Chapter 9
THE PERIPHERAL DEEP SEAS
Paul A. TYLER
INTRODUCTION
The major oceans of the world occupy over 50% of the
surface of the planet. It is in these major oceans that
the greatest depths are found and in which the greatest
research effort has been concentrated (see Chapters
5 to 8). There are, however, a number of seas peripheral
to the main oceans that have basins of oceanic-depth,
are of great interest and have been considered worthy
of study. The largest of these is the Mediterranean,
which itself has peripheral seas. The ‘newest’ ocean
is the Red Sea, with its bottom water warmer than
the surface waters as a result of hydrothermal activity.
In the Americas, the Caribbean has many features
similar to those of the Mediterranean, whilst the Gulf of
Mexico is the only deep peripheral sea known to have
deep chemosynthetically-supported communities. The
deep basins of the Indonesian seas were the only deep
peripheral seas to be sampled by the Challenger. In the
Far East are the Sea of Okhotsk and the Sea of Japan
with their deep basins, the former linked with the main
Pacific and the latter with very limited exchange.
Because these seas border the main ocean, they
are relatively close to land and the local climate has
discernable effects either in the local hydrography, or
through contributions of allochthonous organic matter
from terrestrial vegetation. Major rivers, as well as
aeolian particle transport, may also have a significant
impact on sedimentation. In nearly all cases, however,
the deep-sea ecosystem within these peripheral seas is
driven by the vertical flux of surface production to the
seabed. A notable exclusion is the Black Sea. This sea
is omitted as it is anoxic below ~250 m and does not
have a ‘deep-sea’ metazoan fauna. Recent reviews of
the Black Sea have been published by Izdar and Murray
(1991) and Ivanov and Oguz (1997).
The deep peripheral seas can be examined from a
variety of perspectives. Their morphology is a function
of their geological history, whilst their hydrography
is driven by heat and salt balance as well as by
influx of oceanic waters from the major oceans. In the
Mediterranean and Red Seas evaporation far exceeds
freshwater influx (Table 9.1), and thus surface waters
become dense by increasing salinity. In the Mediterranean the deep water forms when this saline surface
water undergoes winter cooling. In the Caribbean Sea
and the Gulf of Mexico evaporation exceeds freshwater
inflow to a lesser extent (Table 9.1), and the influx of
water from the Atlantic has an effect on both these
areas. On the other hand the Gulf of Mexico makes
a significant contribution to the Atlantic in the form
of the Gulf Stream. Also in warm latitudes are the
deep basins of the Indonesian Seas, where evaporation
exceeds precipitation but there is a strong seasonal
component forced by the monsoon. In the Sea of Japan
evaporation is less than freshwater inflow, and severe
winter cooling is required to form deep water. Lastly
in the very cold Sea of Okhotsk (Okhotskoye More)
there is great freshwater input but the extreme winter
cooling, which results in freezing of the surface waters,
determines the formation of deep water.
HYDROGRAPHY AND PHYSICAL PROPERTIES
The Mediterranean Sea
Morphology
The Mediterranean is the largest of the seas peripheral to the main oceans, and consists of two deep
basins, the western and eastern basin separated by
the Straits of Sicily, a sill of ~400 m depth. The
western extremity of the Mediterranean is the Strait of
Gibraltar with a sill depth of 350 m separating the deep
261
THE PERIPHERAL DEEP SEAS
Paul A. TYLER
INTRODUCTION
The major oceans of the world occupy over 50% of the
surface of the planet. It is in these major oceans that
the greatest depths are found and in which the greatest
research effort has been concentrated (see Chapters
5 to 8). There are, however, a number of seas peripheral
to the main oceans that have basins of oceanic-depth,
are of great interest and have been considered worthy
of study. The largest of these is the Mediterranean,
which itself has peripheral seas. The ‘newest’ ocean
is the Red Sea, with its bottom water warmer than
the surface waters as a result of hydrothermal activity.
In the Americas, the Caribbean has many features
similar to those of the Mediterranean, whilst the Gulf of
Mexico is the only deep peripheral sea known to have
deep chemosynthetically-supported communities. The
deep basins of the Indonesian seas were the only deep
peripheral seas to be sampled by the Challenger. In the
Far East are the Sea of Okhotsk and the Sea of Japan
with their deep basins, the former linked with the main
Pacific and the latter with very limited exchange.
Because these seas border the main ocean, they
are relatively close to land and the local climate has
discernable effects either in the local hydrography, or
through contributions of allochthonous organic matter
from terrestrial vegetation. Major rivers, as well as
aeolian particle transport, may also have a significant
impact on sedimentation. In nearly all cases, however,
the deep-sea ecosystem within these peripheral seas is
driven by the vertical flux of surface production to the
seabed. A notable exclusion is the Black Sea. This sea
is omitted as it is anoxic below ~250 m and does not
have a ‘deep-sea’ metazoan fauna. Recent reviews of
the Black Sea have been published by Izdar and Murray
(1991) and Ivanov and Oguz (1997).
The deep peripheral seas can be examined from a
variety of perspectives. Their morphology is a function
of their geological history, whilst their hydrography
is driven by heat and salt balance as well as by
influx of oceanic waters from the major oceans. In the
Mediterranean and Red Seas evaporation far exceeds
freshwater influx (Table 9.1), and thus surface waters
become dense by increasing salinity. In the Mediterranean the deep water forms when this saline surface
water undergoes winter cooling. In the Caribbean Sea
and the Gulf of Mexico evaporation exceeds freshwater
inflow to a lesser extent (Table 9.1), and the influx of
water from the Atlantic has an effect on both these
areas. On the other hand the Gulf of Mexico makes
a significant contribution to the Atlantic in the form
of the Gulf Stream. Also in warm latitudes are the
deep basins of the Indonesian Seas, where evaporation
exceeds precipitation but there is a strong seasonal
component forced by the monsoon. In the Sea of Japan
evaporation is less than freshwater inflow, and severe
winter cooling is required to form deep water. Lastly
in the very cold Sea of Okhotsk (Okhotskoye More)
there is great freshwater input but the extreme winter
cooling, which results in freezing of the surface waters,
determines the formation of deep water.
HYDROGRAPHY AND PHYSICAL PROPERTIES
The Mediterranean Sea
Morphology
The Mediterranean is the largest of the seas peripheral to the main oceans, and consists of two deep
basins, the western and eastern basin separated by
the Straits of Sicily, a sill of ~400 m depth. The
western extremity of the Mediterranean is the Strait of
Gibraltar with a sill depth of 350 m separating the deep
261