Detrital Layers Research Articles

Selenium removal and mass balance in a constructed flow-through wetland system.

A field study on the removal of Se from agricultural subsurface drainage was conducted from May 1997 to February 2001 in the Tulare Lake Drainage District (TLDD) of San Joaquin Valley, California. A flow-through wetland system was constructed consisting of ten 15- x 76-m unlined cells that were continuously flooded and planted with either a monotype or combination of plants, including sturdy bulrush [Schoenoplectus robustus (Pursh) M.T. Strong], baltic rush (Juncus balticus Willd.), smooth cordgrass (Spartina alterniflora Loisel.), rabbitsfoot grass [Polypogon monspeliensis (L.) Desf.], salt-grass lDistichlis spicata (L.) Greene], cattail (Typha latifolia L.), tule [Schoenoplectus acutus (Muhl. ex Bigelow) A. Löve & D. Löve], and widgeon grass (Ruppia maritima L.). One cell had no vegetation planted. The objectives of this research were to evaluate Se removal efficiency of each wetland cell and to carry out a mass balance on Se. The inflow drainage water to the cells had average annual Se concentrations of 19 to 22 microg L(-1) dominated by selenate [Se(VI), 95%]. Average weekly water residence time varied from about 3 to 15 d for Cells 1 through 7 (target 7 d), 19 to 33 d for Cells 8 and 9 (target 21 d), and 13 to 18 d for Cell 10 (target 14 d). Average weekly Se concentration ratios of outflow to inflow ranged from 0.45 to 0.79 and mass ratio (concentration x water volume) from 0.24 to 0.52 for year 2000, that is, 21 to 55% reduction in Se concentration and 48 to 76% Se removal in mass by the wetland, respectively. The nonvegetated cell showed the least Se removal both in concentration and in mass. The global mass balance showed that on the average about 59% of the total inflow Se was retained within the cells and Se outputs were outflow (35%), seepage (4%), and volatilization (2%). Independent measurements of the Se retained in the cells totaled 53% of the total Se inflow: 33% in the surface (0-20 cm) sediment, 18% in the organic detrital layer above the sediment, 2% in the fallen litter, < 1% in the standing plants, and < 1% in the surface water. Thus, about 6% of the total Se inflow was unaccounted for in the internal compartments.

Selenium Removal from Irrigation Drainage Water Flowing Through Constructed Wetland Cells with Special Attention to Accumulation in Sediments

A flow-through experimental wetland system has been under investigation since 1996 to remove selenium (Se) fromagricultural drainage water in the Tulare Lake Drainage Districtat Corcoran, California, U.S.A. The system consists of ten cellswhich have dimensions of 15 × 76 m continuously flooded andvarious substrates planted. The objectives of this article are topresent the overall performance in Se removal after establishingthe wetland for three years, and to examine factors affecting Seremoval with special attention to accumulation in the sediments.In 1999, The wetland cells reduced Se from inflow water by 32 to65% in concentration and 43 to 89% in mass. Vegetationplays an important role in Se removal as non-vegetated cellshowed the least removal of Se. The inflow drainage water wasdominated by selenate (Se(VI), 91%) with smaller percentages ofselenite (Se(IV), 7%) and organic Se (org-Se(II-), 2%). Theoutflow water from the cells contained an average of 47% Se(VI),32% Se(IV) and 21% org-Se indicating reduction processesoccurring in the wetland cells. The surface sediment appears as alarge sink of Se removal. The highest Se concentration was foundin fallen litter, followed by the fine organic detrital layer onthe sediment surface. The sediment Se concentration dramaticallydecreased with increasing sediment depth. The mass distribution of Se, however, was sediment (0-20 cm) > fine detrital matter >fallen litter. Fractionation of surface sediment (0-5 cm) reveals that elemental Se was the largest fraction (ave. 47%) followedby organic matter-associated Se (34%). Soluble, adsorbed, and carbonate-associated Se accounted for 1.2, 3.1 and 2.5% ofthe total sediment Se, respectively. The major Se sink mechanism in the cells is the reduction of selenate to elemental Se andimmobilization into the organic phase of the sediments.

