Radiometrically determined dates and sedimentation rates for recent sediments in nine North African wetland lakes (the CASSARINA Project)

Sediment cores were collected from nine wetland lakes in Morocco, Tunisia and Egypt for the CASSARINA project investigating environmental change in Northern African wetlands. The cores were dated radiometrically by using natural (²¹⁰Pb) and artificial (¹³⁷Cs and ²⁴¹Am) radionuclides. At sites in Morocco and Tunisia with mean annual rainfall totals ranging from 500–1000 mm yr^–1, fallout records were generally satisfactory and it was possible to develop independent sediment chronologies based on the radiometric data alone. At the Egyptian sites, rainfall was less than 200 mm yr^–1 and fallout records were much less distinct. At these sites the radiometric data could only be used to give an indication of mean sedimentation rates during the past 30–40 years. By using a combination of fallout radionuclide, pollen, and macrofossil stratigraphic records it was however possible to determine a credible sediment chronology spanning the major part of the 20th century. Applying this chronology to records of spheroidal carbonaceous particles (SCP) from the same sediment cores, the onset of significant levels of atmospheric pollution in the Nile Delta is dated in all three cores to the mid 1950s. Results from a number of lakes (Sidi Bou Rhaba, Ichkeul and Korba) revealed high and accelerating siltation rates, threatening their continued existence beyond the next few decades. In contrast, sedimentation rates at all three Nile Delta sites appear to have declined in recent decades, most probably due to the impact of the Nile barrages.     

The inventories of anthropogenic Ph,Zn, As,Cd, and the radionuclides 137Cs and excess 210Pb in lake sediments of the Adirondack region,USA

We determined the inventories of four anthropogenic trace elements, Pb, Zn, As, and Cd, and two radionuclides, ¹³⁷Cs and excess ²¹⁰Pb, in sediment cores collected from eight remote lakes in the Adirondack region of the northeastern United States. The inventories of all six substances vary considerably among the sediment cores, although the lakes and their associated catchments must have received similar cumulative per unit area atmospheric inputs of these substances. These variations are highly correlated, indicating that the trace elements and radionuclides are affected in a coherent way by the processes controlling their deposition to the sediments of these lakes. Assuming that the anthropogenic trace element inventories in each sediment core are enhanced or depleted relative to cumulative atmospheric deposition to the extent indicated by the sediment core inventories of either ¹³⁷Cs or excess ²¹⁰Pb, we produced estimates of the cumulative atmospheric inputs of the four anthropogenic trace elements to this region. Comparison of the excess ²¹⁰Pb normalized anthropogenic Pb, Zn, Cd inventories of the Adirondack sediment cores with excess ²¹⁰Pb normalized inventories of the same substances in a group of South Central Ontario lakes suggests that the Adirondack region has received greater cumulative anthropogenic inputs of Pb, Zn and Cd by a factor of 1.6, 4.5, and 2.9, respectively.     

137Cs as a tracer of recent sedimentary processes in Lake Michigan

To determine recent sediment movement, we measured the levels of ¹³⁷Cs (an artificial radionuclide produced during nuclear weapons testing) of 118 southern Lake Michigan samples and 27 in Green Bay. These samples, taken from 286 grab samples of the upper 3 cm of sediment, were collected in 1975 as part of a systematic study of Lake Michigan sediment. ¹³⁷Cs levels correlated well with concentrations of organic carbon, lead, and other anthropogenic trace metals in the sediment. ¹³⁷Cs had a higher correlation with silt-sized than with clay-sized sediment (0.55 and 0.46, respectively). Atmospherically derived ¹³⁷Cs and trace metals are being redistributed by sedimentary processes in Lake Michigan after being incorporated in suspended sediment. We determined a distribution pattern of ¹³⁷Cs that represents areas of southern Lake Michigan where sediment deposition is occurring.     

Changing suspended sediment and particulate phosphorus loads and pathways in underdrained lowland agricultural catchments; Herefordshire and Worcestershire,U.K.

