Hubert B. Vonhof

Global cooling accelerated by early late Eocene impacts

At Ocean Drilling Program Site 689 (Maud Rise, Southern Ocean), δ18O records of fine-fraction bulk carbonate and benthic foraminifers indicate that accelerated climate cooling took place following at least two closely spaced early late Eocene extraterrestrial impact events. A simultaneous surface-water productivity increase, as interpreted from δ13C data, is explained by enhanced water-column mixing due to increased latitudinal temperature gradients. These isotope data appear to be in concert with organic-walled dinoflagellate-cyst records across the same microkrystite-bearing impact-ejecta layer in the mid-latitude Massignano section (central Italy). In particular, the strong abundance increase of Thalassiphora pelagica is interpreted to indicate cooling or increased productivity at Massignano. Because impact-induced cooling processes are active on time scales of a few years at most, the estimated 100 k.y. duration of the cooling event appears to be too long to be explained by impact scenarios alone. This implies that a feedback mechanism, such as a global albedo increase due to extended snow and ice cover, may have sustained impact-induced cooling for a longer time after the impacts.

Reconstruction of the Miocene western Amazonian aquatic system using molluscan isotopic signatures

Abstract Strontium isotopic ratios and Sr contents of molluscs from the Miocene Pebas Formation in Colombian Amazonia indicate that the bulk of the Pebas Formation was deposited in a freshwater environment, predominantly draining the Andes, with minor contribution from the shield regions. The molluscan oxygen and carbon isotopic composition does not provide a palaeosalinity proxy, but points to a lacustrine palaeoenvironment. The faunal character confirms such an interpretation. Sr isotopic ratios further suggest that a marine incursion that reached the area was composed of a mixture of shield-derived freshwater and seawater, with a maximum salinity of about 5 psu. Faunistic components of this incursion-level support a shield–seawater mixing. The incursion most likely reached western Amazonia from the north, probably along the western Guyana shield.

Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions

Strontium, oxygen, and carbon isotope compositions of well-preserved mollusks (bivalves) indicate a dominantly freshwater depositional setting for the lower Miocene–upper Miocene Pebas Formation in Western Amazonia. Molluscan 87Sr/86Sr ratios identify different freshwater sources. Andean runoff was the dominant water source in Miocene Western Amazonia, though there was occasional influx of waters from cratonic catchments. At only one stratigraphic level, isotope signals indicate increased (mesohaline) aquatic salinities, in concert with a clearly more saline molluscan faunal assemblage. Strontium isotope–based salinity estimates are surprisingly low when compared to other paleosalinity estimates based on the interpretation of (ichno)faunal assemblages and sedimentological structures. We propose that these seemingly contrasting observations can be unified if Miocene Western Amazonia was occupied by a long-lived (lacustrine) wetland system with a restricted connection, via the Los Llanos Basin, to the Caribbean Sea. Abundant runoff supplied fresh water to this system, which effectively blocked the influx of saline waters through the restricted marine connection to the north. Much like modern Lake Maracaibo, such a system could have been the site of microtidal currents and thus could have hosted brackish-water fauna in a dominantly freshwater depositional system.

Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ18O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ18O. In the floodplain lake under study the δ18O variation of the water is controlled by contrasting dry and wet season evaporation–precipitation regimes. Molluscan δ18O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ13C records of water and bivalves are in good agreement, suggesting that both δ18O and δ13C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ13C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.

Modelling oceanic carbon and phosphorus fluxes: implications for the cause of the late Cenomanian Oceanic Anoxic Event (OAE2)

A two-box ocean model with a coupled carbon and phosphorus cycle is used to test which processes can realistically generate a rapid positive 2‰ δ13C excursion in the global carbon reservoir. Internal oceanic processes (enhanced preservation of organic matter under anoxic conditions, enhanced upwelling) cannot produce a substantial δ13C excursion under the assumption that burial rates of C and P are coupled. Loss of nutrients to the sediment acts as a negative feedback mechanism that prevents a substantial perturbation of the carbon cycle. A large and renewable source of nutrients is needed, either from increased continental weathering or from enhanced regeneration of sedimentary organic phosphorus under anoxic conditions. Measurements of total phosphorus and organic carbon in sediments from the Tarfaya Basin, Morocco, suggest that a large decrease in P burial occurred relative to organic carbon at the base of the Cenomanian–Turonian Oceanic Anoxic Event (OAE2), with the right magnitude to quantitatively explain a 2‰ shift in δ13C. It is concluded that the main underlying mechanism that allowed and sustained enhanced carbon burial during the mid-Cretaceous was a perturbation of the oceanic phosphorus cycle.

High-resolution late Maastrichtian–early Danian oceanic 87Sr/86Sr record: Implications for Cretaceous-Tertiary boundary events

A high-resolution late Maastrichtian–early Danian seawater 87 Sr/ 86 Sr reference curve is constructed from two Cretaceous-Tertiary boundary (K-T boundary) sections: Bidart (France) and El Kef (Tunisia). The 87 Sr/ 86 Sr curve shows maxima at 0.3–0.4 Ma before the K-T boundary and at the K-T boundary. The first maximum could mark the onset of a major outflow of the Deccan Traps. The second maximum, a rapid 0.000 06 87 Sr/ 86 Sr, shift, extends from ∼3–4 m below to ∼1 m above the K-T boundary. This profile probably results from diagenetic smoothing of an originally sharp K-T boundary 87 Sr/ 86 Sr anomaly, rather than from a gradual process. The sharp shift could result from (1) the vaporization of the Chicxulub target rocks, (2) global wildfires, and (3) acid-rain leaching of soils and sialic surface rocks. Of these three possibilities, only Sr release by soil leaching combined with increased rainfall associated with the K-T event appears to be sufficiently large to produce the observed K-T 87 Sr/ 86 Sr anomaly.

