Water Isotopes in the Hydrosphere I10/10/05 Lecture outline: 1)the hydrological cycle  D and  18 O variability 3)fractionation processes  18 O,  D of precipitation 5)modeling  18 O precip spectrometer light intake

The Hydrosphere How do 18 O, 16 O (  18 O) and 2 H, 1 H (  D) move through this system?

Water Isotopic Variations Ocean  18 O = 0 ± 2‰  D = 0 ± 16‰ Lake Michigan  18 O = -7‰  D = -54‰ Lake Chad  18 O = -20‰  D = -110‰ Dead Sea  18 O = +4.4‰  D = 0‰ NOTE: water isotopes are always reported with respect to SMOW What processes explain these variations?

Water Isotopic Fractionation – review from last lecture Reminder: Oxygen and hydrogen isotopes are strongly fractionated as they move through the hydrological cycle, because of the large fractionation associated with evaporation/condensation. This fractionation is temperature-dependent. GNIP – global network of isotopes in precipitation Rainwater samples are routinely collected for  18 O and  D analysis all over the world. The data are stored and managed by GNIP, and used to study the processes that fractionate water isotopes.

Rozanski, 1993  18 O of rain near SMOW in tropics, highly depleted in high-latitudes  18 O of rain decreases far from vapor source (Raleigh) and is heavier during winter (temperature) Water Isotopic Fractionation – some data

Temperature effect on the  18 O of precipitation holds for both spatial T variability and temporal variability Rozanski, 1993

But what if we add all the GNIP global  18 O precip data? However, what is happening at higher temperatures? A bit more complicated, but generally strong relationship. Rozanski, 1993

Dansgaard, 1964 Rozanski, 1993 The so-called “amount” effect: more rain, heavier  18 O Empirical relationship – meaning….? It would be difficult to explain a vapor source at +1‰, when the tropical oceans are ~0‰. Thought to be linked to increased evaporation of raindrop in dry, under-saturated environment… (i.e. vapor is -9‰ ish, but the raindrop is enriched as it falls from the sky) Mechanism still unknown – need atmospheric modeler’s help. NOTE: only in tropics (<30  N and S), where “deep convection” takes place

Surface Water Salinity-  18 O relationship - general So  18 O of surface waters, like salinity, is also correlated to evaporation – precipitation. Global precipitation

Fairbanks et al., 1997 Surface Water Salinity-  18 O relationship - tropics Slope of  18 O-salinity relationship is in the deep tropics (<5  N and S), vs elsewhere. Why?

The “Global Meteoric Water Line” – what happens to  18 O happens to  D, but with a different  Craig, 1961 Rozanski, 1993 annual mean  D vs.  18 O of precipitation But month-to-month variations at a given site fall off this line – “deuterium excess”

Why don’t all waters fall on the GMWL? Or…. why do different “source” waters have different ‘deuterium excess’ values? Planetary boundary layer the layer where exchange occurs between the surface and the free atmosphere -evaporation not purely equilibrium process -what other type of fractionation is involved? Water Fact: water vapor above the ocean is -13‰ in  18 O, not the -9.2‰ expected from equilibrium fractionation. Why? 1-3km Given the potential for complicated boundary layer physics, it’s a wonder that the GMWL exists at all!

Deuterium excess Humid regions will show smaller departures from GMWL than arid regions. Generally interpreted as a proxy for the “source” of the moisture.

Noone, D., 2002 Modeling water isotopes in the hydrosphere Full atmospheric General Circulation Model (GCM) with water isotope fractionation included. Goal: quantify physical processes associated with water isotope variability Applications: atmospheric mixing, vapor source regions, impact of climate variability on hydrological cycle, interpretation of paleoceanographic records