Data obtained from a small (150 ha) experimental catchment, Hereford U.K., have shown that land drains contributed >50% of the total catchment suspended sediment (SS) yield over a two year period. In one of the monitored drains, annual sediment yields were 964 and 978 kg ha^–1. Particulate phosphorus (PP) contributed >60% of total phosphorus lost through the drains and at least 73% of the total drain load came from topsoil. A major question that was not answered by the monitoring programme was whether SS and PP loads had increased since the drains had been installed between the 1960s and 1980s. To address this problem, a sediment yield record was reconstructed from the Kyre Pool catchment in Worcestershire that has a similar drainage history and soil types. Reservoir sediments were dated using the ²¹⁰Pb crs method and results suggest a fourfold increase in SS yield (300–1170 kg ha^–1 yr^–1) and PP loads (0.19–0.79 kg ha^–1 yr^–1) since the 1960s. The most recent SS and PP loads are comparable to those obtained from catchment monitoring. The high lake sediment ¹³⁷Cs inventory suggests a significant influx of eroded topsoil and ¹³⁷Cs activities in the most recent lake sediments are comparable to those of monitored land drains, supporting the hypothesis that land drainage has had a major impact on SS and PP yields and pathways.     

Relations between resuspension and the content of 137Cs in freshwater fish in some Swedish lakes

In many Swedish lakes, the fallout of ¹³⁷Cs from the Chernobyl accident in April 1986 has largely accumulated in the sediments. The availability and transfer of deposited ¹³⁷Cs to biota is influenced by factors such as resuspension. The frequency of resuspension and the ¹³⁷Cs-content of different fish species was studied in three shallow lakes in Uppland, central Sweden, and in one deeper lake in northern Sweden. Resuspension was measured by the use of sediment traps. Sedimentation rates measured from the traps in the shallower lakes were 5–10 times higher than normal for this type of lake, indicating that resuspension was an important factor. The decrease of the ¹³⁷Cs-content in muscle tissue of pike, perch and roach was slow in each of the shallow lakes. ¹³⁷Cs decreased by about 30% over a period of 2 years in the shallowest lake (maximum depth 4 m), whereas ¹³⁷Cs decreased by 50% in the deeper lakes (maximum depth 10 m). The slower rates of decline of ¹³⁷Cs in biota from the shallow lakes, are probably a function of sediment dynamics (mainly influenced by lake morphometry, wind direction and strength). They may be influenced, also, by bioavailability of resuspended sediment material. In the deepest northern lake, much of the ¹³⁷Cs-containing material collected in the sediment traps originated from the catchment area. Resuspension was minimal, and the high activity of ¹³⁷Cs in the sediment had no effect on content or decline of ¹³⁷Cs in lake fish.     

湛江高桥红树林湿地有机碳分布及埋藏特征

红树林是世界上单位生产力最高的生态系统之一,其能够持续地固定有机碳,对全球碳平衡和生物地球化学循环有着深远影响。以广东湛江国家级红树林自然保护区高桥核心区为研究区,旨在分析我国典型红树林湿地的固碳潜力,为红树林湿地碳计量提供依据。在垂直于海岸线的两条样线上选取6个不同潮位的样点进行沉积柱取样分析,通过重铬酸钾氧化-外加热法测定有机碳含量,基于放射性同位素~(210)Pb定年推演沉积率,并对湿地有机碳密度和埋藏率进行计算。结果表明:研究区红树林湿地有机碳含量2.14—36.94 g/kg,平均(12.79±9.91)g/kg。红树林湿地有机碳密度为(0.0100±0.0056)g/cm3,空间上差异显著显著,水平方向上两条样线均以中带样点的有机碳密度最大,近陆侧(内带)样点的有机碳密度高于近海侧(外带);垂直方向上,内带和外带样柱的有机碳密度均以表层最高,而且随深度增加而减小。研究区红树林湿地百年尺度上沉积率为6.5—11mm/a,且外带样点沉积速率显著快于内带样点。有机碳埋藏率空间差异大,外带样点为(34.58±7.67)g m-2a-1,而中带样点可达150.56 g m-2a-1。红树林湿地有机碳的分布受潮位的影响大,更高潮位点和表层的有机碳含量和密度更高,而处于低潮位的外带样点的有机碳沉积更快。研究区红树林湿地有机碳含量和密度比更低纬度带低,但均高于地带性陆地植被,且其能够通过持续的沉积过程来捕捉和固定有机碳,固碳潜力大。     

The recent palaeolimnology of Lake Nicholls,Mount Field National Park,Tasmania

Analyses were carried out for diatoms, spherical carbonaceous particles, and magnetic minerals on a short sediment core from a small cirque lake, Lake Nicholls, in the Mount Field National Park, south-west Tasmania.Catchment disturbance is not evident from the fairly constant sediment accumulation rates and magnetic analyses of the sediment record. However, there is evidence for a low level of atmospheric contamination during the industrial period, which reaches a maximum at the present time, but is less than in many areas of north-west Europe. Changes in the diatom assemblages recorded in the sediment core are likely to be related to factors, other than catchment change or atmospheric contamination, such as the response of within lake processes to climate change.Several diatom taxa found in the cores cannot presently be assigned to known species. Although some of these may later be identified following this preliminary study, it seems highly likely that freshwater lakes in south-west Tasmania contain a number of new and perhaps endemic taxa.     

