1,721,173 research outputs found
Evidence for abrupt climate changes in annually laminated marine sediments
No full textAnnually laminated sediments from marine or lacustrine settings represent valuable high-resolution archives of climate change that record variation due to changing precipitation and run-off from land or variation in biological productivity and flux in the water column. Because of their annual resolution such sediments may capture abrupt changes of interannual to decadal scales rivaling corals and ice cores in resolution. Laminated sediments often occur intermittently in the sediment column, and the onset and cessation of laminae commonly record the abrupt crossing of thresholds related to climate change, for example, in the degree of oxygenation of bottom waters. Such records from marginal basins and continental margins have been pivotal in demonstrating that abrupt changes hitherto documented only in high-latitude ice cores are synchronous with climatic change at low latitudes. These insights into global teleconnections have improved our understanding of the mechanisms of rapid climate change. In deep-sea settings, the discovery of the episodic occurrence of laminated diatom-rich sediments in the Equatorial Pacific and Southern Ocean provides evidence for massive climate-related biogeochemical excursions tied to abrupt changes in the input, distribution and availability of nutrients in the oceans
Records of seasonal flux in Holocene laminated sediments, Gulf of California
No full textHolocene laminated sediments from Guaymas Basin (Gulf of California, Mexico) have, in the past, been described as annual couplets of sedimentation. Couplets comprise an alternation of diatom- and terrigenous-rich sediment reflecting winter and summer flux to the sediment respectively. New data from backscattered electron imagery (BSEI) show that a three-component pattern of sedimentation is preserved, comprising (1) a lithogenic lamina consisting of clays, silt and minor diatomaceous material, deposited during the summer and autumn wet season; (2) a mixed flora diatomaceous lamina deposited during the early winter collapse of the thermocline and onset of water column mixing; and (3) a near-monospecific flora lamina of Chaetoceros resting spores or, more rarely, Skeletonema costatum, representing deposition from, or at the end of, the spring coastal upwelling bloom. Recent sediment trap biogenic and terrigenous flux data also show three to four major flux events per year. Direct comparisons are possible between BSEI records and recent sediment trap flux data to facilitate high-resolution palaeoceanaographic and palaeoclimatic reconstructions throughout the Holocene
Preparation and analysis techniques for studies of laminated sediments
No full textLaminated sediments commonly represent the highest resolution continuous records available of marine and lacustrine variability. To exploit this information effectively careful sampling and preparation procedures must be followed, thus the method of analysis chosen must have sufficient resolution to recover the data. The scanning electron microscope, because of its good spatial resolution, is an ideal tool for analyzing laminated sediment fabric. Backscattered electron imagery produces preserved flux data on intra-annual time-scales, which enables high-resolution fabric studies, palaeoenvironmental reconstructions and compilation of data for time-series analysis. This contribution summarizes the techniques used for laminated sediment fabric investigation and discusses the use of the scanning electron microscope, including the best preparation method for thin sections and the procedure for backscattered electron imagery analysis
Letter. Late cretaceous seasonal ocean variability from the arctic
No full textThe modern Arctic Ocean is regarded as barometer of global change and amplifier of global warming1 and therefore records of past Arctic change are of a premium for palaeoclimate reconstruction. Little is known of the state of the Arctic Ocean in the greenhouse period of the late Cretaceous, yet records from such times may yield important clues to its future behaviour given current global warming trends. Here we present the first seasonally resolved sedimentary record from the Cretaceous from the Alpha Ridge of the Arctic Ocean. This “paleo-sediment trap” provides new insights into the workings of the Cretaceous marine biological carbon pump. Seasonal primary production was dominated by diatom algae but was not related to upwelling as previously hypothesised. Rather, production occurred within a stratified water column, involving specially adapted species in blooms resembling those of the modern North Pacific Subtropical Gyre, or those indicated for the Mediterranean sapropels. With increased CO2 levels and warming currently driving increased stratification in the global ocean, this style of production that is adapted to stratification may become more widespread. Our evidence for seasonal diatom production and flux testify to an ice-free summer, but thin accumulations of terrigenous sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter, supporting a wide body of evidence for low temperatures in the Late Cretaceous Arctic Ocean, rather than recent suggestions of a 15 °C mean annual temperature at this time
High diatom production and export in stratified waters – A potential negative feedback to global warming
