Detrital Aggregates Research Articles

Speckled beds; distinctive gravity-flow deposits in finely laminated diatomaceous sediments, Miocene Monterey Formation, California

Distinctive nonlaminated intervals we term speckled beds (SBs) are common in finely laminated diatomites of the Monterey Formation at Celite Quarry, Lompoc, California. SBs have sharp and bedding-parallel upper and lower contacts, tabular geometry, and are less than or equal to 10 cm thick. SBs are characterized by a bedding-parallel arrangement of sand- to granule-size diatomaceous and detrital aggregates that are uniformly distributed in a fine-grained matrix of macerated biosilica and detrital mud. The unique physical properties of diatomaceous sediments provided cohesive substrates and high fluid content that were prerequisite for the formation of SBs. Abundant hybrid fault/vein structures, slump folds, and complexly deformed laminae in the quarry attest to syndepositional deformation of laminated diatomaceous sediments on a subaqueous slope within the oxygen-minimum zone. The unique features of SBs permit an interpretation of stepwise rheological transformation during mass movement, via passage through critical-point transformations, from Bingham plastic (slump and/or slide block) to turbulent fluid (turbidity current) to viscous fluid exhibiting laminar flow (debris flow). The recognition of SBs clarifies the origin of these distinctive yet problematic deposits, and clearly differentiates them from other nonlaminated intervals. SB composition indicates provenance from an oxygen-minimum zone and further illustrates that a hiatus is not required to form cohesive firmgrounds in diatomaceous sediments. The recognition of SBs as recorders of slope failure events complements paleoseismic studies of Holocene diatomaceous laminites from Saanich Inlet, British Columbia (Ocean Drilling Program Sites 1033 and 1034).

Detrital aggregates on the sea floor: chemical composition and aerobic decomposition rates at a time-series station in the abyssal NE Pacific

Seasonal pulses of particulate matter sink to the sea floor and form visible detrital aggregates between July and December at an abyssal station (Sta. M, 4100 m depth) in the NE Pacific. We hypothesized that these detrital aggregates would create localized regions of intense biological activity on the sea floor. Detrital aggregates were collected and incubated on the sea floor using the submersible Alvin in August and three weeks later in September 1994. As a control, samples of the sediments were collected and incubated in situ in April 1995, when no aggregates were present. Detrital aggregates representing a range of sizes, colors and textures were sampled individually with Alvin using tube corers; others were incubated on the sea floor in tube core respirometers. In addition, a number of aggregate locations were documented photographically then sampled 23 and 221–223 d later, after the aggregates were no longer visible. A total of 55 tube cores with aggregates were collected in August 1994, and another 88 were collected in September 1994. Concentrations of total and organic carbon, total nitrogen, chlorophyll a and phaeopigments in the aggregates were similar to those in the sinking particulate matter collected concurrently in sediment traps at 50 m above bottom, but higher than those in the surface sediments. Oxygen consumption of diffuse floc was similar to that of the background sediment, whereas discrete aggregates and radiolarian-patch samples had rates significantly higher than background sediment. Detrital aggregates had no measurable influence on the chemical composition of the underlying sediment on time scales from 23 to 223 d or on sediment oxygen consumption after 222 d. Distinct detrital aggregates monitored visually with a time-lapse camera covered up to 4.9% of the sea floor and contributed substantially to the supply of organic carbon to the sediment community in fall 1994 while producing a minimal impact on sediment community oxygen consumption (SCOC). The annual oxygen consumption associated with detrital aggregates represented a minor contribution (0.34%) to the annual SCOC estimated at Sta. M in 1994–1995.

