Algal Nodules Research Articles

An annotated and illustrated identification guide to common mesophotic reef sponges (Porifera, Demospongiae, Hexactinellida, and Homoscleromorpha) inhabiting Flower Garden Banks National Marine Sanctuary and vicinities.

Sponges are recognized as a diverse and abundant component of mesophotic and deep-sea ecosystems worldwide. In Flower Garden Banks National Marine Sanctuary region within the northwestern Gulf of Mexico, sponges thrive among diverse biological and geological habitats between 16-200+ m deep (i.e., coral reefs and communities, algal nodules, and coralline algae reefs, mesophotic reefs, patch reefs, scarps, ridges, soft substrate, and rocky outcrops). A synoptic guide is presented, developed by studying common sponge species in the region, through direct sampling and in-situ photographic records. A total of 64 species is included: 60 are Demospongiae (14 orders), two are Hexactinellida (one order), and two are Homoscleromorpha (one order). Thirty-four taxa are identified to species and 13 were identified to have affinity with, but were not identical to, a known species. Fifteen taxa could only be identified to genus level, and the species remain as uncertain (incerta sedis), with the potential to represent new species or variants of known species. One specimen received only a family assignation. This study extends geographic or mesophotic occurrence data for eleven known species and includes several potentially new species. This work improves our knowledge of Gulf of Mexico sponge biodiversity and highlights the importance of the region for scientists and resource managers.

Reprint of Comparison of climate signals obtained from encrusting and free-living rhodolith coralline algae

Coralline algae have been used for sclerochronological studies throughout the last decade. These studies have focused on two different growth morphologies of the photosynthetic coralline algae: massive crusts forming small buildups on hard substrate, and free-living branching algal nodules, known as rhodoliths. The latter are generally found on soft-substrate, where they are frequently overturned by water movement and bottom feeding organisms, leaving one side of the rhodolith partially buried in the sediment at any given time. Here we test whether either of these growth morphologies is more suitable for proxy reconstructions by comparing Mg/Ca ratios – a temperature proxy – in multiple replicates of rhodoliths of Lithothamnion glaciale and in rhodoliths as well as encrusting specimens of Clathromorphum compactum. With both species being widespread throughout the Temperate and Arctic regions, we have chosen two North Atlantic localities at Nuuk Fjord, Greenland (Subarctic), and off the southeastern coast of Newfoundland, Canada (Temperate), for this study. Two to three Mg/Ca ratio transects spanning 18years of growth were analysed on multiple specimens with encrusting morphologies and along different sides of rhodoliths using laser ablation inductively coupled mass spectrometry and compared to remotely sensed sea surface temperature (SST) data. The length of the common time span used for comparison was limited by growth interruptions in rhodoliths. Furthermore, our comparison is based on the assumption that rhodolith growth increments are annual – an assumption that has recently been challenged by mesocosm studies. Monthly Mg/Ca values from multiple transects within each individual were compared and in samples from Nuuk fjord significant correlations were found in 4 of 4 encrusting C. compactum, 4 of 4 C. compactum rhodoliths, and 2 of 3 L. glaciale rhodoliths. In Newfoundland significant correlations were found in 6 of 6 encrusting C. compactum comparisons (average: r=0.61, p<0.001), and in 6 of 6 L. glaciale rhodolith comparisons (average: r=0.43, p<0.001) for monthly resolved time series. The monthly Mg/Ca ratios (n=216) from each morphology were compared with instrumental Reynolds SST yielding the following correlations: encrusting C. compactum (r=0.64, p<0.001), C. compactum rhodolith (r=0.62, p<0.001) and L. glaciale (r=0.58, p<0.001). In Newfoundland both morphologies indicate a similar strength in recording SST: encrusting C. compactum (r=0.85, p<0.001) and rhodolith-forming L. glaciale (r=0.84, p<0.001). In summary, Mg/Ca ratios derived from both coralline algal growth forms can yield SST information, however, massive encrusting forms generally yield higher correlations to SST than transects measured on individual rhodoliths, which only allowed for the generation of short uninterrupted time series due to frequent growth irregularities.