Sebkhas as ecological archives and the vegetation and landscape history of southeastern Tunisia during the last two millennia

Sebkhas are temporary lacustrine systems depending on the number and extension of floodings. They may create laminated sediments which can be exploited as ecological archives. Contrary to those of meromictic lakes they are seasonal but not annual. After each flooding a detritus layer and a plasmo-condensed layer of bacteria and algae are formed in the water body or film and during the subsequent dessication a third subaeric layer of evaporites is build up. These laminae show a quasi-textile fabric and they can trap and conserve any dust and fine grained material. A high resolution pollendiagram covering the last two millennia as well as geochemical analysis from a southeastern Tunisian sebkha demonstrate the potentials of these ecological archives.

Short-term changes in phosphorus storage in an oligotrophic Everglades wetland ecosystem receiving experimental nutrient enrichment

Natural, unenriched Everglades wetlands are known to be limited by phosphorus (P) and responsive to P enrichment. However, whole-ecosystem evaluations of experimental P additions are rare in Everglades or other wetlands. We tested the response of the Everglades wetland ecosystem to continuous, low-level additions of P (0, 5, 15, and 30 µ gL −1 above ambient) in replicate, 100 m flow-through flumes located in unenriched Everglades National Park. After the first six months of dosing, the concentration and standing stock of phosphorus increased in the surface water, periphyton, and flocculent detrital layer, but not in the soil or macrophytes. Of the ecosystem components measured, total P concentration increased the most in the floating periphyton mat (30 µ gL −1 : mean = 1916 µ gPg −1 , control: mean = 149 µ gP g −1 ), while the flocculent detrital layer stored most of the accumulated P (30 µ gL −1 : mean = 1.732 g P m −2 , control: mean = 0.769 g P m −2 ). Significant short-term responses of P concentration and standing stock were observed primarily in the high dose (30 µ gL −1 above ambient) treatment. In addition, the biomass and estimated P standing stock of aquatic consumers increased in the 30 and 5 µ gL −1 treatments. Alterations in P concentration and standing stock occurred only at the upstream ends of the flumes nearest to the point source of added nutrient. The total amount of P stored by the ecosystem within the flume increased with P dosing, although the ecosystem in the flumes retained only a small proportion of the P added over the first six months. These results indicate that oligotrophic Everglades wetlands respond rapidly to short-term, low-level P enrichment, and the initial response is most noticeable in the periphyton and flocculent detrital layer.

Chronology of discontinuities and petrology of speleothems as paleoclimatic indicators of the Klamath Mountains, southwest Oregon, USA

Speleothems from Oregon Caves National Monument, a dissolutional cave system located in the Klamath Mountains of southwest Oregon, are composed mainly (>90%) of columnar calcite crystals. Columnar calcites form through syndepositional lateral coalescence of crystallites and precipitate under stable hydrodynamic conditions in humid, temperate climates. Numerous discontinuities from short-lived events (<100 years) punctuate the calcites. Minor fabrics, such as transitional- and randomly-oriented elongate calcites are seeded on these discontinuities and detrital layers. Columnar calcites eventually overrun sub-horizontally oriented crystals as a result of competitive growth. Large (≈750 μm) crystal terminations indicate growth under increased water film resulting from greater flow and/or ponding. 14 Useries dates provide mean growth rates of 5.6 to 27.9 mm/ka during interglacial periods (marine isotope stages 11 through 9, substage 5e, and late stage 2 through stage 1), and indicate short growth intervals during early-to mid-interglacial periods. Several outcrop-scale discontinuities represent periods of prolonged growth stoppage. Growth cessation occurred during the colder phases of stages 8–9 and 6–7, which translate into hiatuses of 77.5 and 99 ka respectively. Given the sensitivity of alpine regions to climate change and the lack of evidence for continental or alpine glaciation, the hiatuses are presumed to be caused by groundwater freezing during extended periglacial conditions, triggered by the lowering of periglacial thresholds by as much as 1800 m during glacial periods.

