The Frasnian-Famennian boundary is recognized as the culmination of a global mass extinction in the Late Devonian. In western New York State the boundary is a distinct horizon within a pyritic black shale bed of the upper Hanover Shale defined by the first occurrence of Palmatolepis triangularis in the absence of Frasnian conodonts. The boundary is characterized by a minor disconformity marked by a lag concentration of conodonts. Iridium at the boundary is 0.11-0.24 ng/g, two to five times background levels of <0.05 ng/g; other Ir enrichments of 0.38 ng/g and 0.49 ng/g occur within 50 cm of the conodont-constrained boundary. Numerous Ir enrichments in the boundary interval suggest extraterrestrial accretion and platinum group element (PGE) concentration at disconformities, or mobilization and concentration in organic-rich/pyritic-rich laminations from cosmic or terrestrial sources. PGE ratios of Pt/Pd and Ku/Ir at the boundary horizon approximate chondritic ratios and are suggestive of an unaltered extraterrestrial source. These values do not conclusively establish a single extraterrestrial impact as the ultimate cause of the Frasnian-Famennian mass extinction, especially given the presence of similar Ir enrichments elsewhere in the section and the absence at the boundary of microtektites and shocked mineral grains.
Life-death (LD) studies of shelly macrofauna are important to evaluate how well a fossil assemblage can reflect the original living community, but can also serve as a proxy for recent ecological shifts in marine habitats and in practice this has to be distinguished using taphonomic preservation pattern and estimates of time-averaging. It remains to be rigorously evaluated, however, how to distinguish between sources of LD disagreement. In addition, death assemblages (DAs) also preserve important information on regional diversity which is not available from single censuses of the life assemblages (LAs). The northern Adriatic Sea is an ecosystem under anthropogenic pressure, and we studied the distribution and abundance of living and dead bivalve and gastropod species in the physically stressful environments (tidal flat and shallow sublittoral soft bottoms) associated with the delta of the Isonzo River (Gulf of Trieste). Specifically we evaluated the fidelity of richness, evenness, abundance, habitat discrimination and beta diversity. A total of 10,740 molluscs from fifteen tidal flat and fourteen sublittoral sites were analyzed for species composition and distribution of living and dead molluscs. Of 78 recorded species, only eleven were numerically abundant. There were many more dead than living individuals and rarefied species richness in the DA was higher at all spatial scales, but the differences are lower in habitats and in the region than at individual stations. Evenness was always higher in death assemblages, and probably due to temporally more variable LAs the differences are stronger in the sublittoral habitats. Distinct assemblages characterized intertidal and sublittoral habitats, and the distribution and abundance of empty shells generally corresponded to that of the living species. Death assemblages have lower beta diversity than life assemblages, but empty shells capture compositional differences between habitats to a higher degree than living shells. More samples would be necessary to account for the diversity of living molluscs in the study area, which is, however, well recorded in the death assemblages. There is no indication of a major environmental change over the last decades in this area, but due to the long history of anthropogenic pressure here, such a potential impact might be preserved in historical layers of the deeper sedimentary record.
A detailed ultra-high-resolution analysis of a 37-cm-long core of Upper Miocene lake sediments of the long-lived Lake Pannon has been performed. Despite a general stable climate at c. 11-9 Ma, several high-frequency oscillations of the paleoenvironments and depositional environments are revealed by the analysis over a short time span of less than 1000 years. Shifts of the lake level, associated with one major 3rd order flooding are reflected by all organisms by a cascade of environmental changes on a decadal scale. Within a few decades, the pollen record documents shifting vegetation zones due to the landward migration of the coast; the dinoflagellate assemblages switch towards "offshore-type" due to the increasing distance to the shore; the benthos is affected by low oxygen conditions due to the deepening. This general trend is interrupted by smaller scale cycles, which lack this tight interconnection. Especially, the pollen data document a clear cyclicity that is expressed by iterative low pollen concentration events. These "negative" cycles are partly reflected by dinoflagellate blooms suggesting a common trigger-mechanism and a connection between terrestrial environments and surface waters of Lake Pannon. The benthic fauna of the core, however, does not reflect these surface water cycles. This forcing mechanism is not understood yet but periodic climatic fluctuations are favoured as hypothesis instead of further lake level changes. Short phases of low precipitation, reducing pollen production and suppressing effective transport by local streams, might be a plausible mechanism. This study is the first hint towards solar activity related high-frequency climate changes during the Vallesian (Late Miocene) around Lake Pannon and should encourage further ultra-high-resolution analyses in the area.
