Concentrations and d13C compositions of n-alkanes, n-alcohols, n-alkanoic acids in a 34-month time series of suspended sediments from the outflow of the Congo River

The concentrations, distributions, and stable carbon isotopes (d13C) of plant waxes carried by fluvial suspended sediments contain valuable information about terrestrial ecosystem characteristics. To properly interpret past changes recorded in sedimentary archives it is crucial to understand the sources and variability of exported plant waxes in modern systems on seasonal to inter-annual timescales. To determine such variability, we present concentrations and d13C compositions of three compound classes (n-alkanes, n-alcohols, n-alkanoic acids) in a 34-month time series of suspended sediments from the outflow of the Congo River. We show that exported plant-dominated n-alkanes (C25-C35) represent a mixture of C3 and C4 end members, each with distinct molecular distributions, as evidenced by an 8.1 ± 0.7 per mil (±1Sigma standard deviation) spread in d13C values across chain-lengths, and weak correlations between individual homologue concentrations (r = 0.52-0.94). In contrast, plant-dominated n-alcohols (C26-C36) and n-alkanoic acids (C26-C36) exhibit stronger positive correlations (r = 0.70-0.99) between homologue concentrations and depleted d13C values (individual homologues average <= -31.3 per mil and -30.8 per mil, respectively), with lower d13C variability across chain-lengths (2.6 ± 0.6 per mil and 2.0 ± 1.1 per mil, respectively). All individual plant-wax lipids show little temporal d13C variability throughout the time-series (1 Sigma <= 0.9 per mil), indicating that their stable carbon isotopes are not a sensitive tracer for temporal changes in plant-wax source in the Congo basin on seasonal to inter-annual timescales. Carbon-normalized concentrations and relative abundances of n-alcohols (19-58% of total plant-wax lipids) and n-alkanoic acids (26-76%) respond rapidly to seasonal changes in runoff, indicating that they are mostly derived from a recently entrained local source. In contrast, a lack of correlation with discharge and low, stable relative abundances (5-16%) indicate that n-alkanes better represent a catchment-integrated signal with minimal response to discharge seasonality. Comparison to published data on other large watersheds indicates that this phenomenon is not limited to the Congo River, and that analysis of multiple plant-wax lipid classes and chain lengths can be used to better resolve local vs. distal ecosystem structure in river catchments.

TOC, TN, and sedimentary d15N and d13C ratios of sediment core SO201-2-85

We present high-resolution records of sedimentary nitrogen (d15Nbulk) and carbon isotope ratios (d13Cbulk) from piston core SO201-2-85KL located in the western Bering Sea. The records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180 kyr. The d15Nbulk record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization (~62-80%) despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction (mterr). This suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today (~84%). d15Nbulk increases from Marine Isotope Stage (MIS) 5.4 and culminates during the Last Glacial Maximum, which indicates that nitrate utilization in the Bering Sea was raised during cold intervals (MIS 5.4, 5.2, 4) and almost complete during MIS 3 and 2 (~93-100%). This is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone, thereby increasing the iron-to-nutrient ratio and therefore the nitrate utilization in the mixed surface layer. Large variations in d15Nbulk were also recorded from 180 to 130 ka BP (MIS 6), indicating a potential link to insolation and sea-level forcing and its related feedbacks. Millennial-scale oscillations were observed in d15Nbulk and d13Cbulk that might be related to Greenland interstadials.

Planktonic foraminiferal d18O and d13C values, derived sea-surface temperatures from d18O and selected planktonic foraminiferal species for core P69 of Hawke Bay off eastern North Island

Recent evidence suggests that the Subtropical Convergence (STC) zone east of New Zealand shifted little from its modern position along Chatham Rise during the last glaciation, and that offshore surface waters north of the STC zone cooled only slightly. However, at nearshore core site P69 (2195 m depth), 115 km off the east coast of North Island and ca 300 km north of the modern STC zone, planktonic foraminiferal species, transfer function data and stable oxygen and carbon isotope records suggest that surface waters were colder by up to 6°C during the late last glacial period compared to the Holocene, and included a strong upwelling signature. Presently site P69 is bathed by south-flowing subtropical waters in the East Cape Current. The nearshore western end of Chatham Rise supports a major bathymetric depression, the Mernoo Saddle, through which some exchange between northern subtropical and southern subantarctic water presently occurs. It is proposed that as a result of much intensified current flows south of the Rise during the last glaciation, a consequence of more compressed subantarctic water masses, lowered sea level, and an expanded and stronger Westerly Wind system, there was accelerated leakage northwards of both Australasian Subantarctic Water and upwelled Antarctic Intermediate Water over Mernoo Saddle in a modified and intensified Southland Current. The expanded cold water masses displaced the south-flowing warm East Cape Current off southeastern North Island, and offshore divergence was accompanied by wind-assisted upwelling of nutrient-rich waters in the vicinity of P69. A comparable kind of inshore cold water jetting possibly characterised most glacial periods since the latest Miocene, and may account for the occasional occurrence of subantarctic marine fossils in onland late Cenozoic deposits north of the STC zone, rather than invoking wholesale major oscillations of the oceanic STC itself.

