Fossil Diatom Assemblages Research Articles

Modern and Fossil Diatom Assemblages from Bol'shoy Lyakhovsky Island (New Siberian Archipelago, Arctic Siberia)

Modern and Fossil Diatom Assemblages from Bol'shoy Lyakhovsky Island (New Siberian Archipelago, Arctic Siberia)

Diatom (Bacillariophyceae) assemblages in salt marshes of south‐central Chile: Relations with tidal inundation time and salinity

SUMMARYTo reconstruct sea‐level history from changes in tidal environments using diatom assemblages, we need to better understand the relations among brackish diatom assemblages and changing environments along elevational gradients from diverse coastal sites. Our statistical analysis reveals relations between environmental variables and brackish benthic diatom assemblages in the little studied region of south‐central Chile. Along four transects across salt marshes at two sites, we identified 224 diatom taxa in 112 samples. Detrended canonical correspondence analysis showed that tidal exposure time index and salinity were appropriately regressed against the abundance of diatom species using unimodal‐based methods. Our tests of classical and inverse regressions of weighted average and weighted averaging partial least squares (WA‐PLS) showed that WA‐PLS resulted in the highest coefficient of determination and the lowest root‐mean square of the error of prediction. Our regression will be useful in reconstructing environmental variables from fossil diatom assemblages in Chile.

Reconstructing a long-term record of microcystins from the analysis of lake sediments.

Based on an analysis of sediment cores from Baptiste Lake (Alberta, Canada), we quantified century-scale trends in cyanobacteria and cyanotoxins, and identified possible drivers of toxigenic cyanobacteria. We measured concentrations of microcystins and pigments preserved in the sediment as proxies of toxigenic cyanobacteria and phytoplankton communities, respectively, while fossil diatom assemblages were used to infer past nutrient concentrations. Microcystins were detected in older sediments (ca. 1800s), pre-dating any significant alteration to the watershed. This demonstrates that toxigenic cyanobacteria may not be a recent phenomenon in eutrophic ecosystems. The dominant variants of microcystin throughout the sediment core were microcystin-LA and microcystin-LR. Other congeners including -LY, -7dmLR, -WR, -LF, -YR, and -LW (-RR was not detected) were mainly found in the upper layers of sediment (post 1980s). Starting in the 1990s, concentrations of microcystins both in the water column and in the sediment record increased in parallel. Total sediment microcystins were strongly correlated with historical nitrogen and phosphorus concentrations inferred from diatom assemblages (r=0.80-0.81, p<0.001, n=22); both nutrients increased over the past two decades coincident with the intensification of agriculture. Microcystins also tracked the rise in cyanobacterial pigments present throughout the core. In contrast, we found no relationship between climate-related variables and sediment microcystin concentrations, although such relationships were detected over the monitoring record with respect to water column concentrations. Overall, the rise in sediment microcystins was much greater than the rise in sediment cyanobacteria and diatom inferred nutrient concentrations. Furthermore, we demonstrate that the reconstruction of the microcystin sediment record can provide important insight for the development of realistic lake management goals. Applying this analytical approach to different lakes and regions of the world, where both natural and anthropogenic gradients vary, has the potential to markedly improve our understanding of long-term drivers of cyanotoxin production.

Stratigraphic and Paleoenvironmental Reconstruction of a Mid-Pliocene Fossil Site in the High Arctic (Ellesmere Island, Nunavut): Evidence of an Ancient Peatland with Beaver Activity + Online Appendix Figures S1 and S2 (See Article Tools)

