Behind the Organic Veil: Assessing the Impact of Chemical Deflocculation on Organic Content Reduction and Lacustrine Arcellinida (Testate Amoebae) Analysis.

Arcellinida (testate lobose amoebae) are widely used as bio-indicators of lacustrine environmental change. Too much obscuring organic material in a gridded wet Petri dish preparation makes it difficult to observe all specimens present and slows quantification as the organic material has to be carefully worked through with a dissection probe. Chemical deflocculation using soda ash (Na2CO3·H2O), potassium hydroxide (KOH), or sodium hexametaphosphate ((NaPO3)6) has previously been shown to disaggregate and reduce organic content in lake sediments, but to date, no attempt has been made to comparatively evaluate the efficiency of these deflocculants in disaggregating organic content and their impact on Arcellinida analysis in lacustrine sediments. Here, we assess the effectiveness of soda ash, potassium hydroxide, and sodium hexametaphosphate treatments on removing organic content and the impact of those digestions on Arcellinida preservation in 126 sample aliquots subdivided from three sediment samples (YK-20, YK-25, and YK-57) collected from three lakes near Yellowknife, Northwest Territories, Canada. Following treatment, cluster analysis and Bray-Curtis dissimilarity matrix (BCDM) were utilized to determine whether treatments resulted in dissolution-driven changes in Arcellinida assemblage composition. Observed Arcellinida tests in aliquots increased drastically after treatment of organic-rich samples (47.5-452.7% in organic-rich aliquots and by 14.8% in aliquots with less organic matter). The BCDM results revealed that treatment with 5% KOH resulted in the highest reduction in observed organic content without significantly affecting Arcellinida assemblage structure, while soda ash and sodium hexametaphosphate treatments resulted in marginal organic matter reduction and caused severe damage to the arcellinidan tests.

Ecosystem state shifts during long-term development of an Amazonian peatland.

The most carbon (C)-dense ecosystems of Amazonia are areas characterized by the presence of peatlands. However, Amazonian peatland ecosystems are poorly understood and are threatened by human activities. Here, we present an investigation into long-term ecohydrological controls on C accumulation in an Amazonian peat dome. This site is the oldest peatland yet discovered in Amazonia (peat initiation ca. 8.9 ka BP), and developed in three stages: (i) peat initiated in an abandoned river channel with open water and aquatic plants; (ii) inundated forest swamp; and (iii) raised peat dome (since ca. 3.9 ka BP). Local burning occurred at least three times in the past 4,500 years. Two phases of particularly rapid C accumulation (ca. 6.6-6.1 and ca. 4.9-3.9 ka BP), potentially resulting from increased net primary productivity, were seemingly driven by drier conditions associated with widespread drought events. The association of drought phases with major ecosystem state shifts (open water wetland-forest swamp-peat dome) suggests a potential climatic control on the developmental trajectory of this tropical peatland. A third drought phase centred on ca. 1.8-1.1 ka BP led to markedly reduced C accumulation and potentially a hiatus during the peat dome stage. Our results suggest that future droughts may lead to phases of rapid C accumulation in some inundated tropical peat swamps, although this can lead ultimately to a shift to ombrotrophy and a subsequent return to slower C accumulation. Conversely, in ombrotrophic peat domes, droughts may lead to reduced C accumulation or even net loss of peat. Increased surface wetness at our site in recent decades may reflect a shift towards a wetter climate in western Amazonia. Amazonian peatlands represent important carbon stores and habitats, and are important archives of past climatic and ecological information. They should form key foci for conservation efforts.

Lacustrine Arcellinina (Testate Amoebae) as Bioindicators of Arsenic Contamination.