The use of fallout 137Cs and 239,240Pu for dating of lake sediments

The distribution of ¹³⁷Cs and ^239,240Pu in sediment core samples of the Finnish lakes Laukunlampi, Lovojärvi and Pääjärvi were determined. The sediment samples were collected using dry ice and liquid nitrogen freezing methods. The sediments of these lakes are annually laminated. A clear maximum concentration of ¹³⁷Cs and ^239,240Pu was found in sediment layers formed during 1962–1964, the years of maximum fallout, and the middle of the 1950's can be estimated from the ¹³⁷Cs and ^239,240Pu profiles. The highest concentrations, 11 500 and 820 pCi kg^–1 dry wt for ¹³⁷Cs and ^239,240Pu, respectively, were found in the sediment of Laukunlampi. The vertical distribution was similar for ¹³⁷Cs and ^239,240Pu in the lakes investigated. A slight migration of ^239,240Pu and ¹³⁷Cs was found and the migration of ¹³⁷Cs seems to be higher than that of ^239,240Pu. The advantages of ¹³⁷Cs dating method are rapidity and simplicity. ^239,240Pu is preferable when the sample size is small. The agreement found between ¹³⁷Cs and ^239,240Pu dates and the annual laminae show that these fallout radio isotopes can be used for dating sediments formed during the past 25 years.     

Cesium-137 sediment depth profiles and inventories in Adirondack Lake sediments

Depth distributions and inventories of¹³⁷Cs (mCi km²) were determined in sediment from several fresh water lakes in the New York State Adirondack Preserve. Included were Big Moose and Darts Lakes, part of the North Branch of the Moose River system, as well as North, Sagamore, South, and Woods Lakes and the seepage pond, Tamarack Lake. Comparisons were made between the¹³⁷Cs inventories in these lakes and large inpoundments in the Adirondacks (Hinkley, Great Sacandaga, Stillwater and Cranberry Lake Reservoirs) and other large impoundments and lakes located in various regions of the U.S., especially Cayuga Lake, Ithaca, NY.None of the Adirondack Lakes had¹³⁷Cs distributions with depth in sediment that closely resembled the deposition pattern of weapons testing as a function of time. All of the natural lakes and small impoundments, including the seepage pond, were found to have significantly lower inventories of¹³⁷Cs than expected; while the large reservoirs were generally enhanced in¹³⁷Cs. We suggest that more than one mechanism may be responsible for the low sediment inventories: for the majority of lakes, flushing of¹³⁷Cs out of the lakes during periods of thermal stratification and ice thaw; and for the seepage pond, remobilization of¹³⁷Cs into the water column due to biological recycling.     

Effects of sulfate concentration in the overlying water on sulfate reduction and sulfur storage in lake sediments

We investigated the effects of sulfate concentration on sulfate reduction and net S storage in lake sediments using³⁴S as a tracer. The water overlying intact sediment cores from the hypolimnion of Mares Pond, MA, was replaced with two Na₂ ³⁴SO₄ solutions at either ambient (70 M) or elevated (260 M) sulfate concentrations. The ³⁴S of the added sulfate was 4974 . Over two months, the net sulfate reduction rate in the ambient sulfate treatment was zero, while the net rate for the high sulfate treatment was 140 moles/m²/d. The water overlying the cores was kept under oxic conditions and the sediment received no fresh carbon inputs, thus the net rate reported may underestimate the in situ rate. Gross sulfate reduction rates calculated by isotope dilution were approximately 350 moles/m²/d for both treatments. While the calculation of gross sulfate reduction rates in intact sediment cores can be complicated by differential diffusion of³⁴S and³²S, isotopic fractionation, and the possible formation of ester sulfates, we believe these effects to be small. The results suggest that sulfate reduction is not strongly sulfate-limited in Mares Pond. The difference in net sulfate reduction rates between treatments resulted from a decrease in sulfide oxidation and suggests the importance of reoxidation in controlling net S storage in lake sediments. In both treatments the CRS and organic S fractions were measurably labelled in³⁴S. Below the sediment surface, the CRS fraction was the more heavily labelled storage product for reduced sulfides.     