Comparison of climate signals obtained from encrusting and free-living rhodolith coralline algae

Coralline algae have been used for sclerochronological studies throughout the last decade. These studies have focused on two different growth morphologies of the photosynthetic coralline algae: massive crusts forming small buildups on hard substrate, and free-living branching algal nodules, known as rhodoliths. The latter are generally found on soft-substrate, where they are frequently overturned by water movement and bottom feeding organisms, leaving one side of the rhodolith partially buried in the sediment at any given time. Here we test whether either of these growth morphologies is more suitable for proxy reconstructions by comparing Mg/Ca ratios – a temperature proxy – in multiple replicates of rhodoliths of Lithothamnion glaciale and in rhodoliths as well as encrusting specimens of Clathromorphum compactum. With both species being widespread throughout the Temperate and Arctic regions, we have chosen two North Atlantic localities at Nuuk Fjord, Greenland (Subarctic), and off the southeastern coast of Newfoundland, Canada (Temperate), for this study. Two to three Mg/Ca ratio transects spanning 18years of growth were analysed on multiple specimens with encrusting morphologies and along different sides of rhodoliths using laser ablation inductively coupled mass spectrometry and compared to remotely sensed sea surface temperature (SST) data. The length of the common time span used for comparison was limited by growth interruptions in rhodoliths. Furthermore, our comparison is based on the assumption that rhodolith growth increments are annual – an assumption that has recently been challenged by mesocosm studies. Monthly Mg/Ca values from multiple transects within each individual were compared and in samples from Nuuk fjord significant correlations were found in 4 of 4 encrusting C. compactum, 4 of 4 C. compactum rhodoliths, and 2 of 3 L. glaciale rhodoliths. In Newfoundland significant correlations were found in 6 of 6 encrusting C. compactum comparisons (average: r=0.61, p<0.001), and in 6 of 6 L. glaciale rhodolith comparisons (average: r=0.43, p<0.001) for monthly resolved time series. The monthly Mg/Ca ratios (n=216) from each morphology were compared with instrumental Reynolds SST yielding the following correlations: encrusting C. compactum (r=0.64, p<0.001), C. compactum rhodolith (r=0.62, p<0.001) and L. glaciale (r=0.58, p<0.001). In Newfoundland both morphologies indicate a similar strength in recording SST: encrusting C. compactum (r=0.85, p<0.001) and rhodolith-forming L. glaciale (r=0.84, p<0.001). In summary, Mg/Ca ratios derived from both coralline algal growth forms can yield SST information, however, massive encrusting forms generally yield higher correlations to SST than transects measured on individual rhodoliths, which only allowed for the generation of short uninterrupted time series due to frequent growth irregularities.

Living carbonate habitats in temperate California (USA) waters: distribution, growth, and disturbance of Santa Catalina Island rhodoliths