Sediment community response to a temporally varying food supply at an abyssal station in the NE pacific

Long time-series measurements were made to examine the effect of episodic inputs of organic matter to the benthic boundary layer on the sediment community at a site in the eastern North Pacific (Sta. M). Chlorophyll a and pheopigments were used to assess sediment enrichment, and the response of the sediment community was examined with seasonal measurements of sediment community oxygen consumption (SCOC) and sediment ATP from February 1992 to July 1996. In addition, macrofaunal density and biomass were examined in 26 sediment grab samples taken over a 2-year period. In order to determine whether the presence or absence of detrital aggregates correlated with macrofaunal density and biomass, 54 tube core samples were collected with the submersible Alvin at times when detrital aggregates were present on (August/September of 1994) and absent (April 1995) from the sea floor. Large, seasonal inputs of particulate organic carbon (POC) generally resulted in sediment enrichment and significantly correlated with increases in SCOC. Summer maxima in SCOC occurred consistently over our study period, indicating that this trend is a regular seasonal occurrence at Sta. M. Protozoans, primarily agglutinating foraminiferans, and five of the six dominant metazoan taxa exhibited seasonal increases in density during winter months after detrital aggregates had disappeared from the sea floor and approximately eight months after peak SCOC and POC measurements, over a 2-year period. Alvin tube core samples showed that relative protozoan density and biomass increased significantly over a 4-week period following an input of phytodetritus, indicating that these organisms may respond to organic matter inputs on time scales as short as weeks.

Deep-sea epibenthic echinoderms and a temporally varying food supply: results from a one year time series in the N.E. Pacific

Abundance and distribution of epibenthic echinoderms and detrital material at an abyssal site (4100 m) in the N.E. Pacific were examined between June 1994 and June 1995. Camera-sled surveys, time-lapse photographs, and observations from the submersible Alvin revealed a sequence of detrital deposition events. Detrital aggregates were present in summer and fall of 1994 but rare in winter and spring 1995. A layer of white flocculent material covered the sea floor from July through November 1994, constituting the first direct observations of a detrital carpet in this area. Large radiolarian patches also were observed from August 1994 through February 1995. Distribution and abundance of echinoderm species were estimated from camera-sled surveys and showed few consistent temporal patterns. Abundance varied spatially, and distributions generally could not be distinguished from random, in contrast to the clumped pattern observed for detrital aggregates. Statistically, the spatial distributions of several echinoderm species correlated significantly with the distributions of detrital aggregates, but these correlations were not consistent across transects or taxa. The lack of consistent correlation between echinoderm distribution and abundance and the presence of aggregate material is not surprising when the long duration of extensive detrital coverage of the sea floor is considered.

Nitrate respiration associated with detrital aggregates in aerobic bottom waters of the abyssal NE Pacific

Rates of nitrate utilization in tube core respirometers (TCR) placed over aggregates on the seafloor at an abyssal site (Station M) in the eastern North Pacific Ocean increased at times of high particle flux. In the presence of aggregates, both oxygen and nitrate were used in respiration. The ratio of O 2 : NO 3 concentrations in ambient waters was 3.9, while O 2 : NO 3 utilization rates in TCR overlying and TCR aggregate pore waters were 2.6 and 0.6, respectively. We postulated that denitrification was occurring in microzones of the particle-rich oxygenated (135 μM) waters. To test this, nitrate respiration was measured aboard a ship in oxygen-minimum (∼26 μM) water supplemented with particulate matter collected by a surface net tow. Dissolved oxygen consumption occurred immediately, followed by nitrate utilization while oxygen was still present. Calculations from cell densities indicated 0.6 μM of the original 42 μM of nitrate was assimilated into bacterial biomass during 36 h of incubation, suggesting the major portion of the utilized nitrate was used in respiration. Nitrate utilization rates in the in situ incubation study and those of the shipboard experiment were 3.1 and 2.7 μM d -1, respectively. The results of the present studies suggest nitrate respiration occurs in microzones of aggregates in oxygenated bottom waters at times of high particle flux and causes some loss of fixed nitrogen.