Phosphorus Loading Effects on Extracellular Enzyme Activity in Everglades Wetland Soils

The impact of P loading to the northern Florida Everglades has been implicated in changing vegetation patterns, peat accretion rates, and other soil physico‐chemical properties. This investigation focused on determining the influence of P loading on the activities of various extracellular enzymes along a P‐enrichment gradient and relating measured enzyme activities to soil physico‐chemical parameters. Alkaline phosphatase activity (APA) was the only enzyme affected by P loading and was negatively related to soil P concentrations and microbial biomass C and P. Arylsulfatase, β‐d‐glucosidase, protease, and phenol oxidase were not affected by P loading and were not related to measured soil C, N, S, and P physical and chemical parameters. All enzyme activities were highest in the surface detritus layer and decreased with soil depth. Due to significant relationships between APA and soil and microbial P parameters, APA appears to be a useful indicator for assessing impacts of P enrichment in wetland soils.

A New Tanarctid Arthrotardigrade with Buoyant Bodies

Four cruises have revealed a very rich tardigrade fauna inhabiting the shell gravel at the Faroe Bank and Bill Bailey Bank in the North Atlantic. This study presents the description of Tanarctus bubulubus sp. n., which is one of the many new tanarctids from the Faroe Bank. The new species is found from 104 m to 200 m and is always in clean carbonate sand to shell gravel. Currently 10 species of Tanarctus are described from various localities around the world. At the Faroe Bank 9 species new to science have been found together with T. gracilis Renaud-Mornant, 1980 and T. heterodactylus Renaud-Mornant, 1980 originally described from the Western Atlantic. T. bubulubus sp. n. (‘the balloon animal’) has been investigated using LM, SEM and TEM and is characterised by sense organs on the fourth pair of legs (P4) which terminate in nine to ten balloons each. The surface of the balloons is adhesive and in whole-mount preparations the balloons attach strongly to the coverslip. The epicuticle of the balloons consists of only a very thin membrane-like layer, which may be folded or smooth, depending of the degree of extension of the balloon. The stalk of P4 is hollow and seems to lack sensory structures. The internal space of the balloon is enclosed with a secondary membrane (inner trilaminary layer?), to which some osmiophilic granulae are attached. The balloon is filled with liquid that may be different from seawater. It is hypothesised that the balloons are used as buoyancy devices, which allow the animal to move easily in the detritus layer on top of the sediment. Live animals are observed upside-down in the water column floating with the head downwards. The dorsal surface of the animal is often covered with coccoliths of the species Emiliania huxleyi, camouflaging the animal in the sediment.

Creation of functional ground models in an urban area

Rationale The ability to see the ground surface in an urban environment is limited by access and the amount of natural ground exposure. This is typically less than 10% by area. The assessment of the probable ground structure and the behaviour of critical ground elements can be achieved by comparison with non-urban environments exhibiting similar landforms. This model can be verified by a detailed study of historic maps and a review of existing local subsurface information. The site The City of Cardiff has largely been constructed in the last 150 years. The area studied relates to the whole city land area lying to the south of the A48T and between the west banks of the rivers Ely and Rhymney. Area model The area lies substantially on the Severn Levels, which comprise a recent tidal coastal margin zone of the Severn estuary. The area consists of a glaciated land surface comprising Mercia Mudstone and Dolomitic Conglomerate draped by a thin and partly discontinuous layer of glacial detritus that includes two identifiable layers of till and a consistent layer of fluvio-glacial sands and gravels which forms the dominant aquifer. These strata reflect Devensian and Flandrian glacial advance and recession. To the south of the historic natural shoreline a series of post-glacial foreshore alluvia are superimposed on the late-glacial surface and form a confining layer to the underlying fluvio-glacial sand and gravel aquifer. These alluvia represent a series of more recent marine transgressions modified by sea defence construction works of Bronze Age to late mediaeval antiquity.

