Page 37 - (DK) Ocean - The Definitive Visual Guide

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Salinity Pressure

Salinity is an expression of the amount of salt in a fixed mass of seawater. Scientists measure pressure in units called bars.
It is determined by measuring a seawater sample’s electrical conductivity At sea level, the weight of the atmosphere
and averages about 35 grams of salt per kilogram of seawater. Salinity exerts a pressure of about one bar. Underwater,
varies considerably over the surface of oceans – its value at any particular pressure increases at the rate of one bar for
spot depends on what processes or factors are operating at that location every 10m (33ft) increase in depth, due to the
that either add or remove water. Factors that add water, causing low weight of the overlying water. This means that
salinity, include high rainfall, river input, or melting of sea-ice. Processes at 70m (230ft), for example, the total pressure
that remove water, causing high salinity, include high evaporative losses is eight bars or eight times the surface pressure.
and sea-ice formation. At depth, salinity is near constant throughout This pressure increase poses a challenge to
the oceans. Between the surface and deep water is a region called a human exploration of the oceans. To inflate
halocline, where salinity gradually increases or decreases with depth. their lungs underwater, divers have to breathe
Salinity affects the freezing point of seawater – the higher the pressurized air or other
salinity, the lower the freezing point. gas mixtures, but doing DECOMPRESSION STOP
so can cause additional To avoid a condition called
problems (arising from “bends” that can arise from
the dissolution of excess decompressing too quickly,
gas in body tissues). on their way to the surface
scuba divers make one or
These problems limit more timed stops to
the depths attainable. release excess gas.

NATURAL ADAPTATION
Elephant seals can dive to depths
of up to 1,550m (5,100ft). They
have evolved various adaptations
for coping with the high pressure,
KEY
including collapsible ribcages.
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EASY FLOATING GLOBAL SALINITY
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In some enclosed seas Surface salinity is highest
where evaporative in the subtropics, where 35 DISCOVERY
losses are high and there evaporative losses of 34 DECOMPRESSION
is little rainfall or river water are high, or in 33
inflow, the sea- water enclosed or semi-enclosed 32
can become so saline basins (such as the After working underwater for
31
and dense that floating Mediterranean). It is hours at a time, professional
30
becomes easy. This is lowest in colder regions divers routinely undergo
29
the case here in the or where there are large controlled decompression in a
Dead Sea. inflows of river water. under 29 purpose-built pressure chamber.
parts per These facilities are also used
thousand (‰) to treat pressure-related diving
Density illnesses and for research into
diving physiology.
The density of any small portion of seawater depends primarily on its
temperature and salinity. Any decrease in temperature or increase in PRESSURE CHAMBER
salinity makes seawater denser – an exception being a temperature drop The person being decompressed may have
below 4˚C (39˚F), which actually makes it a little less dense. In any part to breathe a special gas mixture while the
ambient pressure is slowly reduced.
of the ocean, the density of the water increases with depth, because
dense water always sinks if there is less dense water below it.
Processes that change the density of seawater cause it to either rise or Atlantic Intermediate
sink, and drive large-scale circulation in the oceans between the surface Atlantic Central Water: Water: cool layer of
warm warm, low-density intermediate density,
and deep water (see p.60). Most important is water carried towards surface surface waters in the forms and sinks in
Antarctica and the Arctic Ocean flow tropics and subtropics north Atlantic, then
Antarctic Intermediate moves south
fringes. This becomes denser as it Water: cool layer
cools and through an increase in of intermediate
density, sinks and
its salinity as a result of sea-ice moves north
formation. In these regions large
quantities of cold, dense, salty
water continually form and sink
towards the ocean floor.
DENSITY LAYERS IN ATLANTIC
The oceans each contain distinct, named INTRODUCTION
water masses that increase in density from
the surface downwards. The denser, cooler
masses sink and move slowly towards the Antarctic Bottom Water: mid-ocean ridge North Atlantic Deep Water:
Equator. The cold, high-density deep and coldest and densest layer, cold, dense water, forms
bottom waters comprise 80 per cent of the forms close to Antarctica, and sinks in north Atlantic,
sinks then moves north
then moves south
total volume of the ocean.