PP52B-01
Stable Hydrogen Isotopes of Terrestrial n-alkanes record the Mid Younger Dryas Event in European Lakes
Compound-specific hydrogen isotope measurements (expressed as δD values) on lacustrine long- chained n-alkanes are a potentially promising means of reconstructing variations in the isotopic composition of continental precipitation in the geological past. Long-chain n-alkanes are produced in the leaf waxes of terrestrial higher plants and have been shown to preserve the hydrogen isotope composition of precipitation being influenced by atmospheric moisture source, evaporation as well as plant physiology. Since δD values are determined directly on carbon-bound hydrogen, which is non-exchangeable even at higher temperatures biological and environmental source conditions can be preserved over geological timescales. Here, we present a record of n-alkane δD values, \( nC27 - nC31 \), from the Ammersee, southern Germany and Lac d'Annecy, French Alps spanning the time period from the Oldest Dryas to Preboreal (14 ka to 11 ka BP). We compare biomarker hydrogen isotopic signatures to oxygen-isotope ratios of precipitation inferred from deep-lake ostracods from the Ammersee as well to GRIP \(δ18\)O values. The remarkable climate shift at the onset of the Younger Dryas (YD) (12.9 to 11.6 ka BP) is recorded in the \(δ18\)O values as well as the hydrogen isotope signal with a pronounced drop of approx. - 40‰ in D at the Ammersee and a minor negative excursion of - (20‰) in D at Lac d'Annecy. This depletion is consistent with a colder and dryer climate and the expansion of grasslands in the lake catchments as recorded in a significant increase of grass-derived \( nC31 \). A sudden remarkable deuterium enrichment of about 100‰ in D is apparent in both lakes and marks the mid-Younger Dryas event (MYDE), which is also reflected in the GRIP and Ammersee \(δ18\)O values. δD values during this event reach or even exceed the following Holocene level. The simultaneous appearance of this event in different locations suggests a climate driven by mechanisms like the thermohaline circulation known for the Holocene. The transition to the Preboreal following the decline of the D content after the MYDE is characterized by a positive δD shift of approx. 60‰. This increase is most likely the onset ot the Holocene which is characterized by 10‰ heavier δD values compared to the Oldest Dryas. In general, hydrogen isotope ratios of n-alkanes cover a wide range of values implying a great sensitivity to precipitation as well as vegetation changes and thus wide ranging climate driven fluctuations. Furthermore, the similarity of both lake sediment records suggests that profound regional climatic changes have occurred during the YD, particularly during the MYDE. δD values of Lac d'Annecy are about 40‰ enriched throughout the whole record either suggesting a higher level of continentality or higher evaporation rates within the catchment. The timing and magnitude of the MYDE at three far distant locations over the northern hemisphere supports the suggestion that the Holocene-type thermohaline circulation started at this point with a rapid climate change followed by weakening mechanisms and terminating into warmer Holocene conditions.
PP52B-02 INVITED
Multi-Isotope (13C, 14C, D/H) Insights into Abrupt Climate Shifts in the Tropical Atlantic Basin
Organic geochemical investigations of climate evolution in the tropical Atlantic, which models suggest was sensitive to changes in both the high latitude Atlantic and low latitude Pacific sectors during the last glacial cycle, promise to clarify the origins of high frequency variability observed in many proxy records across the globe. We reconstruct precipitation changes in the western tropical Atlantic over the past 200,000 years using the stable carbon and hydrogen isotopic compositions of higher plant leaf waxes preserved in the Cariaco Basin. Results indicate that less rain fell over northern South America during stadial periods relative to interstadials. Moreover, the severity of these dry events was modulated by precessionally driven oscillations in local insolation, such that droughts concurrent with periods of increased solar heating were less severe than those associated with insolation minima, and vice versa. Detailed molecular level radiocarbon measurements in the modern environment indicate that a significant portion of these leaf wax compounds are pre-aged by several thousand 14C years within the adjacent drainage basin before ultimate incorporation in marine sediments. Climate records based on their corresponding stable isotopic signatures might therefore be offset in both timing and amplitude from the paleoenvironmental shifts they document. Nevertheless, much of the original climatic information is likely preserved by the remaining component, which is delivered to the ocean within ten to twenty years of biosynthesis. Together these findings suggest that while additional care must be taken in the interpretation of phase and amplitude relationships, reconstructions based on vascular plant biomarkers offer tremendous insight into the origins of abrupt climate change in the past.
