lecture 3 - into to geochemical palaeoclimate indicators

Question 1

respectively, what are oxygen and carbon isotopes used for?

Answer 1

oxygen isotopes are used for palaeotemperature and determining ice volume history

carbon isotopes are used for understanding the carbon cycle

Question 2

describe the glacial effect on oxygen

Answer 2

in a cold climate, more ice is present which means average sea levels are lower. this is because water that would be in the ocean is not stored in the form of ice.

the lighter oxygen 16 isotope is preferentially trapped in ice, which means there is more of the heavier oxygen isotope present in the ocean.

on the other hand, in a warmer climate, less ice is present, meaning average sea levels are higher. this is because existing ice has melted as a result of higher temperatures. hence, the lighter oxygen 16 isotope is now present in the ocean

Question 3

explain the differences in oxygen isotopes during oceans in glacial and interglacial periods

Answer 3

glacial periods are generally characterised by lower sea levels and big ice caps. as a result, there is less light oxygen 16 isotope present in sea water as it is trapped in ice. hence, sea water has overall heavier oxygen 18 isotopes during glacial periods.

interglacial periods are generally characterised by high sea levels and small ice caps. as a result, there is more light oxygen 16 isotope present in sea water. this means that during interglacial periods, sea water has overall lighter oxygen 16 isotopes.

Question 4

what are some of the problems associated with using oxygen isotopes in marine calcite (what mostly makes up the shells of marine organisms) as a palaeoclimate indicator?

Answer 4

1. calcite becomes more altered over time
2. older limestones = less reliable oxygen isotopic values
3. oxygen isotopic composition of sea water may have steadily changed over time

Question 5

what have oxygen isotopes indicated about long term climate trends in the Cenozoic?

Answer 5

according to oxygen isotopes, climate during the Cenozoic has become increasingly cold over the last 50 millions years

Question 6

describe what carbon isotopes are used for (as palaeoclimate indicators) and what they indicate:

Answer 6

carbon isotopes are used to analyse and study the long term carbon cycle and climate.

photosynthesis produces organic matter with more light carbon isotopes, which means that if large amounts of organic matter are produced, the leftover seawater is HEAVIER (more carbon isotopes).

Question 7

what are some of the benefits of using carbon isotopes as a geochemical palaeoclimate indicator?

Answer 7

• carbon isotopes are not as easily altered by later processes compared to oxygen isotopes
• carbon isotopes are more useful in older periods, even Precambrian
• variations must relate to changes in long term carbon cycle which relates to oceanography and climate; provides a deep and more hollistic understanding of past climates