Material transfer from water to sediment in Florida lakes

We describe statistical relationships between chemical aspects of surficial sediments from 34 Florida lakes, and trophic state of the overlying waters. Trophic state is expressed by Carlson's TSI-Chl a. The objectives of the study are two-fold: 1) to understand processes that govern the transfer of material across the mudwater interface, and 2) to develop transfer functions for inferring historical TSI measures in chemically analyzed, ²¹⁰Pb-dated cores. Simple regression of organic matter content or nutrient (C, N, P) concentration in surface sediments vs. TSI yields nonsignificant or weak positive correlations. However, using a novel application of ²¹⁰Pb assay, net accumulation rates of the materials are estimated and show a better correlation with trophic state. Cation (Ca, Mg, Fe, K) and sulfur concentrations in surface muds are poorly related to corresponding TSI's. Net accumulation rates of these elements (Ca, Mg, Fe, K, S) are positively correlated (P < 0.01) with TSI-Chl a. Chemical data from the Florida surficial sediment survey suggest that inferred levels of past trophic state should be based on net accumulation rates of chemical constituents rather than their simple sediment concentrations.     

The last 1000 years of natural and anthropogenic low-oxygen bottom-water on the Louisiana shelf, Gulf of Mexico

The relative abundance of three species of low-oxygen tolerant benthic foraminifers, the PEB index, in foraminiferal assemblages from sediment cores is used to trace the history of low-oxygen bottom-water conditions on the Louisiana shelf. Analyses of a network of box cores indicate that the modern zone of chronic seasonal hypoxia off the Mississippi Delta began to develop around 1920 and was well established by 1960. The pattern of development over the last century is consistent with the interpretation that the formation of modern chronic hypoxia is related to anthropogenic activities resulting in increased transport of nutrients to the Louisiana shelf.The PEB index in two gravity- and box core pairs (MRD05-4 and 05-6) indicates that low-oxygen bottom-water events have occurred periodically on the Louisiana Shelf for at least the last 1000 ¹⁴C years. The pre-1900 low-oxygen bottom-water events are likely caused by intervals of increased Mississippi River discharge and widespread wetland export. The PEB record in gravity cores indicates that the pre-1900 low-oxygen bottom-water events were not as well developed or as geographically extensive as the modern hypoxia zone. We conclude that the development of low-oxygen bottom-water on the Louisiana shelf is a natural process that has been negatively modified by human activities in the last 100 years.     

Yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.) in Peru

Two field experiments were performed to evaluate the nitrogen fixation potential of twenty common bean cultivars and breeding lines during summer and winter seasons of 1986 and 1988, respectively. The ¹⁵N isotope dilution method was used to quantify N₂ fixation. The cultivars and breeding lines were variable in terms of their N₂ fixation. The cv. Caballero was very efficient, with more than 50% N derived from the atmosphere and 60–80 kg N ha^–1 fixed in both seasons. Other cultivars were less efficient, since the poorest ones derived less than 30% of their nitrogen from the atmosphere and fixed less than 20 kg N ha^–1. After additional testing the best cultivars may be used directly by the farmers for cultivation. The experiments have provided information about which genotypes may be used to breed for enhanced fixation in common bean.     

Sediment supply and accumulation in a small Moroccan lake: an historical perspective

Dayat-er-Roumi is a small lake (73 ha) located in the sub-humid region of NE Morocco. Lake sediment and catchment soils are examined to identify areas of sediment supply to the lake. Magnetic measurements and clay mineralogy are used to show that recent lake sediment is derived predominantly from eroded topsoil. ²¹⁰Pb dating of one sediment core provided a chronology for the calculation of sediment accumulation rates. The rate of sediment accumulation increased slowly throughout the earlier part of the 20th century and then rapidly in the mid-1970's. These increases are attributed to human disturbances in the catchment, initially by olive planting and then by drainage of a wetland area.     