PP52B-03
The Isotopic History of Western North American Grasslands
Over the last 20 years, researchers have detected a rise in carbon isotope values of fossil mammals, paleosol minerals, and organic substrates roughly 6 to 8 Ma, which indicates an increase in the relative abundance of C4 grass at localities around the world. Prior research has shown that this increase in C4 abundance did not occur in cool high latitude regions, nor did it occur at sites around the Mediterranean. Similarly, modern California and Nevada do not support native C4 grasses. These states have a water regime similar to the Mediterranean region, with hot and dry summers, and most precipitation occurring during the cool winter months. Because the amount of C4 grass in North American ecosystems correlates positively with growing season temperature, it is not surprising that these western states, which have a winter growing season, do not support C4 plants. This does not necessarily imply, however, that C4 plants have never been favored in these western states. We used the carbon isotope composition of fossil ungulate tooth enamel as a proxy for C4 plant abundance in California and Nevada from 18 Ma to the present. Our carbon isotope data suggest that whereas there are very few native C4 species living in California or Nevada today, C4 plants may have been present in the region as early as the Late Miocene. Furthermore, there is an indication that C4 plants may have increased in abundance in the region during Pleistocene glacial periods.
PP52B-04
A 60,000-yr organic geochemical record of precipitation, temperature, and vegetation change in Southeast Tropical Africa
Tropical Africa - influenced by two major monsoon systems and a site of deep atmospheric convection - is a climatically important place. While a number of paleoclimate records illuminate the broad evolution of tropical African climate across the Holocene, very little is known about the nature of African climate during the last glacial period. Furthermore, most paleoclimate records from this region lack the resolution to discern millennial and centennial-scale climatic events. To improve our knowledge of Late Quaternary climate change in the Southeast African tropics, we present a high- resolution, 60,000-year organic geochemical record of climate history from the sediments of Lake Tanganyika. Eight meters of sediment were analyzed at 200-500 year resolution for compound-specific carbon and hydrogen isotopes of plant leaf waxes as well as TEX86 paleotemperature. In addition, we analyzed bulk organic carbon and nitrogen isotopes and opal concentrations at 50-100 year resolution to reconstruct lake productivity. Together, these proxies yield a wealth of information concerning tropical African climate. The organic geochemical proxies allow the reconstruction of tangible climatic variables: rain and heat. Since the isotopic composition of rainfall in East Africa is primarily controlled by the amount effect, the hydrogen isotopic composition of long-chain leaf waxes provides a powerful proxy for rainfall amount. In addition, the novel TEX86 paleotemperature proxy, which records the thermal history of the surface waters of Lake Tanganyika, is influenced primarily by variations in air temperature. All proxies indicate large and abrupt changes throughout the past 60,000 years, suggesting that East African climate is highly sensitive to global climatic change. More specifically, the hydrogen isotopes show large changes in precipitation concurrent with Northern high-latitude millennial scale climate changes such as the Younger Dryas and Heinrich Event 1, potentially highlighting the role that the Indian and African monsoon systems play in synchronizing high and low latitude climates. The TEX86 record shows large, rapid, 2-3˚C shifts in lake surface temperature during Marine Isotope Stages 1 and 3, and the carbon isotopic composition of leaf waxes suggests that East African vegetation also responds to climatic variability. The relationship between lake surface temperature and lake productivity (as indicated by organic carbon and opal concentrations) is not straightforward, suggesting that thermally-induced stratification is not the sole important factor controlling production in Lake Tanganyika. This novel record is a prime example of the invaluable paleoclimatic information that can be gathered utilizing organic geochemical methods, and highlights the interplay between high and low-latitude climate systems with respect to millennial-scale climate change.