A high-resolution multi-proxy analysis was conducted on a 1.5-m-long core of Tortonian age (~ 10.5 Ma; Late Miocene) from Austria (Europe). The lake sediments were studied with a 1-cm resolution to detect all small-scale variations based on palynomorphs (pollen and dinoflagellate cysts), ostracod abundance, geochemistry (carbon and sulfur) and geophysics (magnetic susceptibility and natural gamma radiation). Based on an already established age model for a longer interval of the same core, this sequence can be limited to approx. two millennia of Late Miocene time with a resolution of ~ 13.7 years per sample. The previous study documented the presence of solar forcing, which was verified within various proxies on this 1.5-m core by a combination of REDFIT spectra and Gaussian filters. Significant repetitive signals ranged in two discrete intervals corresponding roughly to 55–82 and 110–123 years, fitting well within the lower and upper Gleissberg cycle ranges.
In order to detect whether the end-Frasnian worldwide biotic crisis is related to an extraterrestrial impact, the global stratotype section of the Frasnian-Famennian boundary and auxiliary sections within the stratotype area have been examined for impact indicators: iridium. Ni-rich spinel bearing spherules and glassy microtektites. This area is particularly well suited to the search for discrete events because it exhibits biostratigraphically continuous sections of sedimentologically homogenous off-shore deposits. Different environmental settings on oxygenated deep-water seamounts, such as the stratotype section at Coumiac, and in oxygen-depleted depressions (La Serre section) are available. The latter is investigated in more detail because it is the least condensed across the boundary, which is determined by the first occurrence of the typical morphotype of Palmatolepis triangularis, the indicator of the first Famennian conodont biozone. Samples from the biostratigraphically defined boundary and adjacent levels failed to provide significantly high Ir values and no Ni-rich spinel or microtektite has been recovered. This is in contradiction with the results of earlier investigations carried out by H. Geldsetzer on the same section. In contrast, the values of Ir concentrations that we measured are always very low or not detectable. The small overabundances observed in some samples, which are about two orders of magnitude lower than what is currently observed at the Cretaceous-Tertiary boundary, are probably due to the accumulation of the normal flux of cosmic dust during periods of relatively low depositional rates or to a terrestrial origin. At present, we have no evidence that an extraterrestrial impact occurred at the F-F transition.
Pollen analyses have been proven to possess the possibility to decipher rapid vegetational and climate shifts in Neogene sedimentary records. Herein, a c. 21-kyr-long transgression-regression cycle from the Lower Austrian locality Stetten is analysed in detail to evaluate climatic benchmarks for the early phase of the Middle Miocene Climate Optimum and to estimate the pace of environmental change.Based on the Coexistence Approach, a very clear signal of seasonality can be reconstructed. A warm and wet summer season with c. 204-236 mm precipitation during the wettest month was opposed by a rather dry winter season with precipitation of c. 9-24 mm during the driest month. The mean annual temperature ranged between 15.7 and 20.8 °C, with about 9.6-13.3 °C during the cold season and 24.7-27.9 °C during the warmest month. In contrast, today's climate of this area, with an annual temperature of 9.8 °C and 660 mm rainfall, is characterized by the winter season (mean temperature: -1.4 °C, mean precipitation: 39 mm) and a summer mean temperature of 19.9 °C (mean precipitation: 84 mm).Different modes of environmental shifts shaped the composition of the vegetation. Within few millennia, marshes and salt marshes with abundant Cyperaceae rapidly graded into Taxodiaceae swamps. This quick but gradual process was interrupted by swift marine ingressions which took place on a decadal to centennial scale. The transgression is accompanied by blooms of dinoflagellates and of the green alga Prasinophyta and an increase in Abies and Picea. Afterwards, the retreat of the sea and the progradation of estuarine and wetland settings were a gradual progress again.Despite a clear sedimentological cyclicity, which is related to the 21-kyr precessional forcing, the climate data show little variation. This missing pattern might be due to the buffering of the precessional-related climate signal by the subtropical vegetation. Another explanation could be the method-inherent broad range of climate-parameter estimates that could cover small scale climatic changes.