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 560:135-145 (2016) - DOI: https://doi.org/10.3354/meps11919 Living carbonate habitats in temperate California (USA) waters: distribution, growth, and disturbance of Santa Catalina Island rhodoliths Paul A. Tompkins*, Diana L. Steller Moss Landing Marine Laboratories, 8272 Moss Landing Rd., Moss Landing, California 95039, USA *Corresponding author: ptompkins25@gmail.com ABSTRACT: Rhodoliths are globally widespread, free-living coralline red algal nodules (Rhodophyta). Living beds of rhodoliths create carbonate-based biogenic benthic habitats, which are sensitive to human disturbance and slow to recover. Despite their ecological importance, few quantitative assessments of the distribution and anthropogenic impacts on rhodoliths exist for the eastern Pacific Ocean. Following historical reports of beds in the Channel Islands off California, USA, we mapped bed distributions, examined rhodolith habitat sensitivity to human disturbance, and determined seasonal growth rates to assess rhodolith recovery potential at Santa Catalina Island. Rhodolith material from 7 beds covering 23000 m2 of live rhodoliths and 43000 m2 of dead rhodolith sediment were mapped. Beds were patchy, dominated by small Lithothamnion australe rhodoliths (mean ± SD: 10.6 ± 0.8 mm in diameter), and 6 beds were disturbed by mooring arrays. Surveys of mooring frequency and experimental manipulation of mooring chains showed that the cover of live rhodoliths, their physical structure, and the diversity and abundance of associated benthic species were negatively related to this prevalent anthropogenic disturbance. Axial growth rates of Santa Catalina Island rhodoliths were relatively slow, seasonally variable, and similar to those reported in other rhodolith studies (annual mean: 1.25 ± 0.62 mm yr-1). Due to their importance as biogenic habitat, sensitivity to disturbance, and slow recovery potential, California rhodolith beds should be considered a priority for monitoring efforts and restoration and should be included within marine protected area planning. KEY WORDS: Rhodophyta · Disturbance · Lithothamnion australe · Coralline algae · Maerl · Channel Islands Full text in pdf format PreviousNextCite this article as: Tompkins PA, Steller DL (2016) Living carbonate habitats in temperate California (USA) waters: distribution, growth, and disturbance of Santa Catalina Island rhodoliths. Mar Ecol Prog Ser 560:135-145. https://doi.org/10.3354/meps11919 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 560. Online publication date: November 24, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Overview of coralline red algal crusts and rhodolith beds (Corallinales, Rhodophyta) and their possible ecological importance in Greenland

Coralline red algae are a globally distributed and abundant group of shallow marine benthic calcifiers. They can form important ecosystems that provide a three-dimensional habitat to a large variety of marine organisms. While the study of coralline red algae has traditionally been focused on warm-water habitats, numerous recent reports have now described widespread coralline red algal ecosystems from high-latitude regions, particularly in the Northern Hemisphere. In fact, it is becoming increasingly evident that coralline red algae are likely the dominant marine calcifying organisms on the seafloor of the Arctic and subarctic photic zone. This article gives a first overview of the distribution of coralline red algal crusts and rhodolith (free-living coralline red algal nodules) grounds in Greenland and the first report of rhodoliths in East Greenland. Museum data and recent sampling information have been compiled to develop a distribution map of coralline genera and rhodolith communities. The depth range of coralline red algae in Greenland has been extended by 27 m, from 50 to 77 m depth. In addition, rhodoliths of the normally crust-forming species Clathromorphum compactum are described for the first time from a sheltered Greenland fjord. Based on the data compiled here, it becomes clear that rhodolith communities are a widespread feature of the Greenland shallow shelf areas. Gaining a better understanding of the distribution of these hitherto poorly understood high-latitude ecosystems is essential due to their function as spawning areas and nursery grounds for commercially important fish and invertebrates.

The leaking bucket of a Maldives atoll: Implications for the understanding of carbonate platform drowning

Abstract Seismic and multibeam data, as well as sediment samples were acquired in the South Male Atoll in the Maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits. Multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15–20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea-level lowstand. Such depocenters are located in current-protected areas flanking the reef rim of the atoll or in infillings of karst dolinas. Much of the 50 m deep sea floor in the lagoon interior is current swept, and has no or very minor sediment cover. Erosive current moats line drowned patch reefs, whereas other areas are characterized by nondeposition. Karst sink holes, blue holes and karst valleys occur throughout the lagoon, from its rim to its center. Lagoonal sediments are mostly carbonate rubble and coarse-grained carbonate sands with frequent large benthic foraminifers, Halimeda flakes, red algal nodules, mollusks, bioclasts, and intraclasts, some of them glauconitic, as well as very minor ooids. Finer-grained deposits locally are deposited in current-protected areas behind elongated faros, i.e., small atolls which are part of the rim of South Male Atoll. The South Male Atoll is a current-flushed atoll, where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim. With an elevated reef rim and tower-like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record.