Arcellininids (testate amoebae) were examined from 61 surface sediment samples collected from 59 lakes in the vicinity of former gold mines, notably Giant Mine, near Yellowknife, Northwest Territories, Canada to determine their utility as bioindicators of arsenic (As), which occurs both as a byproduct of gold extraction at mines in the area and ore-bearing outcrops. Cluster analysis (Q-R-mode) and detrended correspondence analysis (DCA) reveal five arcellininid assemblages, three of which are related to varying As concentrations in the sediment samples. Redundancy analysis (RDA) showed that 14 statistically significant environmental parameters explained 57 % of the variation in faunal distribution, while partial RDA indicated that As had the greatest influence on assemblage variance (10.7 %; p < 0.10). Stress-indicating species (primarily centropyxids) characterized the faunas of samples with high As concentrations (median = 121.7 ppm, max > 10000 ppm, min = 16.1 ppm, n = 32), while difflugiid dominated assemblages were prevalent in substrates with relatively low As concentrations (median = 30.2 ppm, max = 905.2 ppm, min = 6.3 ppm, n = 20). Most of the lakes with very high As levels are located downwind (N and W) of the former Giant Mine roaster stack where refractory ore was roasted and substantial quantities of As were released (as As2O3) to the atmosphere in the first decade of mining. This spatial pattern suggests that a significant proportion of the observed As, in at least these lakes, are industrially derived. The results of this study highlight the sensitivity of Arcellinina to As and confirm that the group has considerable potential for assessing the impact of As contamination on lakes.

Mediation of arsenic mobility by organic matter in mining-impacted sediment from sub-Arctic lakes: implications for environmental monitoring in a warming climate.

Arsenic (As) is commonly sequestered at the sediment-water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12665-022-10213-2.

Intra-lake response of Arcellinida (testate lobose amoebae) to gold mining-derived arsenic contamination in northern Canada: Implications for environmental monitoring.

Arcellinida (testate lobose amoebae) were examined from 40 near-surface sediment samples (top 0.5 cm) from two lakes impacted by arsenic (As) contamination associated with legacy gold mining in subarctic Canada. The objectives of the study are two folds: quantify the response of Arcellinida to intra-lake variability of As and other physicochemical controls, and evaluate whether the impact of As contamination derived from two former gold mines, Giant Mine (1938-2004) and Tundra Mine (1964-1968 and 1983-1986), on the Arcellinida distribution in both lakes is comparable or different. Cluster analysis and nonmetric multidimensional scaling (NMDS) were used to identify Arcellinida assemblages in both lakes, and redundancy analysis (RDA) was used to quantify the relationship between the assemblages, As, and other geochemical and sedimentological parameters. Cluster analysis and NMDS revealed four distinct arcellinidan assemblages in Frame Lake (assemblages 1-4) and two in Hambone Lake (assemblages 5 and 6): (1) Extreme As Contamination (EAC) Assemblage; (2) High calcium (HC) Assemblage; (3) Moderate As Contamination (MAC) assemblages; (4) High Nutrients (HN) Assemblage; (5) High Diversity (HD) Assemblage; and (6) Centropyxis aculeata (CA) Assemblage. RDA analysis showed that the faunal structure of the Frame Lake assemblages was controlled by five variables that explained 43.2% of the total faunal variance, with As (15.8%), Olsen phosphorous (Olsen-P; 10.5%), and Ca (9.5%) being the most statistically significant (p < 0.004). Stress-tolerant arcellinidan taxa were associated with elevated As concentrations (e.g., EAC and MAC; As concentrations range = 145.1-1336.6 mg kg-1; n = 11 samples), while stress-sensitive taxa thrived in relatively healthier assemblages found in substrates with lower As concentrations and higher concentrations of nutrients, such as Olsen-P and Ca (e.g., HC and HM; As concentrations range = 151.1-492.3 mg kg-1; n = 14 samples). In contrast, the impact of As on the arcellinidan distribution was not statistically significant in Hambone Lake (7.6%; p-value = 0.152), where the proportion of silt (24.4%; p-value = 0.005) and loss-on-ignition-determined minerogenic content (18.5%; p-value = 0.021) explained a higher proportion of the total faunal variance (58.4%). However, a notable decrease in arcellinidan species richness and abundance and increase in the proportions of stress-tolerant fauna near Hambone Lake's outlet (e.g., CA samples) is consistent with a spatial gradient of higher sedimentary As concentration near the outlet, and suggests a lasting, albeit weak, As influence on Arcellinida distribution in the lake. We interpret differences in the influence of sedimentary As concentration on Arcellinida to differences in the predominant As mineralogy in each lake, which is in turn influenced by differences in ore-processing at the former Giant (roasting) and Tundra mines (free-milling).

