Boaz Lazar

Effect of aragonite saturation, temperature, and nutrients on the community calcification rate of a coral reef

(1) In this study we investigated the relations between community calcification of an entire coral reef in the northern Red Sea and annual changes in temperature, aragonite saturation and nutrient loading over a two year period. Summer (April-October) and winter (November-March) average calcification rates varied between 60 ± 20 and 30 ± 20 mmolm � 2 � d � 1 , respectively. In general, calcification increased with temperature and aragonite saturation state of reef water with an apparent effect of nutrients, which is in agreement with most laboratory studies and in situ measurements of single coral growth rates. The calcification rates we measured in the reef correlated remarkably well with precipitation rates of inorganic aragonite calculated for the same temperature and degree of saturation ranges using empirical equations from the literature. This is a very significant finding considering that only a minute portion of reef calcification is inorganic. Hence, these relations could be used to predict the response of coral reefs to ocean acidification and warming.

Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers

Abstract The effect of early diagenesis on trace element abundance in coral skeleton was studied in live coral heads (Porites) from the Nature Reserve Reef (NRR), Elat, Gulf of Aqaba, northern Red Sea. Petrography of the corals shows diagenetic features of dissolution, recrystallization, and secondary aragonite precipitation (pore filling), which are most extensive in the oldest part of the coral. Coral porewaters were extracted with a special setup and were analyzed for chemical composition. The total alkalinity and Sr deficit in pore water as compared to reef water is consistent with both precipitation of secondary aragonite and recrystallization of primary skeleton. The present rate constant of pore filling by secondary aragonite was estimated by a water replacement experiment to be 0.0015 y−1, which equals to pore filling rate of 1.5 ± 0.3 kg aragonite per year. The corals show clear seasonal fluctuations in Sr/Ca ratios that are interpreted as reflecting changes in sea surface temperature (SST). Yet, the secondary aragonite is characterized by a significantly higher Sr/Ca ratio than the average ratio in primary aragonite. Thus, measuring a mixed sample of pristine and secondary aragonite may produce erroneous (about 2°C lower) SST estimates by the Sr/Ca thermometers. It appears that the Sr/Ca ratio, a major proxy for paleo-environmental marine studies, is sensitive to subtle pore-filling and replacement of the original coral matrix by secondary aragonite in the marine environment.

Echinoid Bioerosion as a Major Structuring Force of Red Sea Coral Reefs

Echinoid bioerosion is ecologically important as a limiting factor for reef growth. Diadema setosum and Echinometra mathaei are the most abundant sea urchins in the Gulf of Eilat, Red Sea. Bioerosion by these urchins was estimated from field experiments with urchins placed in underwater chambers. A novel methodology was developed for measuring the CaCO3 content of fecal pellets and total gut contents. This method is based on measurement of the amount of gas formed by total acidification of a mixed sample. The population density of the urchins was determined for the reef flat (depth 0.6-1 m) and the reef slope (depth 6-8 m). D. setosum, which erodes 310 mg individual-1 d-1, is much less abundant on the reef flat (0.1 individuals m-2) than on the reef slope (6.4 individuals m-2). In contrast, E. mathaei, which erodes 120 mg individual-1 d-1, is more dominant on the reef flat (10.5 individuals m-2) than on the reef slope (3.7 individuals m--2). We estimate that echinoid bioerosion converts to carbonate sediments 7%-11% of the total reef flat calcification and 13%-22% of the total reef slope calcification. These findings emphasize the importance of echinoid bioerosion as a structuring force in the coral reef, and its potential importance to the dynamics of reef development. However, they imply that some previous studies may have overestimated the role of echinoid bioerosion, thus possibly underestimating internal bioerosion.