Decomposition responses to phosphorus enrichment in an Everglades (USA) slough

The effects of phosphorus (P) enrichment ondecomposition rates were measured in a Ploading experiment conducted in an oligotrophicmarsh in the northern Everglades, USA. In thisstudy, eighteen 2.5 m2 enclosures(mesocosms) were placed in a pristineopen-water (slough) wetland and subjectedweekly to 6 inorganic P loads; 0, 0.2, 0.4,0.8, 1.6 and 3.2 g·m−2g·yr−1. Phosphorus accumulated rapidly in the benthicperiphyton and unconsolidated detrital (benthicfloc) layer and significantly higher Pconcentrations were recorded after 1 yr of Paddition. In contrast, a significant increasein surface soil (0–3 cm) TP concentrations wasmeasured in the surface soil layer only after 3yr of loading at the highest dose. Plantlitter and benthic floc/soil decompositionrates were measured using litter bags,containing sawgrass (Cladium jamaicenseCrantz) leaves, and cotton (cellulose) strips,respectively. Litter bag weight losses weresimilar among treatments and averaged 30% atthe end of the 3 yr study period. Litter Nconcentrations increased over time by anaverage of 80% at P loads 120% at Ploads ≥ 1.6 g·m−2·yr−1.In contrast,litter P concentrations declined up to 50% inthe first 6 months in all P loads and onlysubsequently increased in the two highestP-loaded mesocosms. Cotton strip decaydemonstrated that benthic floc and soilmicrobial activity increased within 5 mo of Paddition with more significant treatmenteffects in the benthic than the soil layer. The influence of soil microbial transformationswas shown in porewater chemistry changes. While porewater P levels remained close tobackground concentrations throughout the study,porewater NH4+ and Ca2+increased in response to P enrichment,suggesting that one significant effect of Penrichment in this oligotrophic peat system isenhanced nutrient regeneration.

Geomagnetic paleointensity and environmental record from Labrador Sea core MD95-2024: global marine sediment and ice core chronostratigraphy for the last 110 kyr

Piston core MD95-2024 from the Labrador Rise provides a continuous record of rapidly deposited detrital layers denoting Laurentide ice sheet (LIS) instability. The core also provides a high-resolution record of geomagnetic paleointensity, that is consistent with, but at higher temporal resolution than previous Labrador Sea records. Correlation to the Greenland Summit ice cores (GRIP/GISP2) is achieved by assuming that Labrador Sea detrital layers correspond to cold stadials in the ice cores. This allows a GISP2 official chronology to be placed on MD95-2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes ( 10Be and 36Cl) in Greenland ice cores. Synchronous millennial scale variability observed from the MD95-2024 paleointensity record and a North Atlantic paleointensity stack (NAPIS-75) [C. Laj et al., Philos. Trans. R. Soc. Ser. A 358 (2000) 1009–1025], independently placed on the GISP2 official chronology [C. Kissel et al., Earth Planet. Sci. Lett. 171 (1999) 489–502], further support the sediment to ice core correlation. High-resolution paleointensity and oxygen isotope records from the North Atlantic, Mediterranean, Somali Basin, sub-Antarctic South Atlantic and the relative flux of 10Be in Vostok (Antarctic) ice core are used to derive a common chronostratigraphy that neither violates the regional (millennial) or global (orbital) scale environmental stratigraphics. The resulting correlation circuit places the ice core and marine records on a common GISP2 official chronology, which indicates discrepancies as high as 5 kyr between the GISP2 and SPECMAP time scales. It further demonstrates that LIS instabilities in the Hudson Strait area are synchronous with cooling in the Greenland Summit ice cores and warming in the sub-Antarctic South Atlantic and in the Vostok ice core. Geomagnetic field intensity shows common global variance at millennial time scales which, in view of the out-of-phase interhemispheric climate variability, cannot be attributed to climatic contamination of the paleointensity records.