Neogene terrestrial deposits of sand and gravel with preserved wood and peat accumulations occur in many areas of the High Arctic. The Pliocene-aged Beaver Pond fossil site (Ellesmere Island, NU) is one such site that differs from other sites in the great thickness of its peat layer and the presence of a rich vertebrate faunal assemblage, along with numerous beaver-cut sticks. Although the site has been the subject of intense paleontological investigations for over two decades, there has not been a reconstruction of its depositional history. In this study, measured sections within and surrounding the site established the stratigraphy and lateral continuity of the stratigraphic units. Grain size analysis, loss on ignition, and fossil diatom assemblages were examined to reconstruct paleoenvironmental changes in the sequence. The base of the section was interpreted as a floodplain system. Using modern peat accumulation rates, the maximum thickness (240 cm) of the overlying peat layer is estimated to represent 49 000 ± 12 000 years. From this evidence, we suggest that during the peat formation interval, beaver activity may have played a role in creating an open water environment. The peat unit was overlain by sand, rich in organic matter and charcoal, suggesting environmental change and fire occurrence.

Direct versus indirect climate controls on Holocene diatom assemblages in a sub-tropical deep, alpine lake (Lugu Hu, Yunnan, SW China)

The reconstruction of Holocene environmental changes in lakes on the plateau region of southwest China provides an understanding of how these ecosystems may respond to climate change. Fossil diatom assemblages were investigated from an 11,000-year lake sediment core from a deep, alpine lake (Lugu Hu) in southwest China, an area strongly influenced by the southwest (or the Indian) summer monsoon. Changes in diatom assemblage composition, notably the abundance of the two dominant planktonic species, Cyclotella rhomboideo-elliptica and Cyclostephanos dubius, reflect the effects of climate variability on nutrient dynamics, mediated via thermal stratification (internal nutrient cycling) and catchment-vegetation processes. Statistical analyses of the climate–diatom interactions highlight the strong effect of changing orbitally-induced solar radiation during the Holocene, presumably via its effect on the lake's thermal budget. In a partial redundancy analysis, climate (solar insolation) and proxies reflecting catchment process (pollen percentages, C/N ratio) were the most important drivers of diatom ecological change, showing the strong effects of climate–catchment–vegetation interactions on lake functioning. This diatom record reflects long-term ontogeny of the lake-catchment ecosystem and suggests that climatic changes (both temperature and precipitation) impact lake ecology indirectly through shifts in thermal stratification and catchment nutrient exports.

A late Miocene methane-seep deposit bearing methane-trapping silica minerals at Joetsu, central Japan

The modern Japan Sea is characterized by active methane seeps associated with gas hydrates, but their ancient counterparts are not fully understood. This study describes a newly discovered methane-seep carbonate block, the ‘Nakanomata Seep Deposit’, from the upper Miocene Nodani Formation in Joetsu City, central Japan. The age of this deposit is constrained to 7.5–6.5Ma based on its fossil diatom assemblage. The deposit contains molluscan fossils typical of methane seeps, including vesicomyid and bathymodiolin bivalves, and provannid gastropods, and it retains an almost entirely aragonitic mineralogy, despite its Miocene age. It is composed of clotted microcrystalline aragonite containing nodules and intraclasts, and is crosscut by vein-like networks of voids and cavities rimmed with acicular aragonite. The δ13C values of the carbonate phases are as low as −41.1‰ and the presence of lipid biomarkers (pentamethylicosane and crocetane) suggests that the deposit originated from the anaerobic oxidation of methane. It is suggested that an initially diffuse methane seepage formed the micritic nodules, followed by a more rapid and intense methane seepage that led to the development of abundant voids in the sediment; finally, the sediment was cemented by microcrystalline aragonite and void-lining acicular aragonite. The seep deposit also contains peculiar globular silica minerals and authigenic quartz. During their precipitation, these globular silica minerals may have trapped methane gas bubbles, and the minerals may be pseudomorphs after silica clathrate. Sufficient increase in pH and supersaturation of silica, which led to the dissolution and subsequent precipitation of these silica minerals, could have resulted from the degassing of carbon dioxide, promoted by an effective supply of methane, and its supersaturation, thus forming gas bubbles.