Carbon geochemistry of marine-derived brines: I. 13C depletions due to intense photosynthesis

The carbonate system of marine-derived brines and its stable isotope composition are described. The study was conducted in the evaporation pans of a solar salt production plant (simulated sabkha environment) in which ca. 50% of the evaporation area is dominated by microbial mat communities (MMC). MMC are the modern equivalents of fossil stromatolites as old as Early Archean; hence, investigation of the impact of their photosynthetic activity on the carbonate system in the aquatic environment is relevant to the research of the ancient carbon cycle. Total dissolved CO2 (CT) decreases by ca. 50% of its original value as brine concentration increases from 1.5 to 4 times “mean” seawater. Roughly 70% of the original total alkalinity is precipitated as calcium carbonate at this salinity range. The relations between total alkalinity AT and CT (both normalized to salinity) suggest that the brines are depleted in CT by up to 50% with respect to equilibrium with atmospheric CO2. This large CT deficit is driven by the intense photosynthetic activity of the MMC. Considerable depletion in 13C was observed despite the photosynthetic activity which normally causes a 13C enrichment in CT. δ13CT values down to −9%. were observed in brine concentration range of 2 to 6 times “mean” seawater. We suggest that the CT deficit is causing an invasion of isotopically light CO2 from the atmosphere into the brine driven by chemical enhancement similar to that observed by Baertschi (1952) and Craig (1953, 1954) in alkaline solutions. Mass balance calculation for one of the evaporation pans is compatible with the suggested mechanism of chemically enhanced atmospheric invasion of 12C enriched CO2. This kinetic isotope fractionation may serve as an alternative explanation to that of fresh water runoff for some negative δ13C values of laminated carbonates from evaporitic sections found in the geological record. In addition, at least part of the large scatter observed in the δ13C vs. age curve for carbonates and organic matter from Precambrian stromatolitic environments may be explained by this mechanism rather than by late metamorphism and maturation processes.

Marine87Sr/86Sr ratios from the Jurassic to Pleistocene: evidence from groundwaters in Israel

Abstract Oceanic 87 Sr/ 86 Sr ratios during Jurassic to Pleistocene have been determined by analysing fresh waters from marine limestone and dolomite aquifers. The results are in good agreement with published data from well preserved fossil material. The 87 Sr/ 86 Sr ratios obtained are 0.7070 for Lower to Middle Jurassic, 0.7075 for Late Cretaceous, 0.7080 for Lower to Middle Eocene and 0.7087 for Pleistocene aquifer waters. The value of 87 Sr/ 86 Sr for the Eimer and Amend isotopic standard was 0.7082.

Six Month In Situ High-Resolution Carbonate Chemistry and Temperature Study on a Coral Reef Flat Reveals Asynchronous pH and Temperature Anomalies.

Understanding the temporal dynamics of present thermal and pH exposure on coral reefs is crucial for elucidating reef response to future global change. Diel ranges in temperature and carbonate chemistry parameters coupled with seasonal changes in the mean conditions define periods during the year when a reef habitat is exposed to anomalous thermal and/or pH exposure. Anomalous conditions are defined as values that exceed an empirically estimated threshold for each variable. We present a 200-day time series from June through December 2010 of carbonate chemistry and environmental parameters measured on the Heron Island reef flat. These data reveal that aragonite saturation state, pH, and pCO2 were primarily modulated by biologically-driven changes in dissolved organic carbon (DIC) and total alkalinity (TA), rather than salinity and temperature. The largest diel temperature ranges occurred in austral spring, in October (1.5 – 6.6°C) and lowest diel ranges (0.9 – 3.2°C) were observed in July, at the peak of winter. We observed large diel total pH variability, with a maximum range of 7.7 – 8.5 total pH units, with minimum diel average pH values occurring during spring and maximum during fall. As with many other reefs, the nighttime pH minima on the reef flat were far lower than pH values predicted for the open ocean by 2100. DIC and TA both increased from June (end of Fall) to December (end of Spring). Using this high-resolution dataset, we developed exposure metrics of pH and temperature individually for intensity, duration, and severity of low pH and high temperature events, as well as a combined metric. Periods of anomalous temperature and pH exposure were asynchronous on the Heron Island reef flat, which underlines the importance of understanding the dynamics of co-occurrence of multiple stressors on coastal ecosystems.

Temporal Changes in Radiocarbon Reservoir Age in the Dead Sea-Lake Lisan System

The Holocene Dead Sea and the late Pleistocene Lake Lisan were characterized by varying radiocarbon reservoir ages ranging between 6 and 2 ka in the Dead Sea and between 2 ka and zero in Lake Lisan. These changes reflect the hydrological conditions in the drainage system as well as residence time of (super 14) C in the mixed surface layer of the lake and its lower brine. