Holocene climate variability in the Southern Ocean recorded in a deep-sea sediment core off South Australia

To understand Holocene climatic development and to determine drivers of climatic changes and climate variability, high-resolution marine proxy records are required from key oceanic locations. However, information on the Holocene climate development from the Southern Hemisphere is still rare and mainly based on terrestrial archives. Here, we present data with a high-resolution of circa 35 years from sediment cores taken east of the Great Australian Bight, where it is possible to determine Southern Ocean Holocene climate and the longer-term trends of the El Niño–Southern Oscillation (ENSO) conditions. For this purpose, we used the oxygen-isotope records of two planktonic foraminifer species Globigerinoides ruber and Globigerina bulloides which inhabit different water masses as well as faunal counts of planktonic foraminifers. After the ocean frontal systems off southern Australia were pushed northward by orbitally-forced insolation changes during the early Holocene, the data indicate increasing ENSO variability during the mid to late Holocene when the fronts shifted polewards again. A strong circa 1550 year cycle is found in the Globigerina bulloides record which reflects the wider Southern Ocean signal with prominent cold phases centred at circa 9.2, 7.3, 5.8, 4.3, 2.7, 1.4 ka BP and, possibly the Little Ice Age, which have global counterparts.     

Diagenetic Alteration of Magnetic Signals by Anaerobic Oxidation of Methane Related to a Change in Sedimentation Rate

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (κ) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS⁻ is generated by the process of AOM. The released HS⁻ reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm kyr⁻¹ during glacial times to 5 cm kyr⁻¹ in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ∼9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.     

A ~565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

International Journal of Earth Sciences - In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565 Ma has been detected. Such...     

Effects of Hypoxia on Fish Survival and Oyster Growth in a Highly Eutrophic Estuary

Human land use activities around estuaries can result in high levels of eutrophication. At Elkhorn Slough estuary, a highly eutrophic California estuary, we investigated the effects of impaired water quality on two stress-tolerant estuarine species, a common fish, the staghorn sculpin, Leptocottus armatus and a foundational invertebrate, the Olympia oyster, Ostrea lurida. We caged the two indicator species at six wetlands with different levels of water quality impairment, four of which had restricted tidal flow. We also recorded water quality parameters simultaneously at all sites using YSI sondes, and sampled nutrients and chlorophyll-a monthly, building on the National Estuarine Research Reserve System-wide Monitoring Program. We found that the monitored environmental variables predicted ecological responses by the indicator species. In particular, we found that the duration and severity of hypoxia were negatively correlated with fish survival and oyster growth. Further, our results corroborate previous studies that artificial tidal restriction leads to increased hypoxia stress. We conclude that large diurnal fluctuations in dissolved oxygen and extended nighttime hypoxia can have lethal and sub-lethal effects even on stress-tolerant organisms in the estuary. While laboratory experiments have often shown such effects, it is relatively rare to demonstrate negative effects of oxygen variation with in situ experiments, which provide stakeholders with concrete evidence for impaired water quality at local wetlands. Tidally restricted sites, which experience the largest fluctuations in dissolved oxygen and longest periods of hypoxia, harbor conditions harmful to vertebrates and invertebrates in the estuary. Reversing the anthropogenically induced low oxygen levels, by restoring more natural tidal exchange and by decreasing agricultural runoff, could improve the survival and growth of important estuarine organisms.     

The distribution and relative toxic potential of organic chemicals in a PCB contaminated bay

For reliable monitoring of environmental improvement, the PCB contaminated bay Orserumsviken on Sweden's Baltic coast was investigated prior to remediation. We examined PCB congener patterns and the relative toxic potential of PCBs in extracts of three matrices: bottom sediment, settling particulate matter and the dissolved phase (obtained from semi-permeable membrane devices). Congener patterns were similar in all matrices. Three fractions were isolated from the extracts: (1) aliphatic and monocyclic aromatic compounds (MAC-fraction), (2) dicyclic aromatic compounds (DAC-fraction), including PCBs, and (3) polycyclic aromatic compounds (PAC-fraction), including PAHs. Total extracts and fractions were injected into newly fertilised rainbow trout eggs. At larval stage, hepatic EROD activities were quantified. Though high, the PCB contamination could explain only an estimated 1-5% of the total EROD induction. The order of EROD induction potential was: total extract > PAC-fraction > DAC-fraction > MAC-fraction in all matrices, suggesting that in Orserumsviken PACs made a larger contribution to the EROD induction potential than PCBs and other DACs.     