An aperiodic collision of the Earth with extra-terrestria] ice/volatile bodies is proposed as a mechanism to produce rapid changes in the geologic record. Due to the volatile nature of these bodies, evidence for their impacts, particularly in the ocean might be subtle and best seen as 'spikes' in the geochemical or fossil record against normal background. Differing effects would result depending on the site of the major break-up of the object: in the atmosphere, on land, or in the ocean. This paper focuses on the effects of adding material to the seas, oceans, and atmosphere. The treatment is largely qualitative, however mass balance calculations were used to estimate the relative mass needed to affect changes in a variety of reservoirs. Although actual impactors probably have a variable composition, the effects of water-, C-, N-, and S-containing objects are discussed. In the atmosphere, effects could include increased rain acidity, increased levels of nutrients, and enhanced greenhouse warming/cooling. Oceanic effects might include increased oceanic productivity (nitrogen-containing objects). As a result of increased chemical weathering and/or greenhouse effects, increased temperatures coupled with enhanced productivity could result in wider-spread oceanic anoxia or altered calcite/aragonite stability. Possible examples of such impacts from the geologic record and potential biotic effects are given.
Growth increment analysis coupled with stable isotopic data (δ18O/δ13C) from Early Pliocene (ca 4.7 Ma) Austrochlamys anderssoni from shallow marine sediments of the Cockburn Island Formation, northern Antarctic Peninsula, suggest these bivalves grew through much of the year, even during the coldest parts of winter recorded in the shells. The high frequency fluctuation in growth increment width of A. anderssoni appears to reflect periodic, but year-round, agitation of the water column enhancing benthic food supply from organic detritus. This suggests that Austrochlamys favoured waters that were largely sea ice free. Our data support interpretation of the Cockburn Island Formation as an interglacial marine deposit and the previous hypothesis that Austrochlamys retreated from the Antarctic as sea ice extent expanded, this transition occurring during climate cooling in the Late Pliocene
The Early–Middle Pleistocene lacustrine sediments at Gesher Benot Ya'aqov (GBY) in the Upper Jordan Valley span ca. 100 Ka across the Matuyama–Brunhes Boundary (MBB) at 780 Ka and oxygen isotope stages (OISs) 20 to 18, provide a continuous record of their sedimentary history and prevailing climate. The lacustrine environment is investigated using ecological and stable isotope characteristics of gastropods belonging to the family Viviparidae supplemented by other molluscan and floral indicators. The sedimentary framework of the 34 m thick sequence is bounded by channel conglomerates at its base and top, and consists of six sedimentary cycles. Each one starts with shore sediments (coquina and sand) and ends with deeper water sediments (black mud or gray mud).
The stratigraphic distribution of warm-water molluscs, planktonic foraminifers and diatoms in Plio-Pleistocene strata (3.5–0.8 Ma) of the Sea of Japan coast of Japan is the basis for reconstructing the history of the Tsushima Current. The results indicate: (1) the Tsushima Current flowed at 3.5, 3.2, 2.9, 2.4 and 1.9 Ma, in the 3.5–1.7 Ma interval; (2) the current flowed at every interglacial highstand, except for MIS 25, 23 and 21.3, in the 1.71–0.8 Ma interval; (3) the salinity and volume of the Tsushima Current during the earlier interval may have been significantly lower and smaller, respectively, than in the latter interval; (4) the Late Pliocene altitude of the southern part of the Sea of Japan was up to 50 m above present-day sea level.