Deep-water rhodolith bed from central Brazilian continental shelf, Campos Basin: coralline algal and faunal taxonomic composition

Rhodolith beds are distributed from the north-eastern to the southeastern Brazilian continental shelf, constituting the largest extension of coralline algal de-posits in the world. Little is known about the deep rho-dolith beds within the Campos Basin: the largest oil pro-duction area in the country and a priority area for marine life conservation. This study illustrates a deep rhodolith bed covering about 15km2 of a 40km2-area in the Peregrino oil field sampled at 100-106m water depth. Coralline algae are the dominant components on the living rhodolith surfaces associated with subordinate bryozoans, cnidarids, brachiopods and porifers. In some inner parts of the coralline algal nodules, encrusting acervulinid fora-minifera are the main nodule contributors. Through accel-erator mass spectrometric analysis, radiocarbon age esti-mates show that the range in ages between the living outer rhodolith parts and within 3mm from it the rhodoliths is ca. 4,700 years. This suggests that a proportion of fossil rhodoliths had been recolonized after periods of burial and/or erosion. The present-day Peregrino rhodolith bed played a fundamental ecological role in the Brazilian con-tinental shelf’s benthic habitats for thousands of years.

Bathymetric distribution of ichnocoenoses from recent subtropical algal nodules off Fraser Island, eastern Australia

Coralline algal nodules living on the subtropical shelf off Fraser Island, eastern Australia, from the inner to the uppermost outer-shelf, from 28 to 117m water depth are pervasively bioeroded. Five ichnogenera have been identifed as nodule borers, comprising one ichnotaxon attributed to bivalves (Gastrochaenolites), one to sponges (Entobia) and three to polychaetes and barnacles (Trypanites, Maeandropolydora, Rogerella). Microtraces comparable to those produced by fungi, algae, bacteria and/or sponges are also present. Two ichnocoenoses have been recognised. The ‘shallow’ water ichnocoenosis (EGTM) occurs at 60m and shallower depths and includes all identified ichnogenera. The ‘deep’ water ichnocoenosis (from 68 to 117m) is characterised by Trypanites and the Trypanites/Maeandropolydora network (TM ichnocoenosis) with a generally higher boring density than in shallower nodules. Decreasing size of algal nodules and reduced thickness of the coralline thalli probably explain the decrease in diversity of ichnogenera with depth, as the larger bioeroders are excluded from the ichnoassemblages. Lower growth rates favour higher density of bioerosion in deeper algal nodules.

The impact of the Mid-Pleistocene Transition on the composition of submerged reefs of the Maui Nui Complex, Hawaii

The submarine reef terraces (L1–L12) of the Maui Nui Complex (MNC—the islands of Lanai, Molokai, Maui and Kahoolawe) in Hawaii provide a unique opportunity to investigate the impact of climate and sea level change on coral reef growth by examining changes in reef development through the Mid-Pleistocene Transition (900–800ka). We present an analysis of the biological and sedimentary composition of the reefs that builds directly on recently published chronological and morphological data. We define nine distinct limestone facies and place them in a spatial and stratigraphic context within 12 reef terraces using ROV and submersible observations. These include oolitic, two coral reef, two coralline algal nodule, algal crust, hemi-pelagic mud, bioclastic and peloidal mud facies. These facies characterise environments from high energy shallow water coral reef crests to low energy non-reefal deep-water settings. Combining the bottom observations and sedimentary facies data, we report a shift in the observed sedimentary facies across the submerged reefs of the MNC from dominant shallow coral reef facies on the deep reefs to coralline algae dominated exposed outcrop morphology on the shallower reefs. We argue that this shift is a reflection of the change in period and amplitude of glacioeustatic sea level cycles (41kyr and 60–70m to 100kyr and 120m) during the Mid-Pleistocene Transition (MPT, ~800ka), coupled with a slowing in the subsidence rate of the complex. The growth of stratigraphically thick coral reef units on the deep Pre-MPT reefs was due to the rapid subsidence of the substrate and the shorter, smaller amplitude sea level cycles allowing re-occupation and coral growth on successive cycle low-stands. Longer, larger amplitude sea level cycles after the MPT combined with greater vertical stability at this time produced conditions conducive to deep-water coralline algae growth which veneered the shallower terraces. Additionally, we compare reef development both within the MNC, and between the MNC and Hawaii. Finally we suggest that climatic forcings such as sea-surface temperature and oceanographic currents may also have influenced the distribution of coral species within the sample suite, e.g., the disappearance of the Acropora genus from the Maui Nui Complex in the Middle Pleistocene.