Macrofaunal communities within and adjacent to a detritus-rich submarine canyon system

Macrofaunal abundance, biomass, diversity and species assemblages within Scripps and La Jolla Submarine Canyons are compared with those on the nearby continental shelf and slope. Our primary objective was to examine the effect of detrital aggregates on infaunal communities within canyons. Two submarines, a remotely operated vehicle (ROV), and a Soutar box-corer were used to collect samples. Within the canyons, organic enrichment by macrophyte detritus was evident from canyon heads down to 550 m, and evidence of strong currents (coarse sediment) was found down to 700 m. Infaunal density and biomass were higher in the canyons than outside at all depths where comparative data were available (100–500 m). Infaunal assemblages in canyons were distinct from those at reference stations. Both the canyon and non-canyons samples showed community differentiation with depth. Species diversity was generally high, but decreased with depth outside of canyons and increased with depth within the canyons. Low diversity at shallow depths within the canyon is attributed to a combination of organic enrichment and physical disturbance. Submarine canyons are commonly found to contain distinct species assemblages or higher faunal densities and/or biomass than nearby non-canyon regions at similar depths. Canyons are regular features along most ocean margins and appear to be important as sites of enhanced secondary production, provide diverse habitats, and act as conduits of coastal detritus to the deep-sea.

234Th and 21Pb evidence for rapid ingestion of settling particles by mobile epibenthic megafauna in the abyssal NE Pacific

Particle-scavenged 234Th and 210Pb can be used to trace the fate of particulate matter reaching the deep-sea floor. We used this technique to demonstrate rapid ingestion of particles arriving at the sea floor (at a depth of 4,100 m) by mobile epibenthic holothuroids (Abyssocucumis abyssorum and Oneirophanta mutabilis). Excess 234Th and 210Pb activities of sediment trap material, detrital aggregates from the sea floor, and animal gut contents all were similar while activities of surface sediments (top 0–5 mm) were considerably lower. A simple calculation using the excess 210Pb concentration of two potential food sources, sediment trap material and surface sediments, shows that ∼91% of gut content material of A. abyssorum must have come from material similar to that found in the sediment trap cups. By setting the sediment trap material as age 0, apparent ages of <0–20 d, >100 d, and 12–13 d were estimated for the aggregates, surface sediments, and A. abyssorum guts, respectively. The population of A. abyssorum at this site could potentially process 0.2–4% of the vertical mass flux (m−2 d−1) during the high particle flux period between June and October 1994. The percentage of vertical flux processed could be substantially greater when all of the mobile epibenthic megafauna species are considered.

Activity patterns of mobile epibenthic megafauna at an abyssal site in the eastern North Pacific: results from a 17-month time-lapse photographic study

Mobile epibenthic megafauna are important components of many deep-sea communities, yet direct observations of their activities and estimates of their potential impact on benthic processes are scarce. To address this deficiency we deployed a time-lapse camera system at an abyssal site in the eastern North Pacific (Sta. M; 34°50′N, 123°00′W; 4100 m depth) to monitor epibenthic megafaunal movements over the 17-month period from February 1990 through July 1991. Photographs, each covering approximately 20 m 2, were taken every hour during this time span, except for the period from late October 1990 through mid-February 1991, when photos were taken every 4 h. Movements of the seven numerically dominant species of mobile epibenthic megafauna were digitized and analyzed for spatial patterns and temporal variation. These movements were markedly non-random for all seven species, with movement patterns falling into three broad categories: run-and-mill, loop and run. The holothuroid Peniagone vitrea displayed looping behavior that in some cases ( n = 8) had a distinct, semi-diurnal component possibly attributable to the influence of external factors such as tidal currents. P. vitrea was also the only species of the seven observed to swim. Temporal variations in movement patterns were also observed, with individuals of the two most abundant species, Elpidia minutissima and P. vitrea, spending less time within the camera's field of view during periods when potential food items, in the form of pelagically-derived detrital aggregates, were visible on the sea floor compared to periods when aggregates were absent. Based on abundances, body sizes and movement patterns, the three most abundant species, E. minutissima, P. vitrea and Abyssocucumis abyssorum, traversed 76.5% of the total area of sea floor covered by all seven species between February 1990 and July 1991. Of the seven species we studied, only two, A. abyssorum and Echinocrepis sp., left distinct traces on the sea floor. These two species accounted for only 32.8% of the total area of sea floor traversed by the mobile megafaunal community at Sta. M. Collectively, the seven numerically dominant species of mobile epibenthic megafauna at Sta. M traversed ca 36 m 2 of sea floor per year within the field of view of our time-lapse camera system ( ca 20 m 2). Our results indicate that mobile epibenthic megafauna have the potential to substantially impact benthic processes at Sta. M, particularly processes involving the utilization, processing and redistribution of particulate organic material at the sea floor.