Coherent late-Holocene climate-driven shifts in the structure of three Rocky Mountain lakes

Large-scale atmospheric pressure centers, such as the Aleutian and Icelandic Low, have a demonstrated relationship with physical lake characteristics in contemporary monitoring studies, but the responses to these phenomena are rarely observed in lake records. We observe coherent changes in the stratification patterns of three deep (&gt;30 m) lakes inferred from fossil diatom assemblages as a response to shifts in the location and intensity of the Aleutian Low and compare these changes with similar long-term changes observed in the δ18O record from the Yukon. Specifically, these records indicate that between 3.2 and 1.4 ka, the Aleutian Low shifted westward, resulting in an increased frequency of storm tracks across the Pacific Northwest during winter and spring. This change in atmospheric circulation ultimately produced deeper mixing in the upper waters of these three lake systems. Enhanced stratification between 4.5 and 3.3 ka and from 1.3 ka to present suggests a strengthened Aleutian Low and more meridional circulation.

Recent and historic eutrophication of an island lake in northern Lake Michigan, USA

Lake Geneserath is located on Beaver Island in northern Lake Michigan, USA. It exhibits evidence of recent eutrophication, despite limited anthropogenic activity in the watershed. The primary goal of this study was to determine the time scale of trophic state change in Lake Geneserath and the influence of multiple stressors. Sediment cores from the shallow northern and deep southern basins of Lake Geneserath were collected and sectioned at 0.5-cm intervals over the topmost 10 cm and at 1.0-cm intervals for the remainder of the core. Total phosphorus, percent organic matter, and chlorophyll derivative concentrations were measured in each section, and core chronology was established using 210Pb. Diatom assemblages were quantified from evenly distributed sections in each core. Geochemical variables in the southern basin core indicate historic (AD ~ 1450–1650) and modern (1848-present) periods of eutrophication, coincident with human disturbance and climate warming events. Geochemical variables in the northern basin core confirm accelerating eutrophication after 1950. The fossil diatom assemblages in both basins shifted from species favoring low nutrients to more nutrient-tolerant taxa. A change from periphytic to planktonic diatom species, concurrent with reduced percent organic matter in the northern basin core, indicates rising water level during the second half of the twentieth century. Lake Geneserath was subject to multiple stressors and its trophic status increased with recent residential development and climate warming.

Reconstruction of Holocene relative sea-level change and residual uplift in the Lake Inba area, Japan

We collected and analyzed fossil diatoms and volcanic ash and determined 14C ages in core samples from the lowlands around Lake Inba in the eastern Kanto Plain, central Japan, an area where late Quaternary sea-level change, fluvial erosion, and tectonic uplift have affected a region of archeological interest. We inferred Holocene paleoenvironmental changes and relative sea levels on the basis of indicators, such as high tide levels, derived from fossil diatom assemblages as well as the stratigraphic positions of 14C-dated samples and the K–Ah tephra. Our results lead to estimates of the upper half tidal range during 6800–7000cal yr BP of 1.7m. The Holocene highstand took place at approximately 6400–6500cal yr BP, and mean sea level reached an elevation of 1.9m. However, the timing of this sea-level rise is earlier in the Lake Inba area than documented in previous studies, and it is suggested that the timing of the Holocene highstand may appear at least 1000years earlier as a result of Holocene residual uplift. After this, sea-level fell abruptly around 4000cal yr BP. We also recognized a sea-level fall corresponding to the Yayoi regression at about 2600cal yr BP, during which the mean relative sea level may have been as low as −2.5m after compensating for uplift, subsidence or geomorphologic effects in this area.

