fact, achieved when the di-unsaturated alkenone is excluded from the computation of the unsaturation index. We propose new indices, termed UK″ [UK″37=C37:3/(C37:3+C37:4) or UK″38=C38:3/(C38:3+C38:4)], for lacustrine, brackish and estuarine waters. Our observation suggests that di-unsaturated alkenones play a less important role than tri- and tetra-unsaturated alkenones in regulating cell functions to temperature variations in lacustrine and brackish waters. We suggest using UK″ indices for paleotemperature reconstructions in the lacustrine and estuarine settings.

Alkenone producers during late Oligocene–early Miocene revisited

This study investigates ancient alkenone producers among the late Oligocene–early Miocene coccolithophores recorded at Deep Sea Drilling Project (DSDP) Site 516. Contrary to common assumptions, Reticulofenestra was not the most important alkenone producer throughout the studied time interval. The comparison between coccolith species‐specific absolute abundances and alkenone contents in the same sedimentary samples shows that Cyclicargolithus abundances explain 40% of the total variance of alkenone concentration and that the species Cyclicargolithus floridanus was a major alkenone producer, although other related taxa may have also contributed to the alkenone production at DSDP Site 516. The distribution of the different alkenone isomers (MeC37:2, EtC38:2, and MeC38:2) remained unchanged across distinct changes in species composition, suggesting similar diunsaturated alkenone compositions within the Noelaerhabdaceae family during the late Oligocene–early Miocene. However, the overall larger cell size of Cyclicargolithus may have implications for the alkenone‐based reconstruction of past partial pressure of CO2. Our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods (>1.85 Ma) predating the first occurrence of contemporary alkenone producers (i.e., Emiliania huxleyi and Gephyrocapsa oceanica).

Hydrogen isotopes in individual alkenones from the Chesapeake Bay estuary

Hydrogen isotope ratios of individual alkenones from haptophyte algae were measured in suspended particles and surface sediment from the Chesapeake Bay (CB) estuary, eastern USA, in order to determine their relationship to water δD values and salinity. δD values of four alkenones (MeC 37:2, MeC 37:3, EtC 38:2, EtC 38:3) from particles and sediments were between −165‰ and −221‰ and increased linearly ( R 2 = 0.7–0.9) with water δD values from the head to the mouth of the Bay. Individual alkenones were depleted in deuterium by 156–188‰ relative to water. The MeC 37 alkenones were consistently enriched by ∼12‰ relative to the EtC 38 alkenones, and the di-unsaturated alkenones of both varieties were consistently enriched by ∼20‰ relative to the tri-unsaturated alkenones. All of the increase in alkenone δD values could be accounted for by the water δD increase. Consequently, no net change in alkenone–water D/ H fractionation occurred as a result of the salinity increase from 10 to 29. This observation is at odds with results from culture studies with alkenone-producing marine coccolithophorids, and from two field studies, one with a dinoflagellate sterol in the CB, and one with a wide variety of lipids in saline ponds on Christmas Island, that indicate a decline in D/ H fractionation with increasing salinity. Why D/ H fractionation in alkenones in the CB showed no dependence on salinity, while D/ H fractionation in CB dinsoterol decreased by 1‰ per unit increase in salinity remains to be determined. Two hypotheses we consider to be valid are that (i) the assemblage of alkenone-producing haptophytes changes along the Bay and each species has a different sensitivity to salinity, such that no apparent trend in α alkenone–water occurs along the salinity gradient, and (ii) greater osmoregulation capacity in coastal haptophytes may result in a diminished sensitivity of alkenone–water D/ H fractionation to salinity changes.

RE‐EXAMINATION OF THE DOUBLE BOND POSITIONS IN ALKENONES AND DERIVATIVES: BIOSYNTHETIC IMPLICATIONS1

By derivatizing with OsO4, we determined the double bond positions of all the diunsaturated alkenones present in different haptophytes and demonstrated that the positions in C37–C40 homologs occur at a fixed carbon number from the carbonyl group, a finding contrary to earlier speculation. These data have allowed us to recognize three “families” of alkenones, for which we propose different biosynthetic pathways. The proposed biosynthesis of classical C37–C40 alkenones starts with acetyl‐ or propionyl‐SCoA and involves classical chain elongation steps with malonyl‐ and methylmalonyl‐SCoA affording alkanoyl‐ACP intermediates, which undergo subsequent Δ14, 21‐desaturation and decarboxylation reactions. Unusual diunsaturated shorter‐chain (C35 and C36) ketones now being produced for yet unknown physiological reasons by our specimen of Emiliania huxleyi (Lohm). Hay et Mohler strain CCMP1742. These compounds exhibit ω15,22 and ω16,23 double bonds, which are at a shorter distance (two carbon atoms) from the carbonyl group than the higher homologs, implying that their biosynthesis must involve an additional chain‐shortening step after Δ14, 21‐desaturation of alkanoyl‐ACP. Alkenones exhibiting a very unusual double bond spacing (three methylene groups instead of five) were detected in Holocene Black Sea sediments, in particulate matter collected in the Ligurian Sea and in Gephyrocapsa oceanica strain JB‐02. The formation of these compounds seems to be linked to the biosynthesis of monounsaturated alkenones previously detected in several haptophytes. Our work demonstrates the value of determining the double bond locations of alkenones whenever possible.