PP52B-05
Abrupt changes in hydrology and vegetation in the West African monsoon region since the Last Glacial Maximum
The West African monsoon is a complex dynamical system that depends on feedbacks between land surface, including vegetation, and the ocean. Modeling and existing paleoclimatic data suggest that the coupling between these system components makes it particularly susceptible to abrupt change. Characterization of each of these components is therefore crucial in understanding their role in West African monsoon variability during different climate regimes. Here we utilize organic geochemical and molecular isotopic approaches to reconstruct vegetation, hydrological and paleoenvironmental changes spanning the last ~30 kyr from the sediments of Lake Bosumtwi, an anoxic lake located in the heart of the West African monsoon region. Prior to 16 kyr, carbon isotopic values of long-chain n-alkanes, synthesized as terrestrial leaf waxes, were as much as 10 per mil higher than during the Holocene, suggesting a dramatic shift from C4 grasslands to tropical forests at that time. The transition between these two states is characterized by abrupt isotopic reversals of up to 3-5 permil, consistent with model predictions of unstable vegetation regimes during periods of changing monsoon strength. Concentrations of levoglucosan, a proxy for fire intensity and frequency, show a complex relationship with vegetation and climate, but appear to be elevated during these abrupt transitions, perhaps as a result of the competing influences of changing fuel loads and aridity. During the glacial period, leaf wax carbon isotopic values are relatively invariant and lack the large-scale shifts evident in isotopic measurements of aquatic compounds and bulk organic matter. These results highlight the importance of a compound-specific approach in reconstructing past environmental changes from carbon isotopic variations in sediment records.
PP52B-06
Penetration of Atlantic Walker Circulation Into East Africa During Early to mid-Holocene: Hydrogen Isotope Evidence From Sacred Lake, Mt. Kenya
The tropics play a very important role in global climate variability, yet the mechanisms behind the tropical climate variation remain poorly understood. Here, we present a high-resolution, well-dated record from Sacred Lake, Kenya, East Africa. We measured D/H ratios of botryococcenes, a class of highly specific biomarkers produced by freshwater algae ( Botrycoccus braunii) in a sediment core obtained from this open lake. Our main goal is to examine changes in East African rainfall amount and moisture source during the past 18kyr BP. During the late Pleistocene and late Holocene, the hydrogen isotope records track local hydrological variations inferred from numerous lake level and pollen records from the region. However, during the early to mid-Holocene (10-5ka cal yr BP), the D/H values from Sacred Lake were as much as 90 per mil heavier than during the late Pleistocene and late Holocene. If the "amount effect" is the main control on the isotopic compositions of rainfall during the early to mid Holocene, Our data would suggest drier conditions, which is inconsistent with the "African Humid Period" inferred by numerous records of the mid-Holocene. We propose that the high isotopic ratios in precipitation in East Africa during the early to mid-Holocene is due to an eastward shift in the large-scale atmospheric circulation of the tropics. In East Africa, this shift involves a major increase in moisture source from the Atlantic Ocean relative to Indian Ocean. Heavier isotope ratios of precipitation originated from Atlantic Ocean result from the intensive convection and recycling of water vapor over the Congo Basin, as opposed to Indian moisture that traverses dry land masses and losses moisture rapidly. In comparison to the late Holocene, the early to mid-Holocene is characterized by relatively northerly positioning of the ITCZ and intense monsoon systems as well as weak ENSO. These factors combine to shift the walker circulation eastward, allowing the Atlantic moisture to penetrate farther into eastern Africa. Our theory is supported by climate model results, paleoclimate records from the Kilimanjaro ice core and Cariaco Basin, and other records from the Indian and Pacific Oceans. For example, the ice core in Kilimanjaro, East Africa suggest that the oxygen isotope ratios of precipitation were ~ 8 per mil higher during early to mid-Holocene than the late Holocene, which is consistent with our results.