Rhodolith‐bearing limestones as transgressive marker beds: fossil and modern examples from North Island, New Zealand

AbstractRhodoliths are nodular structures composed mainly of the superimposed thalli of calcareous red algae. Because their development is controlled by an array of ecological parameters, rhodoliths are a valuable source of palaeoenvironmental information. However, despite their common use in palaeoecological reconstructions, the stratigraphic significance of rhodolith accumulations seldom has been addressed in detail. In a study of Cenozoic rhodolith‐bearing deposits from the North Island of New Zealand, rhodolithic units, usually of limited lateral extent, typically occur above major unconformities at the base of deepening upwards successions. Two types of transgressive rhodolith‐bearing deposits may be distinguished on the basis of texture and rhodolith internal structure: (i) type A deposits are clast‐supported rhodolithic rudstones containing abundant pebbles and cobbles reworked from the substrate, and are characterized by rhodoliths with a compact concentric to columnar internal structure and a high nucleus to algal cover ratio; (ii) type B deposits are rhodolithic floatstones with a matrix usually consisting of bryozoan fragments, benthic foraminifera and echinoid fragments or terrigenous silty fine sand. The rhodoliths of type B units usually have a loose internal framework with irregular to branched crusts. The two contrasting rhodolith‐bearing units are interpreted as characteristic facies of transgressive systems tract deposits, analogous to shell concentrations formed under conditions of low net sedimentation. Type A deposits are correlated with relatively high‐energy settings and/or narrow submerged palaeotopographic lows, whereas type B deposits are interpreted as forming in lower‐energy settings. The association between transgression and development of rhodolithic facies is confirmed by observations of a modern rhodolith production site at Whangaparaoa Peninsula in North Island, where algal nodules grow above a ravinement surface cut during the Holocene sea‐level rise, and also by a review of published fossil examples, many of which show stratigraphic and compositional attributes analogous to those of the New Zealand occurrences. The review indicates that transgressive rhodolith accumulations develop more commonly in, but are not restricted to, non‐tropical settings. It is suggested that a combination of factors, such as low net sedimentary input, nature of the substrate, sea‐level rise and inherited physiography contribute to determine the relationship between rhodolith‐bearing deposits and transgressive settings.

Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.

Coralline algae indicate Pleistocene evolution from deep, open platform to outer barrier reef environments in the northern Great Barrier Reef margin

Coralline algae from a drill core in Ribbon Reef 5 have been used to interpret changes in the depositional palaeoenvironments in the northern Australian Great Barrier Reef over the last 790 ka. Three main coralline algal assemblages, each dominated by members of a particular subfamily, have been distinguished by quantitative analysis: (1) mastophoroid assemblages, usually occurring as crusts on corals, are typical of the shallowest reef settings; (2) lithophylloid assemblages within algal nodules may represent shallow-water, cooler environments or deeper reef subenvironments; and (3) melobesioid assemblages are characteristic of deeper-water platform areas. The algal assemblages between 96 and 210 m b.s.f. (metre below sea floor) record a fluctuating but progressive shallowing-upwards from deep, outer-platform to shallower, non-reefal depositional environments. Two intervening episodes dominated by mastophoroids represent two phases of reef growth. The section above 96 m b.s.f. comprises several stacked reefs in which mastophoroid crusts similar to the present-day shallow-water assemblages predominate.