Radiocarbon in organic compound classes in particulate organic matter and sediment in the deep northeast Pacific Ocean

Radiocarbon (Δ14C) and stable carbon isotopes (δ13C) were measured for total amino acids, carbohydrates, lipids and acid‐insoluble organic fractions separated from sinking particulate organic matter (POMsink), detrital aggregates, sediment floe and sediments collected from the deep Northeast Pacific Ocean. The results show distinct Δ14C signatures among these organic compound classes. Bomb 14C, produced 3–4 decades ago in the atmosphere, was present in all organic fractions in the POMsink in the deep sea. Δ14C values decrease with depth, from the sediment trap POMsink (3450 m) to the sediment (4100 m), with rapid changes occurring at the sediment‐water interface. Total lipid had much ‘older’ Δ14C values than those of amino acid and carbohydrate fractions in detrital aggregates, sediment floe and sediments. These data demonstrate that two processes may be occurring: 1) preferential decomposition of organic matter at the sediment‐water interface, and 2) incorporation of ‘old’ organic carbon into POM and sediment. The alteration of carbon found in these organic compound classes suggests that differences in decomposition and chemical behavior exist for each of these compound classes in the deep ocean.

The deep-sea nematode Thalassomonhystera bathislandica sp. nov. and microhabitats of nematodes in flocculent surface sediments

Thalassomonhystera bathislandica sp. nov. (Nematoda: Monhysteridae) was found in abundance in a few gelatinous phytodetritus aggregates collected from the surface of bottom sediments in the north-eastern Atlantic at depths of about 4000 m. The new species resembles T. islandica (De Coninck, 1943), an inhabitant of saltmarshes. Viscous detrital aggregates, including sediment agglutinations made by the nematodes themselves, are perceived as the preferred habitats of nematode species in fluffy surficial muds. It is suggested that physical constraints prevent nematodes from a fast invasion into freshly sedimented, loose detritus deposits.

Coupling of near‐bottom pelagic and benthic processes at abyssal depths in the eastern North Pacific Ocean

A time‐series site was established in the eastern North Pacific Ocean to study the coupling between near‐bottom pelagic and benthic processes at abyssal depths in an area with well‐documented seasonality in surface water productivity. Particulate total mass (PTM), particulate organic C (POC), particulate total N (PTN), and particulate calcium carbonate [P(CaCO3)] fluxes were measured from sediment trap collections at 600 and 50 m above bottom from February 1990 to October 1991 (606 d). The sea floor at 4,100 m was concurrently monitored with a time‐lapse camera over a 386‐d period from July 1990 to July 1991. During this study, in situ measurements of sediment community oxygen consumption (SCOC) were made and surface sediment samples collected seasonally with a free‐vehicle grab respirometer. PTM, POC, PTN, and P(CaCO3) fluxes exhibited a strong seasonal signal, with episodic peaks from spring through fall and lows in winter. SCOC increased sharply between winter and summer, coinciding with spring peaks in particulate fluxes. Detrital aggregates appeared on the sea floor in pulses over a 6‐month period from July to December 1990, coinciding with periods when particulate fluxes were elevated but delayed ∼1.5 months after the initial peaks in particulate flux. Mobile epibenthic megafauna monitored with time‐lapse photography consisted primarily of holothuroids and echinoids and were twice as active when detrital aggregates were observed on the sea floor as during the rest of the year. Our results show a temporal relationship between the flux of particulate matter entering the benthic boundary layer, the arrival and residence of detritus on the sea floor, and the activity of the sediment community and mobile epibenthic megafauna.