Influence of late-Holocene climate change on the solid-phase speciation and long-term stability of arsenic in sub-Arctic lake sediments.

Sediment cores were collected from two lakes in the Courageous Lake Greenstone Belt (CLGB), central Northwest Territories, Canada, to examine the influence of late-Holocene warming on the transport and fate of arsenic (As) in sub-Arctic lakes. In both lakes, allochthonous As-bearing minerals (i.e. arsenopyrite and scorodite) were identified in sediment deposited during times of both regional warming and cooling, suggesting that weathering of bedrock and derived surficial materials provides a continual source of As to lakes of the CLGB. However, maximum porewater As (84 µg·L-1 and 15 µg·L-1) and reactive organic matter (OM; aquatic and terrestrial-derived) concentrations in each lake are coincident with known periods of regional climate warming. It is inferred that increased biological production in surface waters and influx of terrigenous OM led to the release of sedimentary As to porewater through reductive dissolution of As-bearing Fe-(oxy)hydroxides and scorodite during episodes of regional warming. Elevated sedimentary As concentrations (median: 36 mg·kg-1; range: 29 to 49 mg·kg-1) are observed in sediment coeval with the Holocene Thermal Maximum (ca. 5430 ± 110 to 4070 ± 130 cal. years BP); at these depths, authigenic As-bearing framboidal pyrite is the primary host of As in sediment and the influence of organic matter on the precipitation of As-bearing framboidal pyrite is apparent petrographically. These findings suggest that increased biological productivity and weathering of terrestrial OM associated with climate warming influences redox cycles in the near-surface sediment and enhances the mobility of As in northern lakes. Knowledge generated from this study is relevant for predicting future climate change-driven variations in metal(loid) cycling in aquatic systems and can be used to interpret trends in long-term environmental monitoring data at historical, modern, and future metal mines in northern environments.

Seasonal variation of arsenic and antimony in surface waters of small subarctic lakes impacted by legacy mining pollution near Yellowknife, NT, Canada.

The seasonal variation in lake water arsenic (As) and antimony (Sb) concentrations was assessed in four small (<1.5km2) subarctic lakes impacted by As and Sb emissions from legacy mining activities near Yellowknife, Northwest Territories, Canada. Substantial variation in As concentrations were measured over the two-year period of study in all but the deepest lake (maximum depth 6.9m), including a four-fold difference in As in the shallowest lake ([As]: 172-846µgL-1; maximum depth 0.8m). Arsenic concentrations were enriched following ice cover development in the three shallowest lakes (50-110%) through a combination of physical and biogeochemical processes. Early winter increases in As were associated with the exclusion of solutes from the developing ice-cover; and large increases in As were measured once oxygen conditions were depleted to the point of anoxia by mid-winter. The onset of anoxic conditions within the water column was associated with large increases in the concentration of redox sensitive elements in lake waters (As, iron [Fe], and manganese [Mn]), suggesting coupling of As mobility with Fe and Mn cycling. In contrast, there was little difference in Sb concentrations under ice suggesting that Sb mobility was controlled by factors other than Fe and Mn associated redox processes. A survey of 30 lakes in the region during fall (open-water) and late-winter (under-ice) revealed large seasonal differences in surface water As were more common in lakes with a maximum depth <4m. This threshold highlights the importance of winter conditions and links between physical lake properties and biogeochemical processes in the chemical recovery of As-impacted subarctic landscapes. The findings indicate annual remobilization of As from contaminated lake sediments may be inhibiting recovery in small shallow lakes that undergo seasonal transitions in redox state.

Diatom ecological response to deposition of the 833-850 CE White River Ash (east lobe) ashfall in a small subarctic Canadian lake.