Reconstruction of Lake Balkhash levels and precipitation/evaporation changes during the last 2000 years from fossil diatom assemblages

Environmental changes in arid central Asia are prominently manifest through changes in the water balance. Lake Balkhash, the largest lake in Kazakhstan, is a shallow lake situated in a closed basin. We obtained a 600-cm-long core from the western part of the lake in 2007 and carried out analyses of fossil diatoms and ostracods, geochemistry, and grain size of the sediments. The age control of the core is based on radiocarbon ages of fossil ostracods and, in the uppermost part, the AD 1963 137Cs activity peak and the first appearance of 137Cs in AD 1952. We recognized two main diatom assemblages by detrended correspondence analysis, one characterized by the dominance of freshwater planktonic species, and the other dominated by brackish planktonic, brackish, and marine benthic, and freshwater benthic species. The first assemblage suggests higher lake levels, and the second suggests lower lake levels. The observed assemblage changes are consistent with lake-level observations recorded during the last 120 years. The diatom assemblage data from the core indicate that there were seven periods when the lake level was low during the last 2000 years; moreover, the pH of the lake changed together with the lake level. In addition, during periods when the lake level was low from AD 1260–2000, the abundance of fossil ostracods and the Ca and TOC contents were low in comparison with the periods before AD 1260. The causes of low water levels were essentially the same between Lake Balkhash and the Aral Sea during the studied period. One likely cause of the low water levels, especially those during the last 1200 years, is changes in solar activity. Furthermore, some environmental changes during the past 40 years can be explained by human activities.

Tracking the History and Ecological Changes of Rising Double-Crested Cormorant Populations Using Pond Sediments from Islands in Eastern Lake Ontario.

In the Laurentian Great Lakes region, the double-crested cormorant (Phalacrocorax auritus) has seen a thousand-fold population increase in recent decades. These large colonies of birds now often conflict with socioeconomic interests, particularly due to perceived competition with fisheries and the destruction of terrestrial vegetation in nesting habitats. Here we use dated sediment cores from ponds on islands in eastern Lake Ontario that receive waste inputs from dense colonies of cormorants and ring-billed gulls (Larus delawarensis) to chronicle the population rise of these species and assess their long-term ecological impacts. Modern water chemistry sampling from these sites reveals drastically elevated nutrient and major ion concentrations compared to reference ponds not influenced by waterbirds. Geochemical tracers in dated sediment cores, particularly δ15N and chlorophyll-a concentrations, track waterbird influences over time. Fossil diatom assemblages were dominated by species tolerant of hyper-eutrophic and polluted systems, which is in marked contrast to assemblages in reference sites. In addition to establishing long-term ecological impacts, this multi-proxy paleoecological approach can be used to determine whether islands of concern have been long-term nesting sites or were only recently colonized by cormorant or ring-billed gull populations across the Great Lakes, facilitating informed management decisions about controversial culling programs.

Pliocene and Pleistocene paleoceanography in the western subarctic Pacific based on diatom analyses of ODP Leg 145 Hole 884B and IODP Expedition 323 Holes U1341B and U1343E

Late Pliocene–Pleistocene fossil diatom assemblages from Ocean Drilling Program (ODP) Leg 145 Hole 884B in the western subarctic Pacific were investigated to improve the diatom biostratigraphy. Moreover, the paleoceanographic records at Hole 884B were compared with those in Integrated Ocean Drilling Program (IODP) Expedition 323 Holes U1341B and U1343E in the Bering Sea for the time interval of 2.5–0Ma in order to clarify the differences in the diatom assemblage fluctuations and the history of past ocean circulations around the subarctic Pacific. In Hole 884B, five diatom zones from the Neodenticula koizumii–Neodenticula kamtschatica Zone to the Neodenticula seminae Zone were identified. The decreases in the relative abundance of temperate-water species were recognized at ca. 1.9Ma in Hole U1343E, ca. 2.1Ma in Holes 884B and U1341B. The decreases of the temperate-water species at these sites indicate that a widespread cooling event must have occurred over the subarctic Pacific at ~2Ma. In addition, the continuous low occurrences of the sea-ice related species from around 2Ma onward, which indicate the presence of sea-ice, were observed at Sites 884 and U1341. We also point out that the age differences in decrease of temperate-water species at the three sites (i.e., delay in response of the temperate-water species at Site U1343) may be related to changes of seawater inflow from the North Pacific to Sites U1341 and U1343 and of the East Kamchatka Current from the Bering Sea to Site 884.

