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During the electrolysis of water in an acidified medium, ozone is produced, in association with oxygen, at the anode. This ozone is found to be depleted in heavy isotopes ([sup.18]O and [sup.17]O), with respect to the source water, following a strict mass-dependent rule. Our experiments also suggest that the isotopes are distributed at the apex and base positions of the bent ozone molecule in a random fashion, without obeying the zero-point energy constraint Endowed with these characteristics, the electrolytic ozone provides a source of reference that has a known internal heavy isotope distribution for spectroscopic studies. In addition, this ozone, when subjected to photolytic decomposition, can be used as a source of atomic oxygen with mass-dependent isotope ratios that can be varied by simply changing the water composition. Such an oxygen source is important for studying isotope effects in gas-phase recombination/exchange reactions such as COO + [O.sup.*] [right arrow] [[COOO.sup.*]] [right arrow] [COO.sup.*] + O.

Gross and net [O.sub.2] production between May 1996 and February 1999 was determined in bottle incubation experiments with [H.sub.2][sup.18]O spike and from the change in [O.sub.2] concentration. Carbon fixation rates were obtained from [sup.14]C incubations. In general, production rates determined using the [H.sub.2][sup.18]O-spike were about twice the primary production determined by the [sup.14]C method, where the latter was close to net oxygen evolution. These relationships are similar to results for the open ocean. During the spring bloom, when the dinoflagellate Peridinium was abundant, the ratio of gross [O.sub.2] production to carbon fixation was about 7.5, and net [O.sub.2] production was greater than carbon fixation. The difference between [O.sub.2] gross production and carbon fixation results, at least in part, from uptake by Mehler reaction and from recycling of the [sup.14]C tracer by dark respiration and the alternative oxidase (AOX). We used the difference in isotopic discrimination against [sup.18]O, occurring during [O.sub.2] consumption by various biological pathways, to place constraints on the relative engagement of these pathways. We estimated the overall discrimination against [sup.18]O in the lake from [O.sub.2] isotopic mass balance as 20.5-29 [per thousand]. The only mechanism that can explain the strong overall fractionation in the lake is AOX, which strongly discriminates against [sup.18]O (~31 [per thousand]). Our results show, for the first time, that uptake by AOX is widespread and quantitatively important to oxygen consumption in aquatic systems.

A high-resolution record of carbon isotope changes during the late Holocene was obtained in core GA-112 from the easternmost Mediterranean. Over the past 3600 yr, a gradual [sup.13]C-depletion trend shown by both planktonic and benthic foraminifera corresponds to a global rise in atmospheric C[O.sub.2]. The global and local trends suggest that aridification and biomass destruction in the Nile basin played a major role in the global C[O.sub.2] rise in the late Holocene. Superimposed on the general [delta][sup.13]C trend are four mirror-image cycles in [delta][sup.13]C of planktonic and benthic foraminifera. Higher planktonic [delta][sup.13]C values reflect periods of high productivity off the coast of Israel. Simultaneously, decrease in the benthic [delta][sup.13]C values occurred because of degradation of higher organic-matter fluxes to the sediment. Therefore, large differences between the two records indicate periods of high productivity. The Eastern Mediterranean productivity fluctuations respond to a periodical replenishment of nutrients to the mixed layer via Nile runoff. Historical data of the Nile flood fluctuations for the past ~1400 yr show a remarkable coincidence between high and low Nile floods and high and low Eastern Mediterranean productivity levels, which are correlated with the periodic monsoonal activity over the Nile headwaters. Keywords: Eastern Mediterranean, foraminiferal [delta][sup.13]C, Nile-derived nutrients, productivity cycles, late Holocene.

The naturally occurring isotopes of oxygen are of mass numbers 16,17 and 18, and their ratio in atmospheric O(sub2) relies mainly on the isotopic composition of O(sub2) produced from terrestrial and aquatic plants. An experiment based on estimates of the size of the (super17)O/(super16)O anomaly in tropospheric O(sub2) was undertaken. It is argued that it mainly reflects the influences of biospheric cycling and stratospheric photochemical processes. It is proposed that the magnitude of the tropospheric (super17)O anomaly is useful as a tracer of global biosphere production.

Plant production in the sea is a primary mechanism of global oxygen formation and carbon fixation. For this reason, and also because the ocean is a major sink for fossil fuel carbon dioxide, much attention has been given to estimating marine primary production. Here, we describe an approach for estimating production of photosynthetic oxygen, based on the isotopic composition of dissolved oxygen of seawater. This method allows the estimation of integrated oceanic productivity on a time scare of weeks., Our knowledge of the rate of marine photosynthetic production is based primarily on bottle incubation experiments (1). These experiments provide local instantaneous primary production rates, which often miss the effects [...]