Response of a Lake Michigan coastal lake to anthropogenic catchment disturbance

A paleolimnological investigation of post-European sediments in a Lake Michigan coastal lake was used to examine the response of Lower Herring Lake to anthropogenic impacts and its role as a processor of watershed inputs. We also compare the timing of this response with that of Lake Michigan to examine the role of marginal lakes as ‘early warning’ indicators of potential changes in the larger connected system and their role in buffering Lake Michigan against anthropogenic changes through biotic interactions and material trapping. Sediment geochemistry, siliceous microfossils and nutrient-related morphological changes in diatoms, identified three major trophic periods in the recent history of the lake. During deforestation and early settlement (pre-1845–1920), lake response to catchment disturbances results in localized increases in diatom abundances with minor changes in existing communities. In this early phase of disturbance, Lower Herring Lake acts as a sediment sink and a biological processor of nutrient inputs. During low-lake levels of the 1930s, the lake goes through a transitional period characterized by increased primary productivity and a major shift in diatom communities. Post-World War II (late 1940s–1989) anthropogenic disturbances push Lower Herring Lake to a new state and a permanent change in diatom community structure dominated by Cyclotella comensis . The dominance of planktonic summer diatom species associated with the deep chlorophyll maximum (DCM) is attributed to epilimnetic nutrient depletion. Declining Si:P ratios are inferred from increased sediment storage of biogenic silica and morphological changes in the silica content of Aulacoseira ambigua and Stephanodiscus niagarae . Beginning in the late 1940s, Lower Herring Lake functions as a biogeochemical processor of catchment inputs and a carbon, nutrient and silica sink. Microfossil response to increased nutrients and increased storage of biogenic silica in Lower Herring Lake and other regional embayments occur approximately 20–25 years earlier than in a nearby Lake Michigan site. Results from this study provide evidence for the role of marginal lakes and bays as nutrient buffering systems, delaying the impact of anthropogenic activities on the larger Lake Michigan system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43091/1/10933_2004_Article_1688.pd

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m −3 could have occurred even though 90% or more of the biogenic silica production is recycled.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42471/1/10533_2004_Article_BF02187199.pd

Labile organic carbon dynamics in continental shelf sediments after the recent collapse of the Larsen ice shelves off the eastern Antarctic Peninsula: A radiochemical approach

17 pages, 6 figures, 2 tables, 1 supplementary tableLabile organic carbon (LOC) dynamics (i.e., of recently produced, planktonic material) and sediment dynamics were studied in the seabed using naturally occurring C and Pb measurements in the region where the Larsen Ice Shelves A and B were floating almost two decades ago. A non-steady-state diagenetic model was used to estimate sediment mixing coefficients as well as LOC fluxes to the seabed and LOC turnover times (i.e., mean residence times) in a suite of 14 sediment cores from the continental shelf, including a glacial trough. At four of the stations, cores were collected during 2007 and 2011 cruises, enabling a time-series approach for understanding the evolution of sedimentary processes and LOC dynamics in the deposits below a collapsed ice shelf. Sediment mixing coefficients, based on non-steady-state Pb profiles, varied between 0.01 cm y and 1.6 cm y in these post-ice shelf sediments. These values were similar to those found in polar deep-sea environments, where sedimentary conditions are less dynamic than in shallower provinces. LOC, whose abundance decreased uniformly with depth, was detected to depths ranging from 2 to 16 cm, with LOC seabed inventories varying from 1.5 to 22 mg LOC cm. Excess Pb and LOC fluxes were relatively uniform across the study area suggesting that similar particle fluxes have taken place within the Larsen system since the disintegration of the various ice shelves. The LOC mean residence time at the different stations varied from 3 y to >60 y. The C approach, calculating LOC content based on a two-end member model with planktonic C as the labile carbon end member, most closely correlated with the lipid content of the sediment, which has been considered one of the best descriptors of reactive organic matter readily available to benthic consumers. We suggest that the irregular combination of sea ice coverage, organic matter production and supply to the sea floor introduce scatter in the determination of sediment and LOC dynamics such that short-term temporal (<5 years) and spatial trends could not be readily resolvedThis study was funded by the Spanish Ministry of Economy and Competitiveness through the project CLIMANT (POL2006-06399/CGL; E.I., Principal Investigator) and by the National Science Foundation of the USA (PLR-1341669; D.D., Principal Investigator)Peer Reviewe

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

18 pages, 10 figures, 5 tables, supplementary material https://doi.org/10.1016/j.gca.2021.04.021Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literatureThis work was funded by the Spanish Ministry of Economy and Competitiveness as part of the project ECOWED (CTM2012-39350-C02-01)With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Labile organic carbon dynamics in continental shelf sediments after the recent collapse of the Larsen ice shelves off the eastern Antarctic Peninsula: A radiochemical approach