Coralline algal nodules off Fraser Island, eastern Australia

Calcareous red algal nodules growing on mobile substrates have been sampled from 28 to 117m off Fraser Island in southern Queensland, eastern Australia. This is a subtropical, transitional area between the tropical Great Barrier Reef to the north and temperate, cooler waters to the south. Red algal nodules are the most common components in bioclastic gravels that extend from about 50 to 110 m and locally cover 40–50% of the seafloor. Variations in the overall character and floristic composition of the nodules with depth can be observed. Algal nodules comprise algal covered pebbles/cobbles and rhodoliths in depths shallower than 60 m whereas only rhodoliths occur in deeper settings. No changes in nodule shape occur but shallower algal nodules have larger mean size with higher standard deviation than the deeper ones (39.2 vs. 30.5 mm and 20.5 vs. 6.3 mm s.d.). Living and subrecent red algae in nodules shallower than 60 m are mainly Melobesioideae and peyssonneliaceans with minor Lithophylloideae and Mastophoroideae. Most plants belong to a few species of the generaPhymatolithon andLithothamnion. Below 68 m, rhodoliths are dominated by the family Sporolithaceae, melobesioids and peyssonneliaceans.Sporolithon is the main component below 80 m. Algal growth forms are mostly smooth encrusting to warty with no depth variation. Maximum plant thickness, however, decreases with increasing depth. Thallus thickness in the deeper water samples is more than three times smaller than in those from shallower waters. These data are important for understanding the paleoenvironmental context of deposition of the abundant coralline algal limestones with similar algal nodules found in the geological record.

Phosphorite from the Continental Margin off Madras, Bay of Bengal

A recent survey reveals that phosphorites occur over an area of 800 km2 between 100 and 500 m depths on the outer continental shelf and upper slope ESE off Madras. Appreciable subsurface continuity of phosphorites is not indicated. They mostly occur over two terraces recorded between the 100 and 200, and 350 and 400m isobaths. Sampled phosphorites may be classified under four types: (1) dense conglomeratic phosphorites in the size range 2 5 cm, (2) dark gray phosphatic nodules in the size range 1 3 cm, (3) creamy white calcareous algal nodules with phosphatic laminations in the size range 0.5 3 cm, and (4) recent gastropod shells with phosphatic studs. Conglomeratic phosphorites confined mostly to the upper terrace were recov ered along with calcareous pelecypod and gastropod fossils belonging to Cretaceous to Oligocene age. Gray phosphatic nodules occur over upper as well as lower terraces. The algal nodules are mostly confined to the upper terrace and are less abundant. Acoustic profiles (3.5 kHz) and the recovery of fossils indicate that the upper terrace is a karstic surface of a carbonate platform, probably of Cretaceous to Oligocene age. Petrographic study of the conglomeratic phosphorite shows skeletal matter set in phosphatic matrix and what appears to be the phosphatized limestone. Selective phosphatization of foraminiferal tests is common. Dark gray phosphatic nodules show brownish-gray laminations containing phosphatized skeletal fragments set in phosphatic matrix. Creamy white calcareous algal nodules exhibit alternate gray phosphatic and white calcitic laminations. All the phosphorite types are composed of carbonate fluorapatite and calcite as major minerals, followed by quartz and aragonite. Phosphorous pentoxide content of conglomeratic type varies from 15% to 20%, dark gray nodules from 16% to 19.98%, and calcareous algal nodules from 8.10% to 17.54%. The significant enrichment of Mo and Pb in phosphorites is attributed to their fixation in organic matter under reduced condition. SEM studies indicate a microbial origin for the gray phosphatic and algal nodules. Studies suggest four episodes of phosphatization with intervening reworking and redoposition, probably commencing from the Eocene period. Phosphatization during the late Pleistocene and early Holocene period also can be ruled out.

