foraminiferan Gypsina sp.: by contrast, Gypsina cover
was minimal on the slope rhodoliths. Rhodoliths that
had spilled down from the relatively flat platform
tended to be concentrated in fan-shaped deposits that were
more prevalent near the tops of the slopes. Cover of living
crustose algae on the deeper slope rhodoliths averaged
only 22.8% and was made up of 14.8% unidentified
crustose corallines, 6% Lithophyllum sp., and 2%
Peyssonnelia sp.
Throughout platform depths from 67 to 91 m,
rhodoliths were fairly uniform in composition and abundance. They ranged from 4 to 15 cm in diameter, with an
average diameter of about 9 cm, and were roughly spherical with smooth living surfaces. The rugose rhodoliths
spilling down the steep slopes of the seamount to depths
below 200 m were characteristically smaller (mean of
about 5 cm diameter), much rougher, and pitted by boring
organisms. Cross-sections through the centers of
upperplatform nodules showed outer relatively thin
(1–3 cm thick) well-preserved envelopes overlying dead
laminated crustose layers). These layers surrounded much
thicker cores of biotically altered carbonates (comprised
mostly of corallines, foraminiferans, and corals) that had
been extensively reworked by boring sponges, boring
algae, polychaetes, and pelecypods. Radiocarbon dating
indicated that the outermost unaltered envelopes beneath
actively growing crusts averaged 429 years old
(range = 112 to 880 ybp), while the innermost unaltered
layers showed a mean age of 731 ybp (200–1,100 ybp).
Geology and evolution
Crustose coralline algae appeared in the early Cretaceous
and have been important components of shallow-marine
communities throughout the Cenozoic, mostly showing
long-term increases in species richness during much of
their history. There are currently over 1,600 described species of crustose coralline algae (Woelkerling, 1988) and
$649 fossil species (Aguirre et al., 2000). Interestingly,
Miocene coastal carbonate habitats are characterized by
a worldwide bloom of coralline red algal deposits (termed
rhodalgal facies). These extensively developed facies (i.e.,
characteristic sedimentary deposits) temporarily replaced
corals throughout the tropics and subtropics as dominant
carbonate producers (e.g., Esteban, 1996). By calibrating
modern carbonate assemblages to local oceanographic
conditions in the Gulf of California, Halfar et al. (2004)
demonstrated that the predominance of rhodalgal facies
occurred under mesotrophic to slightly eutrophic conditions. In the Mediterranean region, early to middle Miocene carbonates contain more rhodalgal components
than coral-reef deposits (Esteban, 1996). In addition to
being widespread globally, fossil coralline algae also
exhibited their greatest species richness during the early
and middle Cenozoic (early Miocene peak of 245 species;
Aguirre et al., 2000), with a collapse to a late Pleistocene
low of 43 species. In reviewing 37 representative late
Pliocene studies, Halfar and Mutti (2005) concluded that
although rhodalgal facies were clearly the dominant components at specific study sites, they often were not
Algae, Coralline, Figure 2 Spectrum of simple two-dimensional forms of nongeniculate (crustose) corallines. Upper right image
shows synchronous sloughing in Neogoniolithon fosliei.
ALGAE, CORALLINE
23
was minimal on the slope rhodoliths. Rhodoliths that
had spilled down from the relatively flat platform
tended to be concentrated in fan-shaped deposits that were
more prevalent near the tops of the slopes. Cover of living
crustose algae on the deeper slope rhodoliths averaged
only 22.8% and was made up of 14.8% unidentified
crustose corallines, 6% Lithophyllum sp., and 2%
Peyssonnelia sp.
Throughout platform depths from 67 to 91 m,
rhodoliths were fairly uniform in composition and abundance. They ranged from 4 to 15 cm in diameter, with an
average diameter of about 9 cm, and were roughly spherical with smooth living surfaces. The rugose rhodoliths
spilling down the steep slopes of the seamount to depths
below 200 m were characteristically smaller (mean of
about 5 cm diameter), much rougher, and pitted by boring
organisms. Cross-sections through the centers of
upperplatform nodules showed outer relatively thin
(1–3 cm thick) well-preserved envelopes overlying dead
laminated crustose layers). These layers surrounded much
thicker cores of biotically altered carbonates (comprised
mostly of corallines, foraminiferans, and corals) that had
been extensively reworked by boring sponges, boring
algae, polychaetes, and pelecypods. Radiocarbon dating
indicated that the outermost unaltered envelopes beneath
actively growing crusts averaged 429 years old
(range = 112 to 880 ybp), while the innermost unaltered
layers showed a mean age of 731 ybp (200–1,100 ybp).
Geology and evolution
Crustose coralline algae appeared in the early Cretaceous
and have been important components of shallow-marine
communities throughout the Cenozoic, mostly showing
long-term increases in species richness during much of
their history. There are currently over 1,600 described species of crustose coralline algae (Woelkerling, 1988) and
$649 fossil species (Aguirre et al., 2000). Interestingly,
Miocene coastal carbonate habitats are characterized by
a worldwide bloom of coralline red algal deposits (termed
rhodalgal facies). These extensively developed facies (i.e.,
characteristic sedimentary deposits) temporarily replaced
corals throughout the tropics and subtropics as dominant
carbonate producers (e.g., Esteban, 1996). By calibrating
modern carbonate assemblages to local oceanographic
conditions in the Gulf of California, Halfar et al. (2004)
demonstrated that the predominance of rhodalgal facies
occurred under mesotrophic to slightly eutrophic conditions. In the Mediterranean region, early to middle Miocene carbonates contain more rhodalgal components
than coral-reef deposits (Esteban, 1996). In addition to
being widespread globally, fossil coralline algae also
exhibited their greatest species richness during the early
and middle Cenozoic (early Miocene peak of 245 species;
Aguirre et al., 2000), with a collapse to a late Pleistocene
low of 43 species. In reviewing 37 representative late
Pliocene studies, Halfar and Mutti (2005) concluded that
although rhodalgal facies were clearly the dominant components at specific study sites, they often were not
Algae, Coralline, Figure 2 Spectrum of simple two-dimensional forms of nongeniculate (crustose) corallines. Upper right image
shows synchronous sloughing in Neogoniolithon fosliei.
ALGAE, CORALLINE
23