17 pages, 6 figures, 2 tables, 1 supplementary tableLabile organic carbon (LOC) dynamics (i.e., of recently produced, planktonic material) and sediment dynamics were studied in the seabed using naturally occurring C and Pb measurements in the region where the Larsen Ice Shelves A and B were floating almost two decades ago. A non-steady-state diagenetic model was used to estimate sediment mixing coefficients as well as LOC fluxes to the seabed and LOC turnover times (i.e., mean residence times) in a suite of 14 sediment cores from the continental shelf, including a glacial trough. At four of the stations, cores were collected during 2007 and 2011 cruises, enabling a time-series approach for understanding the evolution of sedimentary processes and LOC dynamics in the deposits below a collapsed ice shelf. Sediment mixing coefficients, based on non-steady-state Pb profiles, varied between 0.01 cm y and 1.6 cm y in these post-ice shelf sediments. These values were similar to those found in polar deep-sea environments, where sedimentary conditions are less dynamic than in shallower provinces. LOC, whose abundance decreased uniformly with depth, was detected to depths ranging from 2 to 16 cm, with LOC seabed inventories varying from 1.5 to 22 mg LOC cm. Excess Pb and LOC fluxes were relatively uniform across the study area suggesting that similar particle fluxes have taken place within the Larsen system since the disintegration of the various ice shelves. The LOC mean residence time at the different stations varied from 3 y to >60 y. The C approach, calculating LOC content based on a two-end member model with planktonic C as the labile carbon end member, most closely correlated with the lipid content of the sediment, which has been considered one of the best descriptors of reactive organic matter readily available to benthic consumers. We suggest that the irregular combination of sea ice coverage, organic matter production and supply to the sea floor introduce scatter in the determination of sediment and LOC dynamics such that short-term temporal (<5 years) and spatial trends could not be readily resolvedThis study was funded by the Spanish Ministry of Economy and Competitiveness through the project CLIMANT (POL2006-06399/CGL; E.I., Principal Investigator) and by the National Science Foundation of the USA (PLR-1341669; D.D., Principal Investigator)Peer Reviewe

A Shift in the Biogenic Silica of Sediment in the Larsen B Continental Shelf, Off the Eastern Antarctic Peninsula, Resulting from Climate Change

5 pages, 4 figuresIn 2002, section B of the Larsen ice shelf, off of the Eastern Antarctic Peninsula, collapsed and created the opportunity to study whether the changes at the sea surface left evidence in the sedimentary record. Biogenic silica is major constituent of Antarctic marine sediment, and its presence in the sediment column is associated with diatom production in the euphotic zone. The abundance of diatom valves and the number of sponge spicules in the biogenic silica was analyzed to determine how the origin of the biogenic silica in the upper layers of the sediment column responded to recent environmental changes. Diatom valves were present only in the upper 2 cm of sediment, which roughly corresponds to the period after the collapse of the ice shelf. In contrast, sponge spicules, a more robust form of biogenic silica, were also found below the upper 2 cm layer of the sediment column. Our results indicate that in this region most of the biogenic silica in the sedimentary record originated from sponge spicules rather than diatoms during the time when the sea surface was covered by the Larsen ice shelf. Since the collapse of the ice shelf, the development of phytoplankton blooms and the consequent influx of diatom debris to the seabed have shifted the biogenic silica record to one dominated by diatom debris, as occurs in most of the Antarctic marine sediment. This shift provides further evidence of the anthropogenic changes to the benthic habitats of the Antarctic and will improve the interpretation of the sedimentary record in Polar Regions where these events occur. © 2013 Sañé et al.Peer Reviewe

Sediment accumulation rates at the edge of the Atlantic: relationships among sea ice, water current and sea floor relief

Scientific Committee on Antarctic Research Open Science Conference (SCAR 2020), 3-7 August 2020Sediment accumulation rates were studied at the southernmost Atlantic Ocean in the vicinities of the Filchner Trough. This is an important region for water mass formation and a marine mammal feeding spot, presumably following high primary production. The region also experiences contrasting sea ice patterns. In spring and summer there are open water conditions at the east of the trough, whereas at the west, the sea surface usually remains covered by multiyear sea ice. Earlier studies showed that sedimentary organic carbon and biogenic silica were more concentrated on the eastern flank of the trough. We attempted to study whether sediment accumulation follow this pattern and its implication for the long-term (hundreds of years) carbon storage in the sediment column. We analyzed 16, 20-cm sediment cores along the axis of the Filchner Trough and the adjacent continental shelf and slope. Sediment accumulation rates (SAR) varied from 8 to 128 cm ky-1. The highest SAR were found on the deepest parts of the axis of the trough and the shelf and slope of its eastern flank, whereas the smallest SAR were found in the shelf and slope of the western flank and at the mouth of the trough. Preliminarily, SAR values matched with the sea-ice pattern and the water current system, showing high values in areas with seasonal open water conditions and the inflow of deep water onto the shelf and low values in areas where multiyear sea ice persists and the outflow of dense cold water towards the deep Weddell Se