Diet of calyptopes of the euphausiidEuphausia pacifica in the Yellow Sea

Foregut contents of calyptopes (the first feeding stage) ofEuphausia pacifica Hansen, collected from the Yellow Sea in June 1989, were examined using scanning electron microscopy with the dry-fracturing method. There was no clear evidence of phytoplankton ingestion by calyptopes. The foregut contents consisted entirely of amorphous organic detritus, crustacean remains and inorganic particles. The inorganic particles adhered to amorphous organic detritus, and bacterial populations were found on peritrophic membranes of fecal pellets which enveloped crustacean remains. These materials seemed to have originated from aggregates of organic and inorganic detritus. It is quite possible that the detrital aggregates are utilized extensively as food by calyptopes ofE. pacifica in the Yellow Sea.

The attachment probabilities of marine snow and their implications for particle coagulation in the ocean

The rate at which suspended particles in the ocean coagulate to form larger, more rapidly sinking aggregates is directly related to the frequency with which the particles stick together once they have collided. The attachment probabilities of marine macroscopic detrital aggregates, known as marine snow, were determined by video recording and analysing individual particle-particle interactions between 660 pairs of aggregates colliding together via gravitational settlement. Attachment probabilities of natural marine aggregates in the size range from 0.2 to 7.6 mm are the highest yet reported for natural particles, ranging from 0.60 (60% of collisions result in attachment) for amorphous detrital aggregates to 0.88 for flocculated diatoms. Attachment probabilities increase with increasing particle volume, surface area of contact and collision velocity. Large size and surface complexity resulting in multi-point contact between colliding particles and the abundance of sticky, microbially-produced exopolymers on the aggregate surfaces probably contribute to high aggregation efficiencies. The high attachment probabilities of suspended macroaggregates suggest that the rate of aggregation of organic particles by physical coagulation in the ocean may be many times higher than previously predicted.

Microbial response to the input of fresh detritus to the deep-sea bed

In order to assess the response of a deep-sea microbial population from the N.E. Atlantic to simulated falls of detrital aggregates we added sterile detritus to deep-sea microbial communities and incubated them under high pressure and low temperature. Rapid colonization, growth, and decomposition rates indicate that the deep-sea benthic microbial community can react quickly to such inputs of organic carbon to the sea bed. Microbial decomposition and transformation of sedimented detrital aggregates may be important in material flow in the deep-sea and may influence the local seawater chemistry.

Microbial response to the input of fresh detritus to the deep-sea bed

In order to assess the response of a deep-sea microbial population from the N.E. Atlantic to simulated falls of detrital aggregates we added sterile detritus to deep-sea microbial communities and incubated them under high pressure and low temperature. Rapid colonization, growth, and decomposition rates indicate that the deep-sea benthic microbial community can react quickly to such inputs of organic carbon to the sea bed. Microbial decomposition and transformation of sedimented detrital aggregates may be important in material flow in the deep-sea and may influence the local seawater chemistry.

Early diagenesis in sediments from Danish coastal waters: Microbial activity and Mn-Fe-S geochemistry

Depth distributions of bacterial respiration of O 2, NO 3 − and SO 4 2− were compared with geochemical data for Mn, Fe and S in coastal sediments from water depths between 26 and 520 m. As water depth increased, the zone of SO 4 2− respiration was found deeper in the sediment and was eventually separated from the surface-located activity of O 2 and NO 2 − respiration. At the deepest station additional SO 4 2− reduction activity was observed in small, detrital aggregates on the sediment surface. Dissolved Mn 2+ and Fe 2+ appeared between the O 2- plus NO 3 −-containing surface layer and the H 2S-plus FeS-containing sediment below. This was a result of Mn and Fe reductions coupled to either the oxidation of sulfide or the mineralization of organic matter. Tracer experiments showed that both FeS, FeS 2 and S 0 were important radiolabelled products of sulfate respiration in this intermediate zone. In the same zone, the overall degradation of organic matter seemed to be underestimated by the assay of SO 2- 4 respiration and additional mineralization by Mn and Fe reductions was likely.