Changes of fossil diatom assemblage and tectonic movements in Tainohama, Tokushima prefecture during the late Holocen

Changes of fossil diatom assemblage and tectonic movements in Tainohama, Tokushima prefecture during the late Holocen

Effects of glacier meltwater on the algal sedimentary record of an alpine lake in the central US Rocky Mountains throughout the late Holocene

The effects of alpine glaciers on the hydrology, physical features, and biogeochemistry of lakes have been investigated over contemporary time scales. However, the influence of these factors on algal communities over longer time scales remains unclear, yet is critical to paleolimnological interpretation of environmental change in alpine regions. We examined sedimentary algal pigments and fossil diatom assemblages in two proximal lakes with equivalent local climates, one glacier-fed and one snow-fed, in the central Rocky Mountains (USA) to determine how glacier meltwater has altered algal records over the last 3,000 years. Differences between the records of the two lakes intensified during the Medieval Climate Anomaly and the Little Ice Age, with the glacier-fed lake exhibiting an overall increase in fossil algal pigment concentrations and greater diatom assemblage turnover. Starting 1,000 years ago, the glacier-fed lake in this study showed evidence of nitrogen enrichment from glacier meltwater, as indicated by increasing relative abundances of Asterionella formosa and, to a lesser extent, Fragilaria crotonensis. Since the Little Ice Age, diatom species richness declined in the glacier-fed lake, and further decreased following the 1950s, while assemblage turnover increased. These results demonstrate that glaciers can strongly alter the algal sedimentary record and should be considered when interpreting high-elevation lake records.

Reconstrucción Paleoambiental de la Planicie Costera de Mar Chiquita (Provincia de Buenos Aires, Argentina) Durante el Holoceno, Basada en Diatomeas

HOLOCENE PALEOENVIRONMENTAL RECONSTRUCTION OF THE MAR CHIQUITA COASTAL PLAIN (BUENOS AIRES PROVINCE, ARGENTINA) BASED ON DIATOMS. Diatom assemblages from a core at the north of Mar Chiquita coastal plain were analyzed in order to identify environmental changes due to salinity and depth. The core contains sediments dated at the base in 5802±70 14C years BP. Four lithological units were defined: greenish-grey silt, brownish-grey sandy silt, heterolithic facies and brown sandy silt. Two hundred diatom species were identified and grouped based on their salinity tolerances and life form. Cluster analysis defined two diatom zones. The lower section of the sedimentary sequence (from the base to 90 cm depth, Zone I) was dominated by Paralia sulcata (Ehrenberg) Cleve and Psammococconeis cf. disculoides (Hustedt) García accompanied by marine-brackish plankton, epipsammon and epiphytes indicating coastal marine conditions. From 90 cm towards the top (Zone II), salinity and depth decrease: P. sulcata decline and appear Planothidium delicatulum (Kützing) Round and Bukht (brackish) and Hippodonta hungarica Grunow (brackish freshwater taxa), benthic and epiphytes diatoms increased. Multidimensional Scaling Analysis (MDS) showed that fossil diatom assemblages were similar to the modern assemblages from the inlet of Mar Chiquita coastal lagoon. These new diatom and sedimentological data support the regional model for the Holocene sea-level fluctuation and allow inferring the existence of a marginal coastal system during the regressive phase that evolved into a brackish-freshwater coastal lagoon and marsh.