Using Radiocarbon to Assess the Abundance, Distribution, and Nature of Labile Organic Carbon in Marine Sediments

19 pages, 4 figures, 1 table, supporting information https://doi.org/10.1029/2020GB006676.-- Data Availability Statement: Original analytical data from this study have been submitted to the following FAIR-aligned data repository (US Antarctic Program Data Center; Dataset ID: 601319; DOI: 10.15784/601319). However, all of this information is provided in the Supplemental Data section of this paper as well (Tables S1 and S2 and Figure S1)Positive 14C gradients have recently been observed within the surface mixed layer of several continental-margin sediments. The best explanation for these positive 14C gradients is the occurrence and rapid degradation of labile organic carbon (LOC) in the upper 5–10 cm of the seabed. Based on a two-component model for sedimentary organic matter (i.e., a planktonic labile component and an older refractory component), bulk 14Corg data were used to determine the abundances of LOC within the surface mixed layers of three cores from the West Antarctic Peninsula (WAP) shelf and one core from San Clemente Basin (California Borderland). LOC contents in surface samples from the four stations varied from 0.5 to 1.1 mg/cm3, comprising 20% (San Clemente Basin) to 80% (WAP, Sta. G) of the total organic carbon. By incorporating a steady state diagenetic model and particle-mixing bioturbation coefficients, the LOC profiles were used to determine LOC turnover times (LOC τ) and LOC e-folding depths. The LOC τ values for the West Antarctic Peninsula sediments varied from 0.09 to 0.59 years, whereas the LOC τ value from the San Clemente Basin core was 63 years. The LOC e-folding depths for the WAP stations varied from 0.8 to 3.4 cm, in contrast to the LOC e-folding depth in San Clemente Basin, which was 4.0 cm. LOC characteristics from the four cores examined in this study were compared to LOC data in the literature as a means of substantiating the overall 14Corg-based approach and justifying model assumptionsThe FOODBANCS-2 Project was funded by the National Science Foundation of the USA, Office of Polar Programs (ANT-0636773; C. R. Smith and D. J. DeMaster, PIs, C. J. Thomas, Co-PI). Support for E. Isla was from the Spanish Ministry of Economy and Competitiveness through the project CLIMANT (POL2006-06399/CGL)With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)Peer reviewe

LOC Manuscript for Global Biogeochemical Cycles Using radiocarbon to assess the abundance, distribution, and nature of labile organic carbon in marine sediments

19 pages, 4 figures, 1 table, supporting information https://doi.org/10.1029/2020GB006676.-- Data Availability Statement: Original analytical data from this study have been submitted to the following FAIR-aligned data repository (US Antarctic Program Data Center; Dataset ID: 601319; DOI: 10.15784/601319). However, all of this information is provided in the Supplemental Data section of this paper as well (Tables S1 and S2 and Figure S1)Positive 14C gradients have recently been observed within the surface mixed layer of several continental-margin sediments. The best explanation for these positive 14C gradients is the occurrence and rapid degradation of labile organic carbon (LOC) in the upper 5–10 cm of the seabed. Based on a two-component model for sedimentary organic matter (i.e., a planktonic labile component and an older refractory component), bulk 14Corg data were used to determine the abundances of LOC within the surface mixed layers of three cores from the West Antarctic Peninsula (WAP) shelf and one core from San Clemente Basin (California Borderland). LOC contents in surface samples from the four stations varied from 0.5 to 1.1 mg/cm3, comprising 20% (San Clemente Basin) to 80% (WAP, Sta. G) of the total organic carbon. By incorporating a steady state diagenetic model and particle-mixing bioturbation coefficients, the LOC profiles were used to determine LOC turnover times (LOC τ) and LOC e-folding depths. The LOC τ values for the West Antarctic Peninsula sediments varied from 0.09 to 0.59 years, whereas the LOC τ value from the San Clemente Basin core was 63 years. The LOC e-folding depths for the WAP stations varied from 0.8 to 3.4 cm, in contrast to the LOC e-folding depth in San Clemente Basin, which was 4.0 cm. LOC characteristics from the four cores examined in this study were compared to LOC data in the literature as a means of substantiating the overall 14Corg-based approach and justifying model assumptionsThe FOODBANCS-2 Project was funded by the National Science Foundation of the USA, Office of Polar Programs (ANT-0636773; C. R. Smith and D. J. DeMaster, PIs, C. J. Thomas, Co-PI). Support for E. Isla was from the Spanish Ministry of Economy and Competitiveness through the project CLIMANT (POL2006-06399/CGL)With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)Peer reviewe