Production of heterotrophic bacteria inhabiting macroscopic organic aggregates (marine snow) from surface waters1

Macroscopic detrital aggregates, known as marine snow, are a ubiquitous and abundant component of the marine pelagic zone. Descriptions of microbial communities occurring at densities 2–5 orders of magnitude higher on these particles than in the surrounding seawater have led to the suggestion that marine snow may be a site of intense heterotrophic activity. We tested this hypothesis using incorporation of [3H]thymidine into macromolecules as a measure of bacterial growth occurring on marine snow from oceanic waters in the North Atlantic and from neritic waters off southern California.Abundances of marine snow ranged from 0.1 to 4.3 aggregates liter−1. However, only 0.1–4% of the bacteria in the water occurred in association with marine snow. Mean thymidine incorporation per cell on aggregates was generally equal to or lower than that of bacteria found free‐living in the surrounding seawater, indicating that attached bacteria were not growing more rapidly than free‐living bacteria. Bacteria inhabiting aggregates were up to 25 times larger than free‐living forms. Thus, the contribution of these attached bacteria to total bacterial production in surface waters was low but occasionally significant, ranging from 3 to 26% and averaging 8 ± 7%

The significance of macroscopic aggregates (marine snow) as sites for heterotrophic bacterial production in the mesopelagic zone of the subtropical Atlantic

Macroscopic detrital aggregates (known as marine snow) harbor greatly enriched populations of bacteria and protozoans, suggesting that they may be an important site of heterotrophic activity in the deep sea. We collected flocculent marine snow larger than 3 mm in diameter in the mesopelagic zone of the subtropical Atlantic from the submersible, Johnson-Sea-Link. Although the bacteria inhabiting these aggregates incorporated as much or more [ 3H]-thymidine per bacterium than free-living forms, were up to 2 orders of magnitude more abundant on marine snow than in the surrounding seawater, and were twice as large as unattached bacteria, the marine snow itself was rare, ranging from 0.5 to 4 aggregates m −3. Thus the contribution of bacteria inhabiting flocculent marine snow to total bacterial production in the mesopelagic zone was insignificant, ranging from 0.01 to 0.39% over depths of 130 to 650 m.

Detrital aggregates in some Iowa lakes and reservoirs

Detrital aggregates in three eutrophic Iowa lakes and four eutrophic Iowa reservoirs were studied with light and scanning electron microscopy to determine if aggregate morphologies and concentrations were similar. Lake and reservoir aggregates were composed of organic and inorganic particles bound together in an organix matrix. Many of the inorganic particles were calcium carbonate. Obvious bacterial and fungal attachment to the aggregates was rare. Aggregate concentrations ranged from 4 to 274 million aggregates per liter. Aggregates smaller than 18 µm diameter dominated the hyperbolic size-frequency distribution of aggregates in all lakes. Reservoir and lake aggregate concentrations did not differ significantly, but mean aggregate concentrations were directly correlated to the mean chlorophyll a concentration of the lakes. These data strongly suggest that detrital aggregate concentrations are influenced by the trophic status of a lake.

Visual Observations of Phosphorus Movement Between Algae, Bacteria, and Abiotic Particles in Lake Waters

The transfer of 33P-PO4 into plankton and the appearance of labeled colloidal material occurs within minutes in lake water. It is difficult to clearly separate the microscopic forms involved in phosphorus transfer by mechanical means since size ranges of algae, bacteria, detritus, and colloidal matter overlap. To circumvent this problem we used microautoradiography and scanning electron microscopy (SEM) to describe the sequence in which bacteria, algae, and suspended particles remove 33P-PO4 from solution and the physical appearance of excreted material suspected of containing labeled phosphorus.Bacteria initially showed much more dense labeling with 33P-PO4 but, since the algal biomass greatly exceeded the bacterial biomass, algal 33P-PO4 incorporation represented the most significant uptake pathway. After 2 h 33P accumulated in both the detrital aggregates which were harboring bacteria and the "capsular" material surrounding Anabaena. When these suspected extracellular release sites were examined by SEM, we observed fibrillar and amorphous particles of colloidal size (.05–0.5 μm). The possible ecological roles of these colloidal substances are discussed.