Tropical climate is variable on astronomical time scale, driving changes in surface and deep-sea fauna during the Pliocene–Pleistocene. To understand these changes in the tropical Indian Ocean over the past 2.36 Myr, we quantitatively analyzed deep-sea benthic foraminifera and selected planktic foraminifera from > 125 μm size fraction from Deep Sea Drilling Project Site 219. The data from Site 219 was combined with published foraminiferal and isotope data from Site 214, eastern Indian Ocean to determine the nature of changes. Factor and cluster analyses of the 28 highest-ranked species distinguished four biofacies, characterizing distinct deep-sea environmental settings. These biofacies have been named after their most dominant species such as Stilostomella lepidula–Pleurostomella alternans (Sl–Pa), Nuttallides umbonifer–Globocassidulina subglobosa (Nu–Gs), Oridorsalis umbonatus–Gavelinopsis lobatulus (Ou–Gl) and Epistominella exigua–Uvigerina hispido-costata (Ee–Uh) biofacies. Biofacies Sl–Pa ranges from ~ 2.36 to 0.55 Myr, biofacies Nu–Gs ranges from ~ 1.9 to 0.65 Myr, biofacies Ou–Gl ranges from ~ 1 to 0.35 Myr and biofacies Ee–Uh ranges from 1.1 to 0.25 Myr. The proxy record indicates fluctuating tropical environmental conditions such as oxygenation, surface productivity and organic food supply. These changes appear to have been driven by changes in monsoonal wind intensity related to glacial–interglacial cycles. A shift at ~ 1.2–0.9 Myr is observed in both the faunal and isotope records at Site 219, indicating a major increase in monsoon-induced productivity. This coincides with increased amplitude of glacial cycles, which appear to have influenced low latitude monsoonal climate as well as deep-sea conditions in the tropical Indian Ocean.
Biogenic records of the marine palaeoproductivity (carbonates, organic carbon, and C37 alkenones) and the molecular stratigraphy of past sea surface temperatures (SSTs; U37K′) were studied at high resolution in two cores of the Iberian Margin. The comparison of these records indicates that the oceanographic conditions switched abruptly during the past 160 kyr between three kinds of regimes. A first regime with high (17–22°C) SST and low productivity typifies the interglacial periods, marine isotopic stages (MIS) 5 and 1. Several periods during MIS 6, 2, and the terminations II and I are characterised by about 4–5°C colder SST and a higher organic matter accumulation, both of which define the second regime. This anticorrelation between SST and marine productivity suggests that these variations are related to the intensity of the coastal upwelling. By contrast with this upwelling behaviour, extremely low biological productivity and very cold SST (6–12°C) occurred during short phases of glacial MIS 6, 4, and 2, and as abrupt events (≈1 kyr or less) during MIS 3. The three oceanographic regimes are consistent with micropalaeontological changes in the same cores based on foraminifera and diatoms.The general trend of these hydrologic changes follows the long-term glacial/interglacial cycle, but the millennium scale variability is clearly related to Heinrich events and Dansgaard–Oeschger cycles. Strengthening of the upwelling corresponds probably to an intensification of the subtropical atmospheric circulation over the North Atlantic which was influenced by the presence of continental ice sheets. However, extreme glacial conditions due to massive discharges of icebergs interrupted the upwelling. Interestingly, both terminations II and I coincided with strong but transient intensification of the upwelling.
Detailed records of the carbon and oxygen isotopic ratios of Neogloboquadrina pachyderma are compared between nine high-latitude sediment cores, from the Northern and Southern Hemispheres, covering the last 140 000 yrs. The strong analogies between the δ 13C records permit to define a δ 13C stratigraphic scale, with three clear cut transitions simultaneous with the oxygen isotopic transitions (125 kyrs.), (65 kyrs.), and (13 kyrs.). The δ 13C records of N. pachyderma in the high-latitude cores, which follow the changes in δ 13C of the surface water TCO2 near areas of deep water formation present trends similar to the benthic foraminifera δ 13C records in cores V19–30 and M12–392, although amplitudes of the isotopic shifts are different. This implies that a large part of the observed variations represents global changes in the carbon distribution between biosphere and ocean.The ratios of N. pachyderma in the North Atlantic cores display larger regional variations at 18 kyrs. B.P. than at present. To explain these differences, we have plotted the 18 kyrs. B.P. δ 13C values of N. pachyderma from 17 cores distributed N of 40°N. Comparison with published surface water temperature distribution at 18 kyrs. B.P. indicates that a strong divergent cyclonic cell, centered approximatively 55°N and 15°W, was active during most of the last ice-age maximum. This hydrology, analogous to the present Weddell Sea, explains the published evidences of bottom water formation, if located on the northern flank of the gyre, and the strong polar front on the southern flank, probable location of intermediate water formation.