Characterization of unusual alkenones and alkyl alkenoates by electron ionization gas chromatography/mass spectrometry

Unusual long-chain, diunsaturated alkenones and alkyl alkenoates exhibiting double bonds separated by three methylene units instead of the more usual five were characterized by electron ionization (EI) gas chromatography/mass spectrometry. In a first step, the positions of the double bonds of these compounds (isolated from Holocene Black Sea sediments) were confirmed after OsO4 treatment and silylation. Mass spectra of the resulting tetratrimethylsilyloxy derivatives allowed unambiguous determination of the positions of unsaturations. The EI mass spectra of the non-derivatized compounds were then compared with those of the alkenones and alkyl alkenoates having double bonds separated by five methylene units. Specific fragment ions resulting from gamma-H rearrangements were found to be prominent in EI mass spectra of these unusual 'Black Sea' diunsaturated alkenones and alkyl alkenoates. These fragment ions can be used to characterize these compounds in natural samples without the need for laborious derivatization treatments.

Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene

The relation between the partial pressure of atmospheric carbon dioxide (pCO2) and Paleogene climate is poorly resolved. We used stable carbon isotopic values of di-unsaturated alkenones extracted from deep sea cores to reconstruct pCO2 from the middle Eocene to the late Oligocene (approximately 45 to 25 million years ago). Our results demonstrate that pCO2 ranged between 1000 to 1500 parts per million by volume in the middle to late Eocene, then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO2 likely allowed for a critical expansion of ice sheets on Antarctica and promoted conditions that forced the onset of terrestrial C4 photosynthesis.

Non-linear effects in the determination of paleotemperature Uk′37 alkenone ratios by chemical ionization mass spectrometry

The performance of gas chromatography coupled to mass spectrometry in the positive chemical ionization mode using ammonia as reagent gas (GC–PCI-MS) in the analysis of C 37 alkenones for paleotemperature estimation has been re-evaluated. In some conditions, the discrepancies observed in the measurement of the U k′ 37 index with this technique as compared with GC equipped with flame ionization detection (GC–FID) cannot be explained by differences in sensitivity between the tri- and diunsaturated alkenones. Thus, at low (currently <0.3) or high (currently >0.4) U k′ 37 values the GC–PCI-MS determinations may be observed to be lower or higher, respectively, than those measured with GC–FID. As shown by analysis of a series of synthetic C 37 alkenone standards these discrepant results can be explained by non linear effects in the GC–PCI-MS response factors. Second-order polynomial functions provide equations that describe better the signal to amount of analyte ratios. Users of GC–PCI-MS should calibrate their instruments with standards of known C 37 alkenone composition in order to minimize non-linear effects.

Variations in Miocene phytoplankton growth rates in the southwest Atlantic: Evidence for changes in ocean circulation

Changes in ocean circulation are often credited as the primary control on large‐scale climate change during the Miocene. This study investigates the latest Oligocene to middle Miocene evolution of Southern Ocean circulation by evaluating stable isotopic trends of shallow‐ and deep‐dwelling planktonic foraminifera, as well as εp records reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altantic Ocean (Deep Sea Drilling Project site 516). Changes in εp at site 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in εp at ∼20.3 Ma is interpreted to reflect an increase in upper water column nutrient concentrations, caused by the onset or strengthening of the Antarctic Circumpolar Current (ACC). Measurable alkenone concentrations disappear by ∼17 Ma, prior to a collapse in surface‐to‐thermocline δ18O and δ13C gradients. This is interpreted as reflecting a severe decrease in mixed layer nutrient concentrations and reduced proto‐Antarctic Intermediate Water influence. The δ18O gradient was reestablished by 14.5 Ma, coincident with the hypothesized East Antarctic ice sheet expansion, suggesting a direct relationship between increased strength of the ACC and the largest climate shift of the middle Miocene.

Miocene evolution of atmospheric carbon dioxide

Changes inpCO2or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late MiocenepCO2from εpvalues based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highestpCO2occurred during the latest Oligocene (∼350 ppmv) but decreased rapidly at ∼25 Ma. The early and middle Miocene was characterized by lowpCO2(260–190 ppmv). Lower intervals ofpCO2correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either highpCO2during the late early Miocene climatic optimum or a sharppCO2decrease associated with EAIS growth. Paradoxically,pCO2increased following EAIS growth and obtained preindustrial levels by ∼10 Ma. Although we emphasize an oceanographic control on Miocene climate, lowpCO2could have primed the climate system to respond sensitively to changes in heat and vapor transport.