Mantle and Crustal Effects on the Geochemistry of Proterozoic Dikes and Sills in Sweden

Chemical and Sr-Nd isotopic data are presented for 14 Proterozoicdolerite intrusions from Sweden. The dolerites are unmetamorphosedand belong to two age groups at 1.26 and 0.95 Ga. The analysedrocks were selected from over 80 samples characterized for initial⁸⁷Sr/⁸⁶Sr ratio, and represent those from each intrusion thathave little to no crustal contamination effect on Sr isolopes.In these tholeiitic to mildly alkaline basalts, MgO varies from4 to 14%, and TiO₂ from 1.0 to 4.7%. Incompatible-element abundancepatterns show similar shapes at widely varying overall enrichmentlevels; La varies from eight to 64 times primitive mantle. Manysamples have apparent Sr anomalies; positive for the less incompatible-elementenriched rocks, negative for those with the highest abundances,Hence Sr varies by a factor of only two, whereas neighboringelements vary by a factor of seven. Small Eu anomalies correlatewith the larger Sr variations. All samples have negative Nbanomalies, correlated with _{Nd 1} values that vary from –0.5to + 3.5 for both dolerite age groups. ⁸⁷Sr/⁸⁶Sr₁ values aresystematically different in the two groups: 1.26-Ga doleritesvary from 0.7026 to 0.7035, and 0.95-Ga dolerites from 0.7041to 0.7051. Rather than urging any particular model of subcontinental mantleevolution, we use the dolerite results to deconvolute the effectsof crustal contamination, differentiation including crypticdifferentiation effects, and mantle-lithosphere interactionson possibly asthenosphere-derived magmas. Problems related toaddition of Rb and ⁸⁷Sr giving false isochrons, as well as raised⁸⁷Sr/⁸⁶Sr from syn- or post-crystallization fluid effects, wereeliminated by the sample selection criteria. In the selecteddolerites, crustal contamination is not a major control on geochemistry,because all incompatible elements, including those of both highand low crustal abundance, are positively and similarly correlated.Overall abundances have been affected by migrations of residualmagma after much olivine and plagioclase crystallization. OnlySr is an approximate indication of the parent magma's enrichment,whereas all other incompatible elements may be depleted or enrichedby factors greater than two. On entering the crust, the magmasvaried by a factor of only two in overall incompatible-elementenrichment. They had Nb anomalies correlated with ^Nd1, and the0.95-Ga dolerites possessed higher ⁸⁷Sr/⁸⁶Sr than had characterizedthe 1.26-Ga dolerites. All mantle effects could be interpretedas resulting from a lithospheric magma source that experiencedaddition of subduction-related components 1.9–1.7 Ga ago,and maybe locally 1.6–1.0 Ga ago. We prefer a model whereasthenosphere-derived melts interact with lithospheric wallrocks, acquiring variable _Nd down to –0.5 and Nb anomalies.Up to two-thirds of the budget of incompatible elements, butnot much Nb, were added to the magmas. Untimately, the contributionof any mantle-source is unconstrained, except that significantlithosphere has to have been involved. The mantle-related geochemicaleffects, which result in only weak constraints on process andsource, are very difficult to isolate in an objective approachto intracontinental basalts, because of superimposed differentiationand contamination effects.     

The compensating effect of glaciers: Characterizing the relation between interannual streamflow variability and glacier cover

Meltwater from glaciers is not only a stable source of water but also affects downstream streamflow dynamics. One of these dynamics is the interannual variability of streamflow. Glaciers can moderate streamflow variability because the runoff in the glacierized part, driven by temperature, correlates negatively with the runoff in the non-glacierized part of a catchment, driven by precipitation, thereby counterbalancing each other. This is also called the glacier compensation effect (GCE), and the effect is assumed to depend on relative glacier cover. Previous studies found a convex relationship between streamflow variability and glacier cover of different glacierized catchments, with lowest streamflow variability at a certain optimum glacier cover. In this study, we aim to revisit these previously found curves to find out if a universal relationship between interannual streamflow variability and glacier cover exists, which could potentially be used in a space-for-time substitution analysis. Moreover, we test the hypothesis that the dominant climate drivers (here precipitation and temperature) switch around the suggested optimum of the curve. First, a set of virtual nested catchments, with the same absolute glacier area but varying non-glacierized area, were modelled to isolate the effect of glacier cover on streamflow variability. The modelled relationship was then compared with a multicatchment data set of gauged glacierized catchments in the European Alps. In the third step, changes of the GCE curve over time were analysed. Model results showed a convex relationship and the optimum in the simulated curve aligned with a switch in the dominant climate driver. However, the multicatchment data and the time change analyses did not suggest the existence of a universal convex relationship. Overall, we conclude that GCE is complex due to entangled controls and changes over time in glacierized catchments. Therefore, care should be taken to use a GCE curve for estimating and/or predicting interannual streamflow variability in glacierized catchments.