Analyses of sedimentological, geochemical and micropaleontological parameters from radiocarbon-dated sediment cores retrieved from the Antarctic Peninsula’s western continental shelf reveal a detailed paleoclimatic and/or paleoceanographic history over the last 15 000 radiocarbon years. Deglaciation of the outer shelf off Anvers Island commenced prior to at least 15 000 yr BP, marked by the deposition of distal glaciomarine diamicton (facies 2) beneath a floating ice shelf, and lasted for 3800 years with increasing diatom abundance and total organic carbon (TOC) over time. A return to colder conditions occurred between 12 800 and 11 600 yr BP with a drop in TOC content and diatom abundance, which is coincident with the Younger Dryas event in the North Atlantic region. At this time, an abrupt increase in percentage sea-ice taxa as well as in the ratio of (Fragilariopsis curta+Fragilariopsis cylindrus)/Thalassiosira antarctica suggests renewed ice-shelf advance. In contrast, the inner shelf was deglaciated somewhat later about 11 000 yr BP, that is, 3000 years after the outer shelf. Prior to 11 000 yr BP, deposition of proximal glaciomarine diamicton (facies 1) close to the grounding line under a floating ice shelf and/or persistent sea ice may have occurred on the inner shelf. After this date, deposition of distal glaciomarine diamicton (facies 2) followed. A climatic optimum is recognized between 6000 and 2500 yr BP, coinciding with a ‘mid-Holocene climatic optimum’ from several other Antarctic sites, e.g. the Palmer Deep. During this time, as the glacial system receded from the shelf, greatly enhanced primary productivity occurred in open marine conditions, resulting in the deposition of diatomaceous mud (facies 3) and causing post-depositional dissolution of calcareous benthic and planktonic foraminifers in sediment. Around 2500 yr BP (the onset of the Neoglacial), diatomaceous sandy mud (facies 4), characterized by a decrease in TOC and diatom abundance, reflects the formation of more extensive and seasonally persistent sea ice, as evidenced by an increase in percentage of sea-ice taxa and in the ratio of (F. curta+F. cylindrus)/T. antarctica. Our results provide evidence of climatic change on the Antarctic Peninsula’s western shelf that helps to refine the existence and timing of late Pleistocene and Holocene millennial-scale climatic events in the Southern Hemisphere.
High-resolution records of planktic foraminifer sea-surface temperature (SST) and biogenic sediment components of carbonate and total organic carbon (TOC) concentrations were determined in an IMAGES giant piston core spanning ∼the last 500 000 years, taken near the western slope of Palawan Island in the southeastern South China Sea (SCS). The records provide information of paleoceanographic and paleoclimatological variations linked to East Asian monsoon systems in the SCS, the largest marginal sea of the western Pacific. Constrained by planktic foraminifer (Globigerinoides ruber) oxygen isotope stratigraphies, the records show a lowering of faunal SST by ∼3°C during glacial stages, indicating significant cooling in the glacial western Pacific climate. In general, they show low-frequency patterns with high SSTs, high carbonate content, and low TOC content during interglacial periods, and exhibit low SSTs, low carbonate content, and high TOC content during glacial periods. The carbonate content variations indicate that the sediment composition is mostly controlled by terrigenous inputs, which are associated with sea-level fluctuations in the SCS during past glacial–interglacial stages. The low SST and high TOC content indicate cooling and high productivity conditions in the surface oceans of the SCS, which also reflect a condition of intensified winter monsoon winds associated with glacial boundary conditions. Some rapid, high-frequency oscillations of the SST and TOC found in the records are coincident with intervals of intensified winter or summer monsoons from the Arabian Sea, implying that the Asian monsoon systems had wider regional effects than previously assumed. Time-series analyses reveal that variations in the SST, carbonate and TOC contents of this record contain statistically significant concentrations of variance at orbital frequency bands, namely 100 kyr−1, 41 kyr−1, and 23 kyr−1, suggesting that both ice volume and orbital solar insolation changes are potential mechanisms for the SCS monsoon variations.
Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250–320 g C m−2 year−1) than during cold stages (210–270 g C m−2 year−1) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed 250 g C m−2 year−1, and during glacials to be as low as 140–180 g C m−2 year−1. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C m−2 year−1 is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.