Morphometric variation of Seminavis pusilla (Bacillariophyceae) and its relationship to salinity in inter‐dune lakes of the Badain Jaran Desert, Inner Mongolia, China

SummaryWe used light and scanning electron microscope analyses to quantify morphometric features (valve length, width, stria density, lineola density and valve curvature) from the observation of valves representing Seminavis pusilla. Cluster analysis based on Gaussian mixture models and the expectation‐maximization algorithm was used for delineating two species, Seminavis pusilla sensu stricto and Seminavis lata (Krammer) Rioual comb. et stat. nov. By comparison with S. pusilla, S. lata is characterized by wider valves and lower stria density. The two species have also markedly different ecology. S. pusilla is most abundant in the most saline lakes of the dataset, while S. lata is most abundant in the less saline lakes. Our results indicate that combining the two species into S. pusilla sensu lato would lead to a loss of ecological information and a decrease of the performance of transfer functions developed for quantitative reconstruction of past salinity from fossil diatom assemblages in sediment cores.

Laurentian Great Lakes phytoplankton and their water quality characteristics, including a diatom-based model for paleoreconstruction of phosphorus.

Recent shifts in water quality and food web characteristics driven by anthropogenic impacts on the Laurentian Great Lakes warranted an examination of pelagic primary producers as tracers of environmental change. The distributions of the 263 common phytoplankton taxa were related to water quality variables to determine taxon-specific responses that may be useful in indicator models. A detailed checklist of taxa and their environmental optima are provided. Multivariate analyses indicated a strong relationship between total phosphorus (TP) and patterns in the diatom assemblages across the Great Lakes. Of the 118 common diatom taxa, 90 (76%) had a directional response along the TP gradient. We further evaluated a diatom-based transfer function for TP based on the weighted-average abundance of taxa, assuming unimodal distributions along the TP gradient. The r2 between observed and inferred TP in the training dataset was 0.79. Substantial spatial and environmental autocorrelation within the training set of samples justified the need for further model validation. A randomization procedure indicated that the actual transfer function consistently performed better than functions based on reshuffled environmental data. Further, TP was minimally confounded by other environmental variables, as indicated by the relatively large amount of unique variance in the diatoms explained by TP. We demonstrated the effectiveness of the transfer function by hindcasting TP concentrations using fossil diatom assemblages in a Lake Superior sediment core. Passive, multivariate analysis of the fossil samples against the training set indicated that phosphorus was a strong determinant of historical diatom assemblages, verifying that the transfer function was suited to reconstruct past TP in Lake Superior. Collectively, these results showed that phytoplankton coefficients for water quality can be robust indicators of Great Lakes pelagic condition. The diatom-based transfer function can be used in lake management when retrospective data are needed for tracking long-term degradation, remediation and trajectories.

Reconstructing phosphorus levels using models based on the modern diatom assemblages of 55 lakes in southern Quebec

This study shows how modern diatom assemblages, obtained from the surface sediments of 55 deep and shallow lakes in southern Quebec and representing a gradient in trophic state and specific conductivity, can be used to reconstruct the historical water column concentrations of total phosphorus (TP) over time. Inference models were developed with the aid of regression analyses based on weighted means. The TP models demonstrate statistical performances equal to or superior to TP models developed in the past for other regions in Canada and elsewhere in the world. A reconstruction of TP values, based on fossil diatom assemblages of Lac Nairne (Charlevoix, QC), was conducted. Comparing the TP inferred values with the measured values and those estimated using export coefficients, the reliability of the model was proven. In addition, the comparison of the results obtained from reconstructions performed with two inference models (Quebec and Ontario lakes) also revealed that models predicted TP and not a collinear variable.

Abrupt climate change and early lake development – the Lateglacial diatom flora at Hässeldala Port, southeastern Sweden

The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13 900 and 11 200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open‐water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom‐based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13 900 to 12 500 cal. a BP. From 12 500 cal. a BP, coincident with the start of climate cooling, to 11 300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build‐up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny.

琵琶湖堆積物コアBIW08-Bにおける過去28万年間の珪藻化石群集変動

琵琶湖堆積物コアBIW08-Bにおける過去28万年間の珪藻化石群集変動