Lithoherms

Introduction

Lithoherms are deep-water carbonate mounds that occur in a broad belt in the northeastern Strait of Florida along deep margin of Little Bahama Bank at depths of 600-800 m. They were first mentioned as haystack-like features but were mapped as downslope-oriented ridges and gullies. In 1975, scientists aboard the submersible Alvin found that they were narrow, elongated mounds ranging from a few meters long and a few centimeters high to 300 m long and 50 m high, oriented north-south, parallel to the Florida Current and the bank margin. The name is a contraction of “lithified bioherm.” Bioherm is a general term for any mound-like accumulation built from the skeletal remains of marine organisms such as corals, mollusks or echinoderms. Lithoherms taper towards their ends, with a sharp crest and steep sides. Larger lithoherms have a blunt upcurrent (southward) nose that may be eroded or covered by the deep-sea branching coral Lophelia pertusa, which can produce a terraced apron of mud and coral debris that extends out onto the flat seafloor between mounds.

Geology

Lithoherms appear to be built up as a series of overlapping layers of hardened limestone crusts, ~10-30 cm thick, with less lithified material between. This onion-like layering suggests that these mounds developed via alternating episodes of sediment accumulation and cementation. Layers visible along the mound flanks are carved into characteristic crescent-shaped embayments, probably by excavating organisms such as clionaid sponges. The lithoherm crests and northern, down-current ends are often eroded into complex, irregular labyrinthine topography. Because living corals occur chiefly on the upcurrent end, and crust samples collected along the entire mound length contain fossil coral fragments (as well as shells and skeletons of both plankton and benthic invertebrates), it is most likely that lithoherms grow forward into the current as they build upward and out to the sides. Although some mound-like features along the southeastern U.S. continental margin have also been called lithoherms, they appear to differ substantially in structure and origin from those in the northeastern Strait of Florida.

Oceanography

Water temperatures in the lithoherm belt range from 5 to 8°C. Although these mounds lie below much of the direct influence of the overlying Florida Current, tidally influenced bottom currents observed from submersibles can exceed 100 cm sec-1. Ripple marks in the sand between mounds are also evidence of bottom currents.

Biology

Lithoherms exhibit a diverse fauna dominated by suspension-feeding, attached organisms. The northbound water flow generates a distinct zonation of different organisms on different parts of the mounds, where they are subject to different flow conditions. On the larger mounds, Lophelia pertusa dominates the upcurrent end. The crests and higher flanks are characterized by gardens of large golden coral fans, the colonial anemone Gerardia sp. Carbon-14 measurements suggest that this species may reach 2000 years old. Lower flanks and downcurrent crests are dominated by often dense lawns of primnoid octocoral fans and stalked crinoids, the latter clinging preferentially to the edges of crusts, so that they often appear to be planted in rows. These three zones overlap, and smaller mounds generally lack the branching corals and golden corals. Other organisms include stylasterid lace corals, brisingid seastars, brittle and basketstars, feather stars (includingthe tiny brooding Comatilia iridometriformis),sea anemones, and over 20 species of sponges. Flat and low-relief hard substrates between mounds harbor many of the same organisms but add other species, such as white precious coral ( Corallium niobe ), fan sponges (Phakellia), and the sea lily Neocrinus blakei.

Habitats


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Porifera


Cnidaria

ZOANTHIDEA – Colonial Anemones

ACTINIARIA – Sea Anemones

SCLERACTINIA – Stony Corals

OCTOCORALLIA – Soft Corals and Gorgonians

ANTIPATHARIA – Black Corals

Crustacea

Echinodermata

CRINOIDEA – Sea Lilies and Feather Stars

ASTEROIDEA – Sea Stars

ECHINOIDEA – Sea Urchins and Sand Dollars

OPHIUROIDEA – Brittle stars, Snake stars and Basket stars

Chondrichthyes