Three Mediterranean deep sea cores have been studied to reconstruct the hydrological conditions at the time of deposition of interglacial and glacial sapropels during the time interval 200–60 kyr BP. The isotopic and sea surface temperature records of planktonic foraminifera were used to estimate the oxygen isotopic composition of surface water (δw) in the Tyrrhenian Sea and Levantine basin. Our results show a strong δw/salinity decrease in both basins associated with each sapropel. These δw decreases reflect a drastic change in the local freshwater budget and were comparable to that observed during the last sapropel (S1) centred at ca. 8 kyr BP. A strong precipitation increase transformed the whole Mediterranean Sea into a non-concentration basin. The low surface salinity was responsible for the water column stratification, while nutrients brought in by the runoff enhanced productivity. It resulted in a significant reduction in the rate of deep water oxygenation and sapropel formation in the eastern basin. In addition, the Mediterranean Sea δ18O record is very similar to that of the Bay of Bengal. As the latter is very sensitive to the summer monsoon rain, this similarity suggests that both glacial and interglacial sapropels result from enhanced monsoon-like precipitation due to an increase in summer insolation driven by precession variations.
A high-resolution age–depth profile is presented for a 16-m deep-sea piston core (EW9709-PC07) using three different methods: magnetostratigraphy, fish-teeth strontium isotope stratigraphy, and radiolarian biostratigraphy. Fish teeth are abundant throughout the core, allowing for precise age determinations by Sr isotope stratigraphy. Magnetostratigraphic ages, though not available for this core, were determined by correlation with the drill core record from adjacent ODP Site 1218. Biostratigraphic ages were independently assigned to the lower 12 m of the core, which contains abundant radiolaria. All three methods define an early Miocene age (∼20 Ma) for the core base. A linear sedimentation rate of ∼2.0 mm/ky was calculated for the lower 10 m of the core, which is dominated by siliceous clays and calcareous ooze. All three methods yield concordant ages over this interval (∼20 to 15 Ma). Tectonic migration of the PC-07 site away from the equatorial high productivity zone produced a significant decrease in sedimentation rates after 15 Ma, diminishing to just ∼0.30 mm/ky in the uppermost 3 m of the core. Correlated magnetic reversal and fish teeth ages are concordant within this upper red clay interval (∼10 to 0.0 Ma), which is dominated by eolian dust accumulation; however, within the 15 to 10 Ma interval, fish teeth ages appear to show more scatter, departing from the magnetic ages by as much as 2–3 million years. Age discrepancies in this dominantly siliceous clay interval are most likely due to uncertainties in magnetostratigraphic age correlations. We conclude from this that the eolian dust component in red clay cores can be reliably dated by the fish teeth strontium technique. For otherwise undatable red clay cores from the vast northern Pacific pelagic clay province, this may prove to be the only available method for developing a regional Cenozoic chrono-stratigraphy.
We present measurements of the maximum diameter of the planktonic foraminifer Neogloboquadrina pachyderma sin. from six sediment cores (Ocean Drilling Program sites 643, 644, 907, 909, 985 and 987) from the Norwegian–Greenland Sea. Our data show a distinct net increase in mean shell size of N. pachyderma sin. at all sites during the last 1.3 Ma, with largest shell sizes reached after 0.4 Ma. External factors such as glacial–interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of N. pachyderma sin. We consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of N. pachyderma sin. to the cold water environment after 1.1–1.0 Ma. Probably, the Mid Pleistocene climate shift and the associated change of amplitude and frequency of glacial–interglacial fluctuations have triggered the evolution of this planktonic foraminifer. Oxygen and carbon stable isotope analyses of different shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis. For paleoceanographic reconstructions, the evolutionary adaptation of Neogloboquadrina pachyderma sin. to the cold water habitat has significant implications. Carbonate sedimentation in highest latitudes is highly dependent on the presence of this species. In the Norwegian–Greenland Sea, carbonate-poor intervals before 1.1 Ma are, therefore, not necessarily related to severe glacial conditions. They are probably attributed to the absence of this not yet polar-adapted species. Further, transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could, therefore, contain a large range of errors if they were applied to samples older than 1.1–1.0 Myrs.
