Droge climate hand out
.pdf
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School
University of Nebraska, Lincoln *
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Course
222
Subject
Geography
Date
Apr 3, 2024
Type
Pages
4
Uploaded by SuperWillpower8653
NRES222 Climate Lab Sp2023 Adapted from Dorian McGee University of South Florida 1 From Coral to Climate Records Much of our quantitative information about past climates comes from analysis and interpretation of stable isotopes in geologic materials. In this lab you will become familiar with the kind of equation that relates the concentration of oxygen isotopes in a coral to the temperature of the water in which it lives. Scenario You are an ocean researcher studying global warming. You are interested in seeing whether water temperatures on a coral reef in your field area have changed over the last 200 years. To do this, you must first establish whether the isotopic ratios in the coral species in this reef provide a good record of temperature. Previous research has established a working equation that can calculate temperatures within 1 °C from isotope ratios for a genus of coral that lives in your study area. Unfortunately, the equation was derived from a species that is different from the two species of the genus that occur at your reef. We will call them Species A and Species B. Will the equation work for either of them? You collect a live specimen of Species A and Species B. You microdrill the aragonite from four years of growth from their annual growth bands and determine the succession of 18
O ratios from the samples using a mass spectrometer. You have records in your marine lab of the water temperature and isotope ratio from a monitoring site at the reef. Does the working equation provide a means to calculate the water temperature from the 18
O ratios in your corals? Comparing Oxygen Isotopes: Background Information δ
18
O Temperature reconstructions rely on variations in the isotope ratio (
R
) of 18
O to 16
O in seawater and the record of those variations in the shells of marine organisms. The 18
O/
16
O ratio of oxygen atoms among the H
2
O molecules of seawater is very small: only about 0.2% of the oxygen atoms are 18
O; the remaining 99.8% are 16
O. Variations in the 18
O/
16
O ratios, of course, are smaller than the ratios themselves. So, instead of using those small values of R
to report the varying 18
O concentrations, geochemists use δ
18
O, a relative difference ratio: δ
18
O = !
!"#$%&’
!"()*)
− 1$ × 1000
The value of δ
18
O is stated as per mille
(‰, parts per thousand in the same way that % is per cent, for parts per hundred). A mass spectrometer, which measures the oxygen-isotope concentration, reports out the per mille δ
18
O of the sample. How it works to estimate past temperatures Because an atom of 16
O has less mass than an atom of 18
O, it is preferentially taken up when seawater evaporates, causing the relative amount of 18
O in the sea water to rise, or become enriched
(i.e., the δ
18
O increases). The 16
O in the evaporated water is then transported through the atmosphere to where it condenses and falls out as precipitation. In the tropics where most corals grow, conditions are typically warm and humid. At high temperatures, evaporation occurs at faster rates such that during warm periods the air is often near saturation with respect to water vapor, causing more frequent precipitation. As a result, when water (containing the 16
O) evaporates from the sea surface, it is not transported far before it precipitates as rain. Precipitation under these conditions is more local and the loss of 16
O in the seawater through evaporation can be roughly equal to the gain of 16
O through precipitation.
NRES222 Climate Lab Sp2023 Adapted from Dorian McGee University of South Florida 2 When air temperatures are cooler (such as during cold fronts, seasonal changes, or global cooling), evaporation is slower and it takes longer for the air to reach saturation. Hence, the water (and contained 16
O) evaporating from the sea surface can travel further before saturation is reached and precipitation occurs. Therefore, 16
O may not be replaced by local precipitation, leading to relatively higher concentrations of 18
O in the remaining water. Cooler atmosphere leads to enriched 18
O in seawater and warmer atmosphere leads to depleted 18
O in seawater. Some marine organisms precipitate calcium carbonate (CaCO
3
) from the seawater to make their shells or exoskeletons. Some of these organisms are able to secrete their hard parts in isotopic equilibrium
with the water. Simply stated, the isotopes incorporated into the CaCO
3
of the organism are of the same ratio as that in the seawater. Since δ
18
O values vary with temperature, we can use these ratios from the CaCO
3
of the organisms that are “recording” in equilibrium with the environment to track temperature trends back over time. Hence, their isotope records are considered to be good proxies for records of past temperatures. Biological factors relating to photosynthesis and metabolism, and external factors such the chemistry of the water, can also influence the isotope values of carbonate organisms. As a result, not all species of carbonate organisms provide a faithful record of environmental δ
18
O. Procedure The dataset you will be using is similar to an actual dataset currently being utilized by the author and data in NASA’s Global Seawater Oxygen-18 Database. Your simulated Excel dataset contains: •
Average daily water temps (as recorded approximately every 15 days for four years from the monitoring station at the modern reef) •
The δ
18
O value of the water at the reef (sampled at the same time the temperature was recorded) •
The δ
18
O value of the living specimens of Species A and B (microsampled at growth intervals of approximately every 15 days over the last four years) 1. Open the Canvas spreadsheet and save it to your drive using the following filename: Lastname_Climatelab 2. Examine the line graph comparing the recorded water temperatures with the recorded δ
18
O of the water. What do you notice about the relationship of δ
18
O
w
to the recorded temperature? The relationship is inversely proportional. 3. Create a scatter plot of δ
18
O
w
vs. temperature. What is the correlation between δ
18
O
w
and the recorded temperature? They appear to be directly proportional because as one decreases, so does the other. What does R
2
indicate? With our R^2 value being very close to 1.0, it means that our oxygen 18 in the water and daily temperature are highly correlated and related to one another.
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