Influence of Phosphorus Loading on Organic Nitrogen Mineralization of Everglades Soils

There have been recent concerns about the anthropogenic phosphorus (P) loading to the naturally oligotrophic Everglades ecosystem. We investigated the effect of P loading on the biogeochemical cycling of nitrogen (N). We investigated the distribution of the potentially mineralizable N (PMN) rate as an indicator of the influence of P loading on selected microbial activities in soil and detritus layers. Soil characteristics measured included bulk density; total C, N, and P; microbial biomass C; and N and extractable NH+4 PMN rates ranged from 10.4 to 325 mg N kg−1 d−1 The highest values of microbial biomass C and N, total P, extractable NH+4, and PMN were observed in the detrital layer, and rates decreased with increasing soil depth. An increase in the size of the microbial pool and heterotrophic activity (organic N mineralization) was found to be related to the P‐loading rate and related to the distribution of total P content in the soil and detrital layers. Extractable NH+4 was a good indicator of PMN rates and total P was found to be significantly correlated to microbial biomass C and N. The stimulatory effect of P enrichment on microbial activity, overall size of the microbial biomass pool, and the PMN rate has led to an increased availability of inorganic N, which could potentially affect macrophyte growth and water quality of the northern Everglades system.

Establishing a chronology for the last 1000 years of laminated sediment accumulation at Lake Barrine, a tropical upland maar lake, northeastern Australia

Radiocarbon dates from the uppermost 115 cm of laminated mud beneath the centre of a small crater lake are affected by the residence-time in the marginal shallows of the organic detritus which, after transport to deep water, forms alternate layers in the sediment column. The delay is estimated from radiocarbon ages of source materials, trapped seston and the upper levels of the sediment column based on the ‘post-bomb’ atmospheric radiocarbon flux. The ‘contaminating’ component does not reside uniquely in any specific fraction of a mud sample, defined by alkali solubility and particle size, but is generally associated with the detrital layers. The lead-210 method, together with proxy indications from charcoal, exotic Solanum pollen and meteorological correlations, effectively replace radiocarbon for the dating of the sediment column from ad 1987 back to 1965. Beyond that, the selection of the youngest radiocarbon age from those in each of eight depth intervals leads to an acceptable estimate of accumulation rate to ad 1060.

Seed germination of two Everglades species, Cladium jamaicense and Typha domingensis

The germination requirements of Cladium jamaicense Crantz and Typha domingensis Pers. were studied under controlled conditions in the laboratory. Treatments included six temperature regimes, (constant temperatures of 15 20, 25, 30°C, and two fluctuating day : night temperature regimes of 25 : 10°C and 30 : 20°C), two light levels (14 : 10 h light : dark photoperiod and 24 h dark environment), two substrates (peat and water) and two O 2 levels (atmospheric and low (4.34%) O 2 concentration) using a complete randomized block design. The average incubation period needed for seeds to germinate was shorter for T. domingensis (1.1–19.5 days) than for C. jamaicense (26–46 days) and the final germination percentage was higher for T. domingensis than for C. jamaicense (85 vs. 42 %). Cladium jamaicense only germinated with fluctuating temperatures whereas T. domingensis germinated at all temperature regimes. Light was required for T. domingensis seeds to germinate, whereas C. jamaicense could germinate at a reduced rate in the dark. Peat substrate had a positive effect on germination in both species. Peat substrate shortened the incubation period for seeds of both species, enhanced germination of T. domingensis, especially at low temperatures, and enhanced germination of C. jamaicense in the dark. Low oxygen availability did not influence the germination of C. jamaicense but enhanced germination of T. domingensis at low temperatures. Although the two species differ significantly in their germination requirements and responses to specific environmental conditions, germination of both these species was enhanced by environmental conditions typical of exposed water saturated mudflats or sediment surfaces. Typha domingensis produces a large number of small seeds, which do not germinate when covered by sediment. In contrast, C. jamaicense produces fewer and larger seeds, with resources able to support growth through a thin layer of sediment or detritus.