An interdisciplinary study of the geologic and archaeological records of a 50 ky year period at the Plio-Pleistocene boundary in Olduvai Basin (Tanzania) revealed that a small groundwater-fed wetland was a consistent focus of hominin activity during periods of climate change (wet/dry cycles). Paleoenvironmental analysis of sedimentary records from 33 excavations in Lowermost Bed II (LMB II), Olduvai Gorge (3° S) revealed evidence of significant climate fluctuations that produced two and one half lake cycles between ~ 1.79 and 1.74 Ma. The region experienced wet-dry cycles in response to combined astronomical forcing effects of eccentricity (~ 100 ky cycle) and precession of the equinox (19 and 23 ky cycles). Rhythmic variability of the monsoon system produced wet periods followed by a transition into dry periods followed by a transition back into wet climate. A high-resolution study of the stratigraphy documented a groundwater-fed wetland complex (~ 1 km2) that flourished during low lake levels (dry periods) and was drowned during high lake levels (wet periods). Stratigraphic analysis (temporal changes) of 916 artifacts from 13 excavations selected from a landscape transect across alluvial fan, spring/wetland and lake margin environments showed no notable differences in artifact type frequency and raw material type when comparing wet periods (as a group) and dry periods (as a group). However, a lateral shift of hominin activity on the landscape did occur between wet and dry periods. The Oldowan tool kit did not vary at spring sites between wet and dry periods, whereas the tool kit did vary at other lake margin sites between wet and dry periods. Both analyses suggest that climate affected the location of an activity, not type of activity. These data suggest that Oldowan hominins adapted their land-use patterns to exploit spring/wetland resources available during dry periods. A switch from wet to dry conditions likely altered surface and groundwater systems affecting food and potable water availability, forcing hominins to adjust their subsistence patterns.
Six lake sediment vibracores from the foothills and mountain valleys of southwestern Alberta were analysed on the basis of AMS radiocarbon dates, tephrochronology and sediment geochemistry. From these results, the magnitude, timing and duration of the early Holocene warm period are presented. At about 10,000 BP, immediately following the Younger Dryas cold period, climate warmed dramatically, precipitation decreased and surface evaporation increased. Previous research has identified this warm interval, but new results have improved resolution of regional scale effects, specific timing and severity. Sedimentation changed from extra-basinal clastic to intra-basinal organic between 10,000 and 9400 BP. Changes in subalpine lakes from sand/silt deposition to biogenic carbonate precipitation suggest decreases in suspended sediment load caused by complete ablation of glacial sediment sources. Peat which formed in lakes of less than 4 m (present depth) indicates climate-induced lake level lowering in the foothills. Water depth and stratigraphic position of the peat suggest that regional water table levels decreased by up to 6.5 m. At Cartwright Lake, an erosional unconformity 6.5 m below the modern lake surface indicates the lake had completely dried out either during or immediately after Mazama tephra time (6800 BP).
A suite of piston cores recovered from Lake Malawi (9–14°S, 34–35°E), east Africa in 1986 has been analyzed for major and minor elements, organic C and N, calcium carbonate and diatoms. An internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera, Stephanodiscus and Melosira, with age control obtained primarily from 14C dating of the carbonate. Differences with time in Fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth. The depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr B.P., indicating less seasonality. Carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column. The carbonate, which precipitates from surface waters, is most abundant during the interval from about 10,000 to 6000 yr B.P. This micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced. Sedimentary evidence suggests that lake levels were 100–150 m lower than present during this period. This record is different from climatic trends in northern intertropical Africa, but appears to also be related to changes in insolation and monsoon circulation. This is the northernmost basin in Africa reported to exhibit a “southern hemisphere” response to the early Holocene northern hemisphere summer insolation maximum. The climatic hingeline north of Lake Malawi (∼ 9°S) implied by our results is significantly south of that indicated by general-circulation model simulations, however. The cores show evidence for periods of abrupt climate change during the interval of generally arid climate.