Microbial Response to Organic Matter Enrichment in the Oligotrophic Levantine Basin (Eastern Mediterranean)

The response of a deep-water benthic microbial community to organic matter (OM) enrichment was studied in the unexplored region of the SW Cretan margin (E. Mediterranean). A food pulse of 0.5 g C m^?2 was simulated by adding ¹³C-labelled diatoms to sediment cores retrieved from 1079 m depth. The diatom addition resulted in a significant increase in the sediment community oxygen consumption (SCOC). After 6 days, ～50 mg C m^?2 of the added material was processed by the microbial community. The major carbon sink was respiration, which accounted for ～96% of the total processed material. The carbon uptake rate (12 mg C m^?2 d^?1) was considerably lower than previously published values in the E. Mediterranean at similar depths. The microbial community in our study site is distinct, as revealed by the unusually high presence of branched phospholipid fatty acids (PLFA). Previous studies have revealed that the slope under investigation may act as a conduit of OM from the shallow shelf to the deep basins, resulting in the prevalence of relatively refractory OM at mid-slope depths. We postulate that sedimentary processes affect the amount of bioavailable sedimentary OM and consequently the structure and physiological state of bacterial community in our study site. The distinct microbial community composition at our station compared to more stable adjacent slopes could explain the limited response of the microbial community to the addition of labile OM. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.     

Biodiversity of benthic invertebrates and organic matter processing in shallow marine sediments: an experimental study

The main objective of this study was to measure the impact of benthic invertebrate diversity on processes occurring at the water-sediment interface. We analyzed the effects of interactions between three shallow water species (Cerastoderma edule, Corophium volutator, and Nereis diversicolor). The impacts of different species richness treatments were measured on sediment reworking, bacterial characteristics, and biogeochemical processes (bromide fluxes, O₂ uptake, nutrient fluxes, and porewater chemistry) in sediment cores. The results showed that the three species exhibited different bioturbation activities in the experimental system: C. edule acted as a biodiffusor, mixing particles in the top 2 cm of the sediments; C. volutator produced and irrigated U-shaped tubes in the top 2 cm of the sediments; and N. diversicolor produced and irrigated burrow galleries in the whole sediment cores. C. edule had minor effects on biogeochemical processes, whereas the other species, through their irrigation of the burrows, increased the solute exchange between the water column and the sediment two-fold. These impacts on sediment structure and solute transport increased the O₂ consumption and the release of nutrients from sediments. As N. diversicolor burrowed deeper in the sediment than C. volutator, it irrigated a greater volume of sediments, with great impact on the sediment cores.Most treatments with a mixture of species indicated that observed values were often lower than predicted values from the addition of the individual effects of each species, demonstrating a negative interaction among species. This type of negative interaction measured between species on ecosystem processes certainly resulted from an overlap of bioturbation activities among the three species which lived and foraged in the same habitat (water-sediment interface). All treatments with N. diversicolor (in isolation and in mixture) produced similar effect on sediment reworking, water fluxes, nutrient releases, porewater chemistry, and bacterial characteristics. Whichever species associated with N. diversicolor, the bioturbation activities of the worm hid the effect of the other species. The results suggest that, in the presence of several species that use and modify the same sediment space, impact of invertebrates on ecosystem processes was essentially due to the most efficient bioturbator of the community (N. diversicolor). In consequence, the functional traits (mode of bioturbation, depth of burrowing, feeding behaviour) of an individual species in a community could be more important than species richness for some ecosystem processes.     

Effects of filamentous algal mats on sulfide invasion in eelgrass (Zostera marina)

The effect of filamentous algae invasion into Zostera marina meadows on water quality, sediment sulfur pools and sulfide invasion into plant tissues was studied experimentally. Sulfide invasion was assessed through analysis of sulfur isotopic composition (δ³⁴S) and total sulfur (TS) concentrations in plant tissues. The algal mats (5 and 10 cm thickness) depleted oxygen in the mats and increased the pools of sulfides in the sediments. Plants exposed to algal mats had δ³⁴S signals closer to the δ³⁴S of sediment sulfide, whereas plants with no mats present had δ³⁴S signals closer to the δ³⁴S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to algal mats. The δ³⁴S varied between the plant tissues with the leaves having more positive δ³⁴S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS concentrations were higher in plants exposed to algal mats suggesting that sulfur derived from sediment sulfide accumulated in the plants. F_sulfide showed that up to 50% of the sulfides in the plants were derived from sedimentary sulfides. The combined effect of water column anoxia in the lower parts of the meadow and high sulfide invasion into the plants lead to significantly reduced growth rates after 3 weeks and the below-ground tissues showed signs of degradation suggesting that algal mats invasion in to Zostera marina meadows can result in seagrass decline.     