Influence of Nitrate and Phosphorus Loading on Denitrifying Enzyme Activity in Everglades Wetland Soils

There has been recent concern about the impact of increased nutrient loading on the northern Everglades ecosystem. We investigated the spatial and temporal distribution of denitrifying enzyme activity (DEA) along a P‐enrichment gradient in the Water Conservation Area 2A (WCA‐2A) and determined the effects of added P and NO−3 on DEA. The DEA in soil and detritus layers was measured under anaerobic conditions four times during 2 yr, using the acetylene blockage technique. The DEA ranged from 0.004 to 7.75 mg N2O–N kg−1 h−1 Highest rates of DEA were found in the detritus and surface (0–10 cm) soils, and rates decreased exponentially with increasing distance from the surface‐water inflow point, where nutrients are loaded to the wetland. Nitrate was found to be limiting, while the addition of P had no effect on the distribution of DEA in these soils. There was a seasonal effect on DEA, with higher activity observed during the summer when temperatures and hydraulic and nutrient loading were highest. Soils from outside the impacted zone demonstrated denitrifying potentials, within 10 h when spiked with inflow concentrations of NO−3, similar to DEA of soils from within the impacted zone. This suggests that soils from outside the impacted zone can increase denitrification rates when exposed to higher NO−3 concentrations in a relatively short time. Agricultural drainage water discharge, and consequent NO−3 loading, has created a zone of elevated DEA proximal to the S‐10C surface‐water inflow point in WCA‐2A.

Discussion on Phosphorous Release from Lake Sediment

对东太湖三种类型9个样品的沉积物-上覆水柱状原样在滤除藻类、蔽光、室温、 微量充气条件下连续培养50d,上覆水中的PO₄^3--P浓度在起初15d内增高了2个数量级,在随后 的33d中又下降了1个数量级并趋于稳定．这一过程有别于一般意义上的沉积物磷释放,本文提 出了另外一种解释．认为前期上覆水中的PO₄^3-积累源于沉积物表面活性有机碎屑层的分解释放 磷,后期上覆水中的PO₄^3-被沉积物吸附沉淀．并提出了以下假设：在自然条件下,沉积物表面活性 有机碎屑层释磷反应强烈,形成很高的PO₄^3-P浓度并向沉积物和上覆水中扩散,在湖水中由于水 生植物吸收维持了较低的PO₄^3--P浓度,在间隙水中由于扩散阻力较高而形成较陡的PO₄^3--P浓度 梯度。与内源磷污染负荷等同的沉积物磷释放是指沉积物磷输出（到湖水中）通量与（自湖水）输入 通量之差值,在东太湖那样湖底条件比较稳定的湖泊中,磷的生物循环有可能严重干扰柱状原样 模拟实验,测定内源磷负荷采用全湖磷平衡法或原位围隔模拟实验法比较可靠．;Nine sediment-water columns were taken from three stations of East Taihu Lake (about lm in depth) in June, 1997. The overlying water of the cores was filtered (0.45μm), and then the cores were placed in a dark room for 50 days. Air-bubbling was used to keep oxidized conditions in the overlying water and upper sediment, without resuspending the sediment. The room temperature was between 23.5 and 31.2℃ , and the pH value of the overlying water and upper sediment was 7.2±0.2. Concentration of PO₄^3--P in the overlying water was measured every 2 or 3 days to follow the P-release process. In the first 15 days,PO₄^3--P level in the overlying water increased 2 orders of magnitude. But in the following 33 days, the PO₄^3--P level decreased 1 order of magnitude and then stabilized. Such a process was also found in the West Lake, Hangzhou and Gehu Lake, Changzhou. We think it not a normal sediment-P-release process, but more complex as follows. In natural conditions of East Taihu Lake, the sediment is covered by a layer of detritus. Biological degradation is so intensive in this layer that PO₄^3- accumulates to a high level and diffuses to both the overlying water and the sediment beneath it. In the sediment a steep PO₄^3--P concentration gradient forms because of the high diffusive resistance. But in the overlying water, PO₄^3--P is used by aquatic plants and is kept at a low level. We are not able to measure PO₄^3--P concentration in the detritus layer, but an indirect evidence was found in the same lake: a PO₄^3--P concentration gradient from 200mg m^-3 in the upper sediment layer to 80mg m^-3 in the sediment of 18cm depth existed when PO₄^3--P concentration in the overlying water was only 22 mg m^-3. In the core experiment, phosphorous biological cycling was broken when phytoplankton was removed from the overlying water by filtration, and thus, PO₄^3- accumulated in the water quickly. After 15 days, the P-release reaction in the detritus layer was about to end, and P-diffusion from overlying water to the sediment triggered. In East Taihu Lake and the other similar lakes with stable bottom conditions, detritus layer may exist and affect P-release core experiment seriously. In such a situation, it is better to measure internal-P-loading by other means, such as whole lake P-budget and in-situ enclosure experiment.