Geology of the Paleocene Sepultura Formation, Mesa de La Sepultura, Baja California

At its Mesa de La Sepultura type area, the upper-lower to upper Paleocene Sepultura Formation lies with apparent disconformity on the lower Maastrichtian, marine upper slope and shelf deposits of the Rosario Formation. In places the uppermost Rosario Formation is marked by a kaolinite-rich paleosol. The Sepultura Formation is a deepening-upward sequence divided into a lower glauconitic-clastic member and an upper limestone member. The lower member has a basal plutonic-volcanic conglomerate overlain by sandstone containing abundant Turritella peninsularis of late-early to early-late Paleocene age. The dominant lithology is calcite-cemented, glauconitic, fossiliferous arkose or lithic arkose of apparent Peninsular Ranges provenance. The beds contain abundant red algal nodules, storm deposits, and iron-stained corrosion surfaces. Terrigenous sediment influx was diverted so that the upper member is virtually pure limestone dominated by branching red algae fragments and forams but devoid of micrite. The Sepultura Formation at Mesa de La Sepultura is a carbonate sediment-dominated record of late-early to late Paleocene deepening that progressed from shallowest marine at the base to 200 m deep shelf waters at the top. During this time, terrigenous coarse sediment deposition was blocked here even though it dominated in areas to the north and South.

An early Holocene reef in the western Atlantic: submersible investigations of a deep relict reef off the west coast of Barbados, W.I.

Submersible observations and collections reveal that a probable relict reef off the west coast of Barbados has a rich cover of sponges, along with algae and scattered corals, on a substrate of algal nodules in a muddy-sand matrix. The collections provide new data on the distributions of these fauna. This relict reef is about 20 km long, has a relief of up to 10 m, and is established at a depth of 80 m. Relict shallow-water features in other areas at similar depths along with data from core holes drilled off the south coast of Barbados suggest that this reef was probably established about 12,000 years ago and existed for no more than 2,000 years, during the Holocene sea-level transgression.

Phosphatised limestones and associated sediments from the western continental shelf of India

The phosphatised limestones and associated sediments of the outer continental shelf of the west coast of India between Goa and Ratnagiri at depths of between 70 and 120 m have been studied. The highest P 2O 5 content (9%, ranging from 2 to 9% depending on location) occur in the algal nodules. Polished and thin sections show that the algal nodules grew with the occasional microbial mat. These mats apparently formed by trapping and binding of sediment particles, which were subsequently phosphatised. Phosphate also occurs as pellets and rounded structures. High-magnesium calcite and carbonate fluorapatite are the major minerals. Aragonite, low-magnesium calcite and quartz are also present in some samples. Microprobe geochemistry of the phosphatised material of the algal nodules reveals the presence of land-derived siliciclastic detritus and the possibility of more than one source for Ca and P. Phosphate also occurred as cavity fillings in coral and bryozoan limestones, but not in oolitic limestones, regardless of their occurrence at the same depths and in the vicinity of the other limestones. The associated sediments are mostly sandy and show smaller concentrations of P 2O 5 (average 0.3%) and organic carbon (average 0.93%) as compared to the regions of upwelling where phosphorites are forming; the correlation exhibited between the P 2O 5 and the organic carbon is negative. The environmental conditions for the formation of phosphatised limestones and possible sources of phosphate are discussed. Sheath organic matter associated with filaments appears to be the major source for phosphorus; the phosphatisation is a microbial process which takes place at shallow depths. The impact of present-day upwelling is insignificant in the phosphatisation of the limestones.