No full textIt is widely held that increased stratification and reduced vertical mixing in the ocean driven by global warming will promote the replacement of diatoms by smaller phytoplankton and lead to an overall decrease in productivity and carbon export. Here we present contrary evidence from a synergy of modern observations and palaeo-records that reveal high diatom production and export from stratified waters. Diatom adaptations to stratified waters include the ability to grow in low light conditions in deep chlorophyll maxima; vertical migrations between nutricline depths and the surface, and symbioses with N2-fixing cyanobacteria in diatom–diazotroph associations (DDA). These strategies foster the maintenance of seed populations that may then exploit mixing events induced by storms or eddies, but may also inherently promote blooms. Recent oceanographic observations in the subtropical gyres, at increasingly high temporal and spatial resolutions, have monitored short-lived but often substantial blooms and export of stratified-adapted diatoms including rhizosolenids and the diazotroph-associated Hemiaulus hauckii. Aggregate formation by such diatoms is common and promotes rapid settling thereby minimizing water column remineralization and optimizing carbon flux. Convergence zones associated with oceanic fronts or mesoscale features may also generate substantial flux of stratified-adapted diatom species. Conventional oceanographic observing strategies and sampling techniques under-represent such activity due to the lack of adequate capability to sample the large sized diatoms and colonies involved, the subsurface location of many of these blooms, their common development in thin <3 m layers and their episodic nature. Superbly preserved “palaeo-sediment trap” records in laminated sediments reveal enhanced production and export from stratified waters mediated by the same taxa. The organic carbon-rich Mediterranean sapropels were the result of production within deep chlorophyll maxima in highly stratified waters dominated by rhizosolenid diatoms but also including H. hauckii. In the Cretaceous when CO2 levels were higher than present, laminated sediments reveal dominant flux also including Hemiaulus and rhizosolenid species. Nitrogen isotope and biomarker studies suggest that, analogous to modern DDA blooms within the subtropical gyres, Hemiaulus blooms in the ancient Mediterranean and within the Cretaceous seas were aided by nitrogen-fixing cyanobacterial symbionts. These lines of evidence suggest that diatom production and associated export of organic carbon, may not decrease, as is widely predicted, but may actually increase with greater ocean stratification, and potentially act as a negative feedback to global warming. However, the key genera involved in such potential feedbacks are underrepresented in both laboratory and field studies and are poorly represented in models. Our findings suggest that a reappraisal is necessary of the way diatoms are represented as plankton functional types (PFTs) in ocean biogeochemical models and that new observing and sampling strategies are also required to study these processes
Late Cretaceous seasonal palaeoclimatology and diatom palaeoecology from laminated sediments
No full textLaminated diatom-rich marine sediments from California and the Arctic Ocean provide a window into the seasonal climate and oceanography of the mid- and high-latitude Late Cretaceous. These remarkable shallow-buried sediments constitute palaeo-sediment traps that record exceptionally well-preserved sequential biogenic and lithogenic flux events. Many of the diatom laminae are composed of a few dominant taxa, and we use a species-based approach for palaeoecological interpretation. Contrary to many earlier interpretations, results indicate that both sites preserve a major flux of taxa adapted to exploit a strongly stratified ocean. The uppermost Maastrichtian Marca Shale of California records a seasonal cycle initiated with a spring bloom flux of diatom resting spores followed by a summer lithogenic sediment input likely driven by monsoonal storms providing river runoff and aeolian input. This is followed by a dominant diatom flux of species that thrived in the summer stratification in deep chlorophyll maxima and were sedimented in the fall when this stratification broke down. Dominant taxa comprising this group include Hemiaulus, Stephanopyxis, Stellarima and Rhizosolenia. The uppermost Campanian CESAR 6 core from the Arctic Alpha Ridge, records a spring bloom flux of resting spores followed by a more dominant summer and fall flux of diatom vegetative cells which were likely concentrated in subsurface summer blooms that generated a “summer export pulse” and by the breakdown of stratification in the fall that gave massive flux of deep chlorophyll maxima species in the fall dump. The dominance and diversity of Hemiaulus in the CESAR 6 core together with widespread evidence of N2-fixation may indicate that some diatom blooms were powered by intracellular N2-fixing cyanobacteria as in the modern oligotrophic ocean. Thin lenses of fine lithogenic sediment that occur mostly in the spring layer represent rafting by winter sea ice and support other evidence that suggests intermittent winter freezing in the Late Cretaceous Arctic
Microfabric analysis of Mn-carbonate laminae deposition and Mn-sulfide formation in the Gotland Deep, Baltic Sea