Heinrich layer in Antarctic marine sediments and its significance to global changes

Results of grain size analysis and rock magnetic measurements of core NP95-1 from Prydz Bay, Antarctica revealed two series of ice-rafted detritus layers, which correspond to cold climatic events, Younger Dryas and Heinrich event 1, occurring at about 11.7-10.3 and 14.3-13.6 kaB.P. respectively. Studies also show that the sequence of paleoclimatic changes in Antarctica can be correlated with that in other parts of the world, and that the millennia1 climate of the earth could change synchronously and globally. In addition, magnetic fabric analysis also shows a close relationship between paleoclimatic change and ocean circulation re-assemblage.

Benthic microalgae and nutrient dynamics in wave-disturbed environments in Marmion Lagoon, Western Australia, compared with less disturbed mesocosms

The relationships between the standing stock of marine benthic microalgae and physical variables and nutrients in the water column and porewaters were explored by comparing a wave-exposed field location at Marmion Lagoon, Western Australia, to less disturbed benthic mesocosms. For field stations in Marmion Lagoon, multiple regressions of chlorophyll a concentrations against the PCA scores (PCA1, PCA2, PCA3) indicated chlorophyll a was only significantly negatively related to PCA1 scores, which itself was dominated by temperature, salinity, DO, NO x , SiO 4 and SRP from the water column. Concentrations of chlorophyll a in surface sediments at field stations were significantly positively correlated to decreased temperature and salinity and negatively correlated to increased water column nutrients over the duration of the experiment. For mesocosms, chlorophyll a concentrations were not significantly related to any of the PC axes, indicating that shifts in temperature, salinity and nutrients in water column and porewaters were not responsible for the observed distribution of chlorophyll a in surface sediments. Benthic microalgae standing stock did not respond in similar ways in mesocosms and field stations to changes in nutrient status in the water column and surface sediments. There was a visible difference in the level of disturbance of surface sediments. In field stations, sand was resuspended and ripples developed on surface sediments in response to winter storm swells. In mesocosms, where disturbance of sediment was reduced, a surface layer of detritus and living benthic microalgae developed on the sediment surface and appeared to reduce the rate of nutrient efflux out of the sediment.

A 200 ka geomagnetic chronostratigraphy for the Labrador Sea: Indirect correlation of the sediment record to SPECMAP

A chronostratigraphic framework for the last 200 ka, based on relative variations of geomagnetic paleointensity, has been derived for the Labrador Sea where δ 18O stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record. Relative paleointensity records derived from three Labrador Sea piston cores are correlated to a recently developed 200 ka paleointensity composite (Sint-200) [Y. Guyodo, J.-P. Valet, Earth Planet. Sci. Lett. 143 (1996) 23–36]. The chronology for Sint-200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the SPECMAP chronostratigraphy. The correlation of Sint-200 to Labrador Sea paleointensity records allows the SPECMAP chronology to be imported into the Labrador Sea. Although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from AMS 14C dates and δ 18O data indicate significant discrepancies, support for the new chronostratigraphic framework is provided by: (1) consistent correlation of Labrador Sea detrital layers to North Atlantic Heinrich layers; (2) general consistency between planktic δ 18O values and the SPECMAP reference curve; and (3) feasible prediction for the age of ash layer 2. As the new chronostratigraphy is tied to SPECMAP, Labrador Sea sediment cores can now be directly compared with any SPECMAP dated record.