A <5 mm thick volcanic ashfall layer associated with the White River Ash (east lobe [WRAe]) originating from the eruption of Mount Churchill, Alaska (833-850 CE; 1,117-1,100 cal BP) was observed in two freeze cores obtained from Pocket Lake (62.5090°N, -114.3719°W), a small subarctic lake located within the city limits of Yellowknife, Northwest Territories, Canada. Here we analyze changes in diatom assemblages to assess impact of tephra deposition on the aquatic biota of a subarctic lake. In a well-dated core constrained by 8 radiocarbon dates, diatom counts were carried out at 1-mm intervals through an interval spanning  1 cm above and below the tephra layer with each 1 mm sub-sample represented about 2 years of deposition. Non-metric Multidimensional Scaling (NMDS) and Stratigraphically Constrained Incremental Sum of Squares (CONISS) analyses were carried out and three distinct diatom assemblages were identified throughout the interval. The lowermost "Pre-WRAe Assemblage (Pre-WRAeA)" was indicative of slightly acidic and eutrophic lacustrine conditions. Winter deposition of the tephra layer drove a subsequent diatom flora shift to the "WRAe Assemblage (WRAeA)" the following spring. The WRAeA contained elevated abundances of taxa associated with oligotrophic, nutrient depleted and slightly more alkaline lake waters. These changes were only apparent in samples within the WRAe containing interval indicating that they were short lived and only sustained for a single year of deposition. Immediately above the WRAe horizon, a third, "Post-WRAe Assemblage (Post-WRAeA)" was observed. This assemblage was initially similar to that of the Pre-WRAeA but gradually became more distinct upwards, likely due to climatic patterns independent of the WRAe event. These results suggest that lacustrine environments are sensitive to perturbations such as deposition of ash fall, but that ecological communities in subarctic systems can also have high resilience and can recover rapidly. If subsampling of the freeze cores was carried out at a more standard resolution (0.5-1 cm) these subtle diatom ecological responses to perturbation associated with the WRAe depositional event would not have been observed. This research illustrates the importance of high-resolution subsampling when studying the environmental impact of geologically "near instantaneous" events such as episodic deposition of ashfalls.

Organic matter control on the distribution of arsenic in lake sediments impacted by ~65years of gold ore processing in subarctic Canada.

Climate change is profoundly affecting seasonality, biological productivity, and hydrology in high northern latitudes. In sensitive subarctic environments exploitation of mineral resources led to contamination and it is not known how cumulative effects of resource extraction and climate warming will impact ecosystems. Gold mines near Yellowknife, Northwest Territories, subarctic Canada, operated from 1938 to 2004 and released >20,000t of arsenic trioxide (As2O3) to the environment through stack emissions. This release resulted in elevated arsenic concentrations in lake surface waters and sediments relative to Canadian drinking water standards and guidelines for the protection of aquatic life. A meta-analytical approach is used to better understand controls on As distribution in lake sediments within a 30-km radius of historic mineral processing activities. Arsenic concentrations in the near-surface sediments range from 5mg·kg-1 to over 10,000mg·kg-1 (median 81mg·kg-1; n=105). Distance and direction from the historic roaster stack are significantly (p<0.05) related to sedimentary As concentration, with highest As concentrations in sediments within 11km and lakes located downwind. Synchrotron-based µXRF and µXRD confirm the persistence of As2O3 in near surface sediments of two lakes. Labile organic matter (S1) is significantly (p<0.05) related to As and S concentrations in sediments and this relationship is greatest in lakes within 11km from the mine. These relations are interpreted to reflect labile organic matter acting as a substrate for microbial growth and mediation of authigenic precipitation of As-sulphides in lakes close to the historic mine where As concentrations are highest. Continued climate warming is expected to lead to increased biological productivity and changes in organic geochemistry of lake sediments that are likely to play an important role in the mobility and fate of As in aquatic ecosystems.

What killed Frame Lake? A precautionary tale for urban planners.