No full textThe manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy-based techniques allows identification of small-scale sedimentary and geochemical features associated with Mn-carbonate laminae, which has significant implications for models of Mn-carbonate formation. Varves occurring in the Littorina sequence contain up to four laminae that may be placed in a seasonal cycle, and kutnahorite laminae occur within varves only as a winter-early spring deposit. This kutnahorite laminae seasonality is in agreement with the seasonal distribution of major Baltic inflow events recorded in historical records, and a direct causal link between inflows and kutnahorite deposition is implied. Benthic foraminifera tests are found to be heavily encrusted in kutnahorite, implying that benthic recolonization during oxidation events occurs concurrently with kutnahorite formation. The relatively common occurrence of small (50 to 100 ?m) hexagonal ?-Mn-sulfide pseudomorphs, associated with 13% of kutnahorite laminae studied, is reported in Gotland Deep sediments for the first time. Although Mn-sulfide crystals are not usually preserved in the sediment, the discovery of Mn-sulfide pseudomorphs suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied
The case of the diatoms and the muddled mandalas: Time to recognize diatom adaptations to stratified waters
No full textModels used to predict future ocean ecosystem and biogeochemical behaviour depend on simplified ecological frameworks allowing the definition of plankton functional types. Foremost among such frameworks has been the mandala of Margalef. His 1978 paper has been increasingly referred to in the past decade as simplified ecological schemes have been sought to help predict the effect of climate change on phytoplankton. However, the mandala is based on an understanding of the subject that is over 40 years old, when observational studies were largely limited to the coastal ocean and to near surface waters. Furthermore, most recent reproductions of the mandala are significant oversimplifications of Margalef’s original. In these simplified mandala-type constructs, diatoms, in particular, have commonly been cast as a single plankton functional type that thrive in turbulent waters and decrease in abundance with increasing stratification. On this basis, it is widely predicted that diatom productivity and hence the effectiveness of the marine biological carbon pump will decrease with climate change that is driving increased stratification of the oceans. But Margalef’s original took a more refined approach and depicted diatom genera that were adapted to more stratified conditions such as those characteristic of the subtropical oligotrophic gyres. If we now draw on the vast advances in observational oceanography of recent decades it is evident that diatoms may thrive, bloom and generate significant export even in the most intensely stratified and apparently oligotrophic conditions. Indeed, some diatom species have unique adaptations to such environments. We therefore suggest that it is time to abandon oversimplified schemes and recognize the diverse ecology of diatoms
Massive Gulf of California diatom blooms mark silica boost from the Southern Ocean and intensification of El Niño and the North American monsoon in the transition to Greenland interstadial 12
No full textLaminated sediments provide a rare opportunity to examine seasonal-scale ocean/atmosphere variation during abrupt climate transitions. Seasonal-scale changes through the transition from Heinrich Stadial 5 to Greenland Interstadial 12 are recorded in a sediment core (MD02-2515) from the Guaymas Basin, Gulf of California. The laminated sediments of the full interstadial have the highest opal concentrations (59.6 wt % biogenic silica) of the entire 55 kyr sequence and contain monospecific concentrations of the tropical oceanic diatom Azpeitia nodulifera that record flux from exceptional bloom events. Enhanced silica supply was likely initiated from the Southern Ocean during Heinrich Stadial 5, when Southern Hemisphere warming, via the bipolar seesaw mechanism, led to increased upwelling around Antarctica and an increase in silica supply to Subantarctic Mode Water formation. The resulting pulse of silica-rich waters was rapidly transferred north to the Equatorial Undercurrent and thence via the subsurface Mexican Coastal Current to the Gulf of California. Overall, the sequence shows a resurgence in El Niño strength and an intensification of the North American Monsoon associated with Northern Hemisphere warming on the transition to Greenland Interstadial 12. Penetration of tropical waters to the Gulf was aided by El Niño events. Diatom production in the summer stratified waters was in the subsurface, tapping nutrients from the nutricline. Marine varves up to 9 mm thick record as many as 8 within-year flux events. Repeated summer flux events of actively reproducing diatoms in subsurface blooms were driven by recurrent intense monsoonal Gulf surge storms. Around 60% of A. nodulifera cells examined were actively dividing and every stage of the cell division cycle is represented. Other diatom species record renewed vigour of winter – spring upwelling associated with strengthening northwesterly winds as the North Pacific high migrated northward. The monsoon intensification, the resurgence in El Niño strength and the strengthened winter-spring northwesterlies were likely all driven by the reduction to a minimum extent of the Laurentide Ice Sheet in Greenland Interstadial 12. The changes recorded in this transition to a warm interstadial may serve as an indicator of future changes in the region driven by global warming
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