Benthic Biological Studies of the Southwest Florida Shelf

Between 1980 and 1987, the U.S. Department of the Interior funded a series of benthic studies of the continental shelf off southwestern Florida. The goal was to gather environmental information in order to make decisions about offshore oil leasing. One major study element was habitat mapping. Fourteen transects were surveyed geophysically (side scan sonar, subbottom profiler) and visually (underwater video and still cameras), and the results were compiled in atlases showing the distribution of substratum types and visually distinct benthic communities. Rock outcrops were rarely seen and usually of low relief (<1 m), but patches of reef-associated sessile epifauna such as sponges, hard corals, gorgonians, ascidians, and bryozoans occupied 31% of the seafloor surveyed. Most of the sessile epifauna were seen on hard bottom covered by a thin sand veneer, or on biogenic rubble layers (shell rubble, coralline algal nodules). A second major study element was benthic station sampling. Fifty-five stations were sampled from two to twelve times each, and over 1,500 species of epibiota and over 1,100 species of macroinfauna were identified. The species composition of both hard- and softbottom communities varied primarily in relation to water depth.

Crustose Coralline Algae from the Flower Garden Banks, Northwestern Gulf of Mexico: Controls on Distribution and Growth Morphology

ABSTRACT The East and West Flower Garden Banks in the northwestern Gulf of Mexico represent the northernmost tropical coral reefs and Caribbean coralline algal flora on the North American continental shelf. Coralline algae are the dominant framework builders and sediment contributors below 50 m on the banks. On the basis of growth form and taxonomic composition, coralline algae on the Flower Garden Banks can be divided into four overlapping depth zones: upper coral reefs (20-30 m), Madracis-algal ridges (30-40 m), algal nodule zone (45-80 m), and deep algal reefs (50-90 m). Corallines on the upper coral reefs are dominated by Hydrolithon, Lithophyllum, and Porolithon. Their growth form as thick, laterally restricted crusts and knobs results from intensive grazing by parrotfish and urchins. Grazing is diminished on the Madracis-algal ridges below the upper reefs, and corallines grow here as laterally extensive plates, binding coral rubble. Dominant corallines on the ridges are Lithophyllum, Paragoniolithon, and Tenarea. Algal nodules (rhodoliths) o the deeper bank slopes are the most widespread algal growth form on the Flower Garden Banks. On the basis of shape, internal structure, and composition, rhodoliths are divided into an upper zone (where they have a spherical shape with concentric laminae) and a lower zone (discoidal shape with irregular laminae). Diminishing current velocity with increasing depth controls nodule morphology. The red alga, Peyssonnelia, and the foraminifer, Gypsina, are two non-coralline components that contribute substantially to nodule growth. Reduced current velocity in the lower nodule zone enables corallines and Peyssonnelia to develop laterally extensive pavements and 1-2 m high algal reefs. Taxonomic composition, high framework porosity, and irregular internal growth morphology f the deep algal reefs distinguish these structures from shallow algal ridges described elsewhere. The 90 m isobath at the Flower Garden Banks approximates the upward limit of a turbid nepheloid layer composed of continually resuspended sediment. Reefal structures within this water layer are dead and are now covered by fine sediment.

Microbial origin of the phosphorites of the western continental shelf of India

Phosphatised algal nodules were recovered by dredging from the western continental shelf off Goa at depths ranging from 70 to 150 m. Energy dispersive X-ray analysis indicates that the phosphorus present in the algal nodules is confined to brownish grains and occurs as cavity fillings. Phosphate grains are predominantly distributed in the outer layers and become less abundant towards the centre of the nodule P 2O 5 and fluorine contents are higher (9.5 and 1.25%, respectively) in the outer laminations while they are lower (5.4 and 0.78%) in the inner layers. SEM studies have revealed that the phosphate grains comprise endolithic filamentous fungi. Phosphatised cellular structures resembling bacteria are also present. It is thus envisaged that the direct role of upwelling compared to the microbial influence is probably of less significance in the phosphatisation of these limestones.