Frame Lake, located within the city of Yellowknife, Northwest Territories, Canada, has been identified as requiring significant remediation due to its steadily declining water quality and inability to support fish by the 1970s. Former gold mining operations and urbanization around the lake have been suspected as probable causes for the decline in water quality. While these land-use activities are well documented, little information is available regarding their impact on the lake itself. For this reason, Arcellinida, a group of shelled protozoans known to be reliable bioindicators of land-use change, were used to develop a hydroecological history of the lake. The purpose of this study was to use Arcellinida to: (1) document the contamination history of the lake, particularly related to arsenic (As) associated with aerial deposition from mine roaster stacks; (2) track the progress of water quality deterioration in Frame Lake related to mining, urbanization and other activities; and (3) identify any evidence of natural remediation within the lake. Arcellinida assemblages were assessed at 1-cm intervals through the upper 30 cm of a freeze core obtained from Frame Lake. The assemblages were statistically compared to geochemical and loss-on-ignition results from the core to document the contamination and degradation of conditions in the lake. The chronology of limnological changes recorded in the lake sediments were derived from 210Pb, 14C dating and known stratigraphic events. The progress of urbanization near the lake was tracked using aerial photography. Using Spearman correlations, the five most significant environmental variables impacting Arcellinida distribution were identified as minerogenics, organics, As, iron and mercury (p < 0.05; n = 30). Based on CONISS and ANOSIM analysis, three Arcellinida assemblages are identified. These include the Baseline Limnological Conditions Assemblage (BLCA), ranging from 17-30 cm and deposited in the early Holocene >7,000 years before present; the As Contamination Assemblage (ACA), ranging from 7-16 cm, deposited after ∼1962 when sedimentation began in the lake again following a long hiatus that spanned to the early Holocene; and the Eutrophication Assemblage (EA), ranging from 1-6 cm, comprised of sediments deposited after 1990 following the cessation of As and other metal contaminations. The EA developed in response to nutrient-rich waters entering the lake derived from the urbanization of the lake catchment and a reduction in lake circulation associated with the development at the lake outlet of a major road, later replaced by a causeway with rarely open sluiceways. The eutrophic condition currently charactering the lake-as evidenced by a population explosion of eutrophication indicator taxa Cucurbitella tricuspis-likely led to a massive increase in macrophyte growth and winter fish-kills. This ecological shift ultimately led to a system dominated by Hirudinea (leeches) and cessation of the lake as a recreational area.

Late Holocene climatic variability in Subarctic Canada: Insights from a high-resolution lake record from the central Northwest Territories.

We examined late Holocene (ca. 3300 yr BP to present-day) climate variability in the central Northwest Territories (Canadian Subarctic) using a diatom and sedimentological record from Danny's Lake (63.48ºN, 112.54ºW), located 40 km southwest of the modern-day treeline. High-resolution sampling paired with a robust age model (25 radiocarbon dates) allowed for the examination of both lake hydroecological conditions (30-year intervals; diatoms) and sedimentological changes in the watershed (12-year intervals; grain size records) over the late Holocene. Time series analysis of key lake ecological indicators (diatom species Aulacoseira alpigena, Pseudostaurosira brevistriata and Achnanthidium minutissimum) and sedimentological parameters, reflective of catchment processes (coarse silt fraction), suggests significant intermittent variations in turbidity, pH and light penetration within the lake basin. In the diatom record, we observed discontinuous periodicities in the range of ca. 69, 88-100, 115-132, 141-188, 562, 750 and 900 years (>90% and >95% confidence intervals), whereas the coarse silt fraction was characterized by periodicities in the >901 and <61-year range (>95% confidence interval). Periodicities in the proxy data from the Danny's Lake sediment core align with changes in total solar irradiance over the past ca. 3300 yr BP and we hypothesize a link to the Suess Cycle, Gleissberg Cycle and Pacific Decadal Oscillation via occasional inland propagation of shifting air masses over the Pacific Ocean. This research represents an important baseline study of the underlying causes of climate variability in the Canadian Subarctic and provides details on the long-term climate variability that has persisted in this region through the past three thousand years.