PP52B-07
Paleoclimate of the Eastern Mediterranean/North Africa during the past 26 cal ka based on organic geochemical investigations of a Nile River Delta sediment core
The Mediterranean Sea is situated within a highly sensitive climatic region, which is influenced by both tropical and mid-latitude climate dynamics, and the paleoenvironmental history of this region is of interest because large human populations occupy the surrounding landmasses. In this study, multiple organic geochemical proxies are examined from a Nile River Delta sediment core (GeoB 7702-3) to investigate the paleoclimatic history of the North Africa/Eastern Mediterranean region during the past ~26 cal ka. Sea surface temperatures were reconstructed using both the TEX86 and alkenone paleothermometers. The TEX86 record exhibits centennial to millennial scale variability and captures global climate events including the Last Glacial Maximum (LGM), Heinrich Event 1 (H1), the Bolling/Allerod and the Younger Dryas (YD). The recently developed Branched and Isoprenoid Tetraether (BIT) index, used to differentiate between marine and terrestrial inputs, closely tracks changes noted in bulk C/N ratios. Overall, these records indicate greater variability during the Late Pleistocene than during the Holocene, with the highest terrestrial inputs noted at approximately the time of the YD and prior to H1. Although it might be expected that fluvial organic matter inputs should be the lowest during the LGM, when the sources of both the Blue and White Nile were severely reduced or desiccated, higher (more terrestrial) BIT values noted at these times may be related to changes in vegetation cover in North Africa. During the Holocene, a major shift in the BIT index to lower (more marine) values marks the onset of deposition of the S1 sapropel layer. The lower BIT values noted during this interval are caused by a dramatic (order of magnitude) increase in the absolute abundance of crenarchaeol, attesting to enhanced marine productivity at this time. Following deposition of the S1 sapropel, absolute abundances of crenarchaeol are generally higher than during the Late Pleistocene, suggesting increased marine productivity in the eastern Mediterranean throughout the Holocene.
PP52B-08
Timing is everything: ecological vs. evolutionary pacing of Triassic-Jurassic carbon cycle disruptions
Eruption of Earth's largest flood basalt, the Central Atlantic Magmatic Province (CAMP) has been proposed as the trigger for a major carbon cycle disruption at the Triassic-Jurassic mass extinction interval at ~201 Ma. Inferred from negative excursions in the carbon isotopic composition (δ13C) of carbonate and organic matter, this perturbation has been linked to massive dissociation of isotopically light, methane-rich gas hydrates caused by volcanogenic CO2-induced global warming. However, both the sequence and duration of the CAMP eruptions relative to the carbon cycle perturbation remain circumstantial and indirect, because the data have been from stratigraphic sections far from the flood basalts and without accumulation rate constraints. Here we use a record of atmospheric (δ13C) from specific molecules (nC25 - nC32 n-alkanes) diagnostic of terrestrial plant leaf waxes from astronomically-paced cyclical lacustrine strata in which CAMP flood basalts are interbedded to directly examine the relationship between the (δ13C) excursions and their durations. We show that the flood basalts postdate the abrupt start of a ~400 ky negative excursion coincident with the initiation of the mass extinction event, but predate a protracted 1.5 m.y. negative excursion. Based on a modified BLAG carbon cycle model, the timing and long durations of our (δ13C) excursions are incompatible with CAMP-triggered gas hydrate release. Instead, we suggest that the (δ13C) pattern is more consistent with a catastrophically-triggered functional reorganization of the biosphere, part of which involved the ascent of dinosaurs to ecological dominance, playing out over evolutionary time.