Question

describe how oxygen isotopes provide a proxy for past sea ice volume

Answer 1

The oxygen-isotope record is commonly used as a proxy for global ice isotopes.Isotopes are atoms of same element that have different number of neutrons.Depending on the climate two types of oxygen isotope (lighter-16O and heavier-18O)vary in water.Scientists compare the ratio of 18O and 16O isotopes in icecores, sediments or fossils .They compare this ratio to a standard ratio of oxygen isotope found in ocean water at a depth of 200-500meters.

More evaporation occurs in warmer regions of ocean and water containing the lighter isotope 16O evaporates more quickly than the water containingthe heavier isotope 18O.Water vapour containing the heavier 18O will condense and precipitate more quickly than water vapour containing the lighter 16O.As water evaporates in warmer regions,it is moved with air by convection towards the polar regions.The water vapour that reaches the polar regions precipitate as snow , eventually becoming ice.When the ocean is colder it takes more energy to evaporate the heavier isotope.The water vapour with heavier isotope condenses and precipitates out first at lower latitudes.This causes the oceans to have more 18O.In warmer climate ocean water contain more 16O because as ice sheet melt,the water with 16O is returned to ocean.

Ocean-floor sediments can also be used to determine the past climate.They reflect the oxygen isotope of the ocean water, because the oxygen in the calcium carbonate shells that are deposited on the ocean floor records the oxygen isotope variations in the ocean at the time of formation.