Recent Increases in Sediment and Nutrient Accumulation in Bear Lake, Utah/Idaho, USA

This study examines historical changes in sediment and nutrient accumulation rates in Bear Lake along the northeastern Utah/Idaho border, USA. Two sediment cores were dated by measuring excess ²¹⁰Pb activities and applying the constant rate of supply (CRS) dating model. Historical rates of bulk sediment accumulation were calculated based on the ages within the sediment cores. Bulk sediment accumulation rates increased throughout the last 100 years. According to the CRS model, bulk sediment accumulation rates were <25mgcm^?2year^?1 prior to 1935. Between 1935 and 1980, bulk sediment accumulation rates increased to approximately 40mgcm^?2year^?1. This increase in sediment accumulation probably resulted from the re-connection of Bear River to Bear Lake. Bulk sediment accumulation rates accelerated again after 1980. Accumulation rates of total phosphorus (TP), total nitrogen (TN), total inorganic carbon (TIC), and total organic carbon (TOC) were calculated by multiplying bulk sediment accumulation rates times the concentrations of these nutrients in the sediment. Accumulation rates of TP, TN, TIC, and TOC increased as a consequence of increased bulk sediment accumulation rates after the re-connection of Bear River with Bear Lake.     

In vitro culture response of apical and axillary shoot-tips excised from crisphead lettuce cores depends on head maturity, not storage time

Summary Successful lettuce (Lactuca sativa L.) breeding requires assessment, selection, and reseue of germplasm from field-grown plants during a limited period of time. Tissue culture methods have proved successful in rescuing selected lettuce genotypes and producing seeds in a disease-free environment. The present study was conducted to determine the response of the apical and axillary shoot-tips of lettuce to storage duration prior to in vitro culture. The study focused on core length as a source of variation between maternal explants. Advanced maturity was obtained by applying an agrotextile row cover over lettuce plants. Apical and axillary buds were derived from covered lettuce with long cores (46±0.8 mm) and from non-covered lettuce with short cores (30±0.7 mm). The cores were stored at 2°C for 0,2, or 13d. Apical and axillary shoot-tips were removed from the cores and placed onto MS medium. After 3 mo. of growth culture, twice as many leaf primordia (6.6 leaves) developed from the apical shoot-tip as compared to those from the axillary shoot-tip (3.0 leaves). More shoot-tips survived when excised from short cores (83%) than when excised from long cores (71%). Root formation was affected by the source of shoot-tip and the size of donor explant. Three times more explants from apical shoot-tips and short cores rooted (71%) than did explants from apical shoot-tips and long cores (27%) or explants from axillary shoot-tips (long cores, 14%; short cores, 19%). Storage of cores for up to 13 d had no effect on the survival rate of shoot-tips, nor on shoot or root formation, suggesting that cores of lettuce plants can be stored prior to in vitro micropropagation without significant loss of regeneration efficiency.     

Organic matter rain rates, oxygen availability, and vital effects from benthic foraminiferal δC in the historic Skagerrak, North Sea

The sediment cores 225514 and 225510 were recovered from 420 and 285 m water depth, respectively. They were investigated for their benthic foraminiferal δ¹³C during the last 500 years. Both cores were recovered from the southern flank of the Skagerrak. The δ¹³C values of Uvigerina mediterranea and other shallow infaunal species in both cores indicate that organic matter rain rates to the seafloor varied around a mean value until approximately AD 1950 after which they increased. This increase might result from changes in the North Atlantic Current System and a co-occurring persistently high North Atlantic Oscillation index state in the 1980s to 1990s, rather than from anthropogenic eutrophication. Using δ¹³C mean values of multiple species, we reconstruct δ¹³C gradients of dissolved inorganic carbon (DIC) within pore waters for the time periods AD 1500 to 1950 and AD 1950 to 2000. The calculated δ¹³C_DIC ranges, interpreted as indicating total organic matter remineralization due to respiration, are generally bigger in Core 225514 than in Core 225510. Since mean δ¹³C values of U. mediterranea suggest that organic matter rain rates were similar at both locations, differences in total organic matter remineralization are attributed to differing oxygen availability. However, oxygen concentrations in the overlying bottom water masses are not likely to have differed significantly. Thus, we suggest that organic matter remineralization was controlled by oxygen availability within the sediments, reflecting strong differences in sedimentation rates at the two investigated core sites. Based on the assumptions that tests of benthic foraminiferal species inhabiting the same microhabitat depth should show equal δ¹³C values unless they are affected by vital effects and that Globobulimina turgida records pore water δ¹³C_DIC, we estimate microhabitat-corrected vital effects for several species with respect to G. turgida: > 0.7‰ for Cassidulina laevigata, > 1.3‰ for Hyalinea balthica, and > 0.7‰ for Melonis barleeanus. Melonis zaandami seems to closely record pore water δ¹³C_DIC.