Q: Why is the
ocean salty? And what are the benefits of its saltiness? (Emma, Paul)
A: Yayyyy! I love this question – it seems so simple,
and then there are all of these nuances that become important and it’s not a
simple story. So here goes: The ocean is salty because it slowly
accumulated salts over the course of its formation, and with the formation of
the hydrologic cycle of the earth.
Nowadays, there is a tidy balance between th continuous onslaught of
salts being delivered from rivers, and the output (export or sequestration) of
those materials. On land, rocks are
weathered through chemical and mechanical weathering that dislodges minerals
from the rocks and mobilizes them into runoff waters such as rain, rivulets,
groundwater and rivers and streams. This
weathered material can take the form of dissolved salts or suspended particles
(sediment). Regardless, a huge amount
of sediment and dissolved minerals are delivered to the ocean at the mouths of
rivers. There are other sources of salt
material, as well: dust mobilized from
the great deserts and deposited in the ocean, hydrothermal vents and underwater
volcanic eruptions, for example. It seemed
counter-intuitive to me that this bit of salt could make a difference in the
enormous ocean, but we’ve used this idea
of salts input and outputs we see today to explain how salty must have formed
on early earth. Some of the ocean’s
salts – especially chlorine – are extremely soluble, and extremely unlikely to
be removed from the water column once they’ve entered. The water in the ocean does not mix and turn
over very quickly, so the salt eventually accumulated, but it took time. The commonly held belief now is that salts
built up in the ocean over a very long time period, enough to make the water
salty, and now the ocean is in a relative steady state with respect to salinity
(the latter is not disputed, and there is a huge amount of evidence that shows
that salinity has been stable for millions, maybe billions of years). When ocean water evaporates, the salts are
left behind. Back to present day
ocean: there is a net balance of salt in
and salt out, while water continues to flow in and evaporate out, and we
experience the salty ancient ocean. Some
of the salts are taken out of the water by organisms (especially calcium
carbonate formation), by volcanic activity on the seafloor, and by the
subduction of tectonic plates, but these are all slow processes.
The salinity of the ocean ranges from brackish in the
estuaries and some bays, with a salinity of less than 1, to high salinities in
a place like the Mediterranean Sea with salinities upwards of 38 (parts per
thousand). Most of the ocean ranges from
between 32 – 36. The units used to
describe salinity are interesting, because we don’t use a measure of actual
salt per volume water anymore. We now
use the conductivity of the water, which indicates the level of salts, and the
measure is a unitless ratio, so sometimes we use “PSU” to denote practical
salinity units, but it’s not necessary to use units and sometimes it’s frowned
upon.
The ocean being salty is important, because we wouldn’t
have life as we know it without it! This
is because of the hydrologic cycle. If
we didn’t have our hydrologic cycle, we wouldn’t be here. We rely on the cycle for our water, and
everything we eat depends on it too. The
ocean and the land are inextricably connected by the water and biogeochemical
cycles of the planet, and we need them all to function.
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