A high-resolution, continuous 10,500 cal. yrs-long macroscopic charcoal record from a peat and lake sediment deposit at Storasjö, in the hemiboreal vegetation zone of southern Sweden, is presented. This record was compared with the microscopic charcoal record from the same core, and tentatively correlated with the macroscopic and microscopic charcoal records from another site (Stavsåkra), situated 30 km West of Storasjö. The charcoal records are also compared with regional climate proxy records with the aim to separate climate — from human-induced fire activity. The results suggest that the major signal of both microscopic and macroscopic charcoal records represents local fire history. The best record of local fire history was obtained from the continuous macroscopic charcoal analysis. A tentative correlation of the charcoal records between the sites indicates that most fire episodes of the early and middle Holocene are probably of regional character. Both sites exhibit three major phases of high fire activity 1) 8700–8300 BC, 2) 7250 BC to ca. 4000 BC, and 3) 750 BC to the 19th century. These three phases are separated by periods with lower or very low fire activity. This general trend is in good agreement with the pattern emerging for Europe from the analysis of the recently developed global charcoal database. Fire appears to have been controlled by climate during the early and middle Holocene and by humans during the late Holocene. Warmer and drier climate during the early and middle Holocene caused frequent and intensive fires, which suggests that natural fire activity might increase under predicted future climate scenarios. The results also suggest that fire was an important disturbance factor in the hemiboreal vegetation zone of Sweden and played an important role in the forest dynamics and characteristics of the flora and fauna of the region.
Glaciolacustrine rhythmites within sediment cores from Lake Superior record the regional recession of the Laurentide Ice Sheet (LIS) from 10,700 to 8900 cal ybp [ca. 9.5–8.0 14C ka]. LIS retreat from Superior opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge. Multiple rhythmite stratigraphies, a time series analysis of the thickness measurements, and high-resolution inorganic carbonate data demonstrate that this is an annual record (varved). The varve thickness records primarily document regional ice margin dynamics; correlative thick varve sequences at 9100 cal ybp [∼ 8.1 14C ka] and 10,400–10,200 cal ybp [∼ 9.2–9.0 14C ka] record two periods of enhanced glaciofluvial discharge, most likely moraine formation (the Nakina and Nipigon). General varve cessation is associated with the circumvention of Lake Agassiz and glacial meltwater into Lake Ojibway at 9040 cal ybp [∼ 8.1 14C ka], although adjacent to the inlets from Lake Nipigon, rhythmic sedimentation persisted for 200 years.Positively identifying Lake Agassiz catastrophic discharge events remains speculative but seems feasible. Following retreat of Marquette ice that had re-advanced to fill the basin, the initial influx of Lake Agassiz water is expected at around 10,600 cal ybp [∼ 9.4 14C ka], but at this time, most of northeastern Lake Superior was covered by ice. Three sets of thick–thin varves in western Lake Superior perhaps record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal ybp [∼ 9.4 14C ka]. Varve formation in Superior coincides with high lake levels in Lake Huron, suggesting that high lake levels in Huron correspond to periods of high Agassiz and/or meltwater flow into Lake Superior.
Tectonic influences on long-term climate change are of considerable current interest and debate. This paper reviews the relationship between multi-million year periods of glaciation (glacio-epochs) over the last 3 Ga of Earth history and phases of supercontinent breakup and assembly. A preferred but not exclusive relationship is evident between glacio-epochs and their mostly glacially influenced marine record, with rifting. The earliest known glaciation (mid Archean ∼ 2.9 Ga) is recorded in the marine Mozaan Group of South Africa deposited along the passive margin of the Kapvaal Craton then part of the early continent Ur. The Paleoproterozoic glacio-epoch, exemplified by the Huronian Supergroup of Ontario, Canada (∼ 2.4 Ga) and strata in northern Europe and the U.S., is associated with rifting of Kenorland. A long Paleo-Mesoproterozoic non-glacial interval (c. 2.3 Ga to 750 Ma?) coincides with continental collisions and high standing Himalayan-scale orogenic belts marking the suturing of supercontinents Nena-Columbia and Rodinia. A near absence of glacial deposits other than at 1.8 Ga, may reflect lack of preservation. The extensive and prolonged Neoproterozoic glacio-epoch records either diachronous glaciations or discrete pulses of cooling between ∼ 750 and ∼ 580 Ma, and is overwhelmingly recorded by substantial thicknesses (1 km+) of glacially influenced marine strata stored in rift basins. These formed on the mid to low latitude (< 30°) oceanic margins of western (Panthalassa: Australia, China, Western North America) and eastern (Iapetus: Northwest Europe) margins of a disintegrating Rodinia. The youngest glacially influenced deposits formed about 580 Ma along the compressional Cadomian Belt exterior to Rodinia (Gaskiers Formation) possibly correlative with the classic passive margin Marinoan deposits of South Australia.