Compilation of 220 sediment core d13C data from the glacial Atlantic Ocean

We compare a compilation of 220 sediment core d13C data from the glacial Atlantic Ocean with three-dimensional ocean circulation simulations including a marine carbon cycle model. The carbon cycle model employs circulation fields which were derived from previous climate simulations. All sediment data have been thoroughly quality controlled, focusing on epibenthic foraminiferal species (such as Cibicidoides wuellerstorfi or Planulina ariminensis) to improve the comparability of model and sediment core carbon isotopes. The model captures the general d13C pattern indicated by present-day water column data and Late Holocene sediment cores but underestimates intermediate and deep water values in the South Atlantic. The best agreement with glacial reconstructions is obtained for a model scenario with an altered freshwater balance in the Southern Ocean that mimics enhanced northward sea ice export and melting away from the zone of sea ice production. This results in a shoaled and weakened North Atlantic Deep Water flow and intensified Antarctic Bottom Water export, hence confirming previous reconstructions from paleoproxy records. Moreover, the modeled abyssal ocean is very cold and very saline, which is in line with other proxy data evidence.

Epibenthic foraminiferal d13C in the Recent deep Arctic Ocean

Low planktic and benthic d18O and d13C values in sediments from the Nordic seas of cold stadials of the last glaciation have been attributed to brines, formed similar to modern ones in the Arctic Ocean. To expand on the carbon isotopes of this hypothesis I investigated benthic d13C from the modern Arctic Ocean. I show that mean d13C values of live epibenthic foraminifera from the deep Arctic basins are higher than mean d13C values of upper slope epibenthic foraminifera. This agrees with mean high d13C values of dissolved inorganic carbon (DIC) in Arctic Bottom Water (ABW), which are higher than mean d13CDIC values from shallower water masses of mainly Atlantic origin. However, adjustments for oceanic 13C-Suess depletion raise subsurface and intermediate water d13CDIC values over ABW d13CDIC ones. Accordingly, during preindustrial Holocene times, the d13CDIC of ABW was as high or higher than today, but lower than the d13CDIC of younger subsurface and intermediate water. If brine-enriched water significantly ventilated ABW, brines should have had high d13CDIC values. Analogously, high-d13CDIC brines may have been formed in the Nordic seas during warm interstadials. During cold stadials, when most of the Arctic Ocean was perennially sea-ice covered, a cessation of high-d13CDIC brine rejection may have lowered d13CDIC values of ABW, and ultimately the d13CDIC in Nordic seas intermediate and deep water. So, in contrast to the idea of enhanced brine formation during cold stadials, the results of this investigation imply that a cessation of brine rejection would be more likely.

Multi-proxy records of Two-Yurts Lake

Within the scope of Russian-German palaeoenvironmental research, Two-Yurts Lake (TYL, Dvuh-Yurtochnoe in Russian) was chosen as the main scientific target area to decipher Holocene climate variability on Kamchatka. The 5x2 km large and 26 m deep lake is of proglacial origin and situated on the eastern flank of Sredinny Ridge at the northwestern end of the Central Kamchatka Valley, outside the direct influence of active volcanism. Here, we present results of a multi-proxy study on sediment cores, spanning about the last 7000 years. The general tenor of the TYL record is an increase in continentality and winter snow cover in conjunction with a decrease in temperature, humidity, and biological productivity after 5000-4500 cal yrs BP, inferred from pollen and diatom data and the isotopic composition of organic carbon. The TYL proxy data also show that the late Holocene was punctuated by two colder spells, roughly between 4500 and 3500 cal yrs BP and between 1000 and 200 cal yrs BP, as local expressions of the Neoglacial and Little Ice Age, respectively. These environmental changes can be regarded as direct and indirect responses to climate change, as also demonstrated by other records in the regional terrestrial and marine realm. Long-term climate deterioration was driven by decreasing insolation, while the short-term climate excursions are best explained by local climatic processes. The latter affect the configuration of atmospheric pressure systems that control the sources as well as the temperature and moisture of air masses reaching Kamchatka.

Accumulation rates of plant-wax-derived long-chain C25 to C35 odd-numbered n-alkanes and d13C values of the n-C31 alkane of ODP Hole 175-1077B

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants-adapted to warm, dry conditions and low atmospheric CO2 concentrations (Collatz et al., 1998, doi:10.1007/s004420050468) -have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature (Pagani et al., 1999, doi:10.1126/science.285.5429.876; Huang et al., 2001, doi:10.1126/science.1060143) and CO2 concentration (Cerling et al., 1993, doi:10.1038/361344a0; Kuypers et al., 1999, doi:10.1038/20659) are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content - driven by tropical sea surface temperature changes and the strength of the African monsoon - controlled aridity on the African continent, and hence large-scale vegetation changes.