Stromatolite-thrombolite associations in a modern environment, Lee Stocking Island, Bahamas

Columnar buildups found in a tidal channel off Lee Stocking Island, Exuma Cays, Bahamas, have been interpreted as modern giant stromatolites growing in a subtidal normal marine environment. However, these organically-formed columns reveal three discernible internal structures: (1) prokaryotic stromatolites comprised of alternating layers of coarse-grained ooids and peloids, and fine micrite that formed exclusively by microbial activity; (2) eukaryotic stromatolites comprised of microbially-induced micritic layers alternating with detrital layers accumulated, bound, and cemented by eukaryotic algae; and (3) thrombolites displaying irregular, clotted fabrics and formed by microbes, algae, and metazoans. Phanerozoic thrombolites, in contrast, have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Eukaryotic organisms overgrow all of the columns at present. Thus, the contemporaneous formation of prokaryotic stromatolites, eukaryotic stromatolites, and thrombolites under identical conditions within the present environment appears unlikely. We suggest that the prokaryotic stromatolites represent forms that began to develop in an intertidal setting with the Holocene flooding of the Great Bahama Bank. The thrombolites, however, began to form under the present, normal-marine subtidal conditions. The eukaryotic stromatolites represent intermediate forms between prokaryotic stromatolites and thrombolites. There is evidence for a gradual change from stromatolite to thrombolite reefs associated with rising sea-level. With the deepening, there would have been a decrease in salinity, an increase in energy, and possibly an increase in nutrient supply; all factors that favor thrombolite growth. We propose that the co-existing stromatolites and thrombolites found off Lee Stocking Island did not grow contemporaneously, but reflect a response to changing environmental controls with changing sea-level.

Accumulation and turnover of carbon in organic and mineral soils of the BOREAS northern study area

Rates of input, accumulation, and turnover of C differ markedly within soil profiles and in soils with different drainage in the BOREAS northern study area. Soil C storage increases from ∼3 kg C m−2 in well‐drained, sandy soils to greater than 100 kg C m−2 in wetlands. Two modes of C accumulation were observed in upland soil profiles. Large annual C inputs (0.06–0.1 kg C m−2 yr−1) and slow decomposition (turnover times of 6–250 years) lead to rapid C accumulation in regrowing surface moss and detrital layers following fire. Deep organic layers that have accumulated over the millennia since the initiation of soil development, and are located below the most recent charred horizon, show slower rates of input (0.015–0.03 kg C m−2 yr−1) and turnover (100–1600 years) and accumulate C about 10 times slower than surface detrital layers. Rates of C input to soils derived from C and 14C data were in accord with net primary production estimates, with highest rates of input (0.14–0.6 kg C m−2 yr−1) in wetlands. Turnover times for C in surface detrital layers were 6–15 years for well‐drained sand soils that showed highest soil temperatures in summer, 30–40 years for wetlands, and 36–250 years for uplands with thick moss cover and black spruce trees. Long (&gt;100 years) turnover times in upland black spruce/clay soils most likely reflect the influence of woody debris incorporated into detrital layers. Turnover times for deep organic and mineral layer C were controlled by drainage, with fastest turnover (80–130 years) in well‐drained sand soils and slowest turnover (&gt;3000 years) in wetlands. Total C accumulation rates, which account for C losses from both deep organic and surface detrital layers, are close to zero for sand/jack pine soils, 0.003–0.01 kg C m−2 yr−1 for moderately to poorly drained sites in mature forest stands, and 0.03 kg C m−2 yr−1 for a productive fen. Decomposition of organic matter more than several decades old accounts for 9–22% of total heterotrophic respiration at these sites. The rates of C accumulation derived here are decadal averages for specific stands and will vary as stands age or undergo disturbance. Extrapolation to larger regions and longer timescales, where burning offsets C gains in moss layers, will yield smaller rates of C storage.