Palaeoecology of testate amoebae in a tropical peatland.

We present the first detailed analysis of subfossil testate amoebae from a tropical peatland. Testate amoebae were analysed in a 4-m peat core from western Amazonia (Peru) and a transfer function developed from the site was applied to reconstruct changes in water table over the past ca. 8,000 years. Testate amoebae were in very low abundance in the core, especially in the lower 125cm, due to a combination of poor preservation and obscuration by other organic matter. A modified preparation method enabled at least 50 testate amoebae to be counted in each core sample. The most abundant taxa preserved include Centropyxis aculeata, Hyalosphenia subflava, Phryganella acropodia and Trigonopyxis arcula. Centropyxis aculeata, an unambiguous wet indicator, is variably present and indicates several phases of near-surface water table. Our work shows that even degraded, low-abundance assemblages of testate amoebae can provide useful information regarding the long-term ecohydrological developmental history of tropical peatlands.

1.8 Billion Years of Detrital Zircon Recycling Calibrates a Refractory Part of Earth's Sedimentary Cycle.

Detrital zircon studies are providing new insights on the evolution of sedimentary basins but the role of sedimentary recycling remains largely undefined. In a broad region of northwestern North America, this contribution traces the pathway of detrital zircon sand grains from Proterozoic sandstones through Phanerozoic strata and argues for multi-stage sedimentary recycling over more than a billion years. As a test of our hypothesis, integrated palynology and detrital zircon provenance provides clear evidence for erosion of Carboniferous strata in the northern Cordillera as a sediment source for Upper Cretaceous strata. Our results help to calibrate Earth's sedimentary cycle by showing that recycling dominates sedimentary provenance for the refractory mineral zircon.

Spheroidal carbonaceous particles are a defining stratigraphic marker for the Anthropocene.

There has been recent debate over stratigraphic markers used to demarcate the Anthropocene from the Holocene Epoch. However, many of the proposed markers are found only in limited areas of the world or do not reflect human impacts on the environment. Here we show that spheroidal carbonaceous particles (SCPs), a distinct form of black carbon produced from burning fossil fuels in energy production and heavy industry, provide unambiguous stratigraphic markers of the human activities that have rapidly changed planet Earth over the last century. SCPs are found in terrestrial and marine sediments or ice cores in every continent, including remote areas such as the high Arctic and Antarctica. The rapid increase in SCPs mostly occurs in the mid-twentieth century and is contemporaneous with the 'Great Acceleration'. It therefore reflects the intensification of fossil fuel usage and can be traced across the globe. We integrate global records of SCPs and propose that the global rapid increase in SCPs in sedimentary records can be used to inform a Global Standard Stratigraphic Age for the Anthropocene. A high-resolution SCP sequence from a lake or peatland may provide the much-needed 'Golden Spike' (Global Boundary Stratotype Section and Point).

The long-term fate of permafrost peatlands under rapid climate warming.

Permafrost peatlands contain globally important amounts of soil organic carbon, owing to cold conditions which suppress anaerobic decomposition. However, climate warming and permafrost thaw threaten the stability of this carbon store. The ultimate fate of permafrost peatlands and their carbon stores is unclear because of complex feedbacks between peat accumulation, hydrology and vegetation. Field monitoring campaigns only span the last few decades and therefore provide an incomplete picture of permafrost peatland response to recent rapid warming. Here we use a high-resolution palaeoecological approach to understand the longer-term response of peatlands in contrasting states of permafrost degradation to recent rapid warming. At all sites we identify a drying trend until the late-twentieth century; however, two sites subsequently experienced a rapid shift to wetter conditions as permafrost thawed in response to climatic warming, culminating in collapse of the peat domes. Commonalities between study sites lead us to propose a five-phase model for permafrost peatland response to climatic warming. This model suggests a shared ecohydrological trajectory towards a common end point: inundated Arctic fen. Although carbon accumulation is rapid in such sites, saturated soil conditions are likely to cause elevated methane emissions that have implications for climate-feedback mechanisms.