Improved age control on early Homo fossils from the upper Burgi Member at Koobi Fora, Kenya.

To address questions regarding the evolutionary origin, radiation and dispersal of the genus Homo, it is crucial to be able to place the occurrence of hominin fossils in a high-resolution chronological framework. The period around 2 Ma (millions of years ago) in eastern Africa is of particular interest as it is at this time that a more substantial fossil record of the genus Homo is first found. Here we combine magnetostratigraphy and strontium (Sr) isotope stratigraphy to improve age control on hominin-bearing upper Burgi (UBU) deposits in Areas 105 and 131 on the Karari Ridge in the eastern Turkana Basin (Kenya). We identify the base of the Olduvai subchron (bC2n) plus a short isolated interval of consistently normal polarity that we interpret to be the Pre-Olduvai event. Combined with precession-forced (~20 kyr [thousands of years]) wet-dry climate cycles resolved by Sr isotope ratios, the magnetostratigraphic data allow us to construct an age model for the UBU deposits. We provide detailed age constraints for 15 hominin fossils from Area 131, showing that key specimens such as cranium KNM-ER 1470, partial face KNM-ER 62000 and mandibles KNM-ER 1482, KNM-ER 1801, and KNM-ER 1802 can be constrained between 1.945 ± 0.004 and 2.058 ± 0.034 Ma, and thus older than previously estimated. The new ages are consistent with a temporal overlap of two species of early Homo that can be distinguished by their facial morphology. Further, our results show that in this time interval, hominins occurred throughout the wet-dry climate cycles, supporting the hypothesis that the lacustrine Turkana Basin was a refugium during regionally dry periods. By establishing the observed first appearance datum of a marine-derived stingray in UBU deposits at 2.058 ± 0.034 Ma, we show that at this time the Turkana Basin was hydrographically connected to the Indian Ocean, facilitating dispersal of fauna between these areas. From a biogeographical perspective, we propose that the Indian Ocean coastal strip should be considered as a possible source area for one or more of the multiple Homo species in the Turkana Basin from over 2 Ma onwards.

Seasonally resolved growth of freshwater bivalves determined by oxygen and carbon isotope shell chemistry

By means of a monitoring experiment in two rivers in the Netherlands, we establish a relationship between seasonally resolved growth rates in unionid freshwater bivalves and their environment. We reconstructed these seasonally resolved growth rates by using relationships of stable isotopes in the shells and their ambient river water. The reconstructed growth rates reveal that shells grow fastest in spring-early summer, when highest food availability occurs in the rivers. In addition, the reconstructed growth rates show that onset and cessation of growth are mainly influenced by water temperature.

OXYGEN ISOTOPE COMPOSITION OF BIVALVE SEASONAL GROWTH INCREMENTS AND AMBIENT WATER IN THE RIVERS RHINE AND MEUSE

Abstract The application of oxygen isotope ratios (δ18O) from freshwater bivalves as a proxy for river discharge conditions in the Rhine and Meuse rivers is investigated. We compared a dataset of water temperature and water δ18O values with a selection of recent shell δ18O records for two species of the genus Unio in order to establish: (1) whether differences between the rivers in water δ18O values, reflecting river discharge conditions, are recorded in unionid shells; and (2) to what extent ecological parameters influence the accuracy of bivalve shell δ18O values as proxies of seasonal, water oxygen isotope conditions in these rivers. The results show that shells from the two rivers differ significantly in δ18O values, reflecting different source waters for these two rivers. The seasonal shell δ18O records show truncated sinusoidal patterns with narrow peaks and wide troughs, caused by temperature fractionation and winter growth cessation. Interannual growth rate reconstructions show an ontogenetic growth rate decrease. Growth lines in the shell often, but not always, coincide with winter growth cessations in the δ18O record, suggesting that growth cessations in the shell δ18O records are a better age estimator than counting internal growth lines. Seasonal predicted and measured δ18O values correspond well, supporting the hypothesis that these unionids precipitate their shells in oxygen isotopic equilibrium. This means that (sub-) fossil unionids can be used to reconstruct spring-summer river discharge conditions, such as Meuse low-discharge events caused by droughts and Rhine meltwater-influx events caused by melting of snow in the Alps.

Belemnite-based strontium, carbon and oxygen isotope stratigraphy of the type area of the Maastrichtian Stage *

Belemnitellid cephalopods from the Maastrichtian stratotype area (southeast Netherlands) are shown to be comparatively well preserved. Although partial diagenetic alteration has been observed, micromilling techniques have permitted the extraction of pristine belemnite calcite, suitable for the reconstruction of strontium (Sr), oxygen (O) and carbon (C) isotope variation of Maastrichtian seawater. A distinct Sr isotope pattern in the Maastricht record can be matched stratigraphically with records from Hemmoor (northern Germany), El Kef (Tunisia) and ODP site 690 (Maud Rise, Antarctica), leading to a new chemostratigraphical age model for the Maastrichtian stratotype section. Our data improve currently applied strontium isotope stratigraphical reference curves by revealing an Sr isotope inflection pattern near the lower/upper Maastrichtian boundary that is a potentially diagnostic feature for intra-Maastrichtian stratigraphical correlation between distant sections. Belemnites further show significant stratigraphical oxygen isotope variation through the Maastrichtian. We interpret this variation to have resulted from palaeoceanographic reorganisations in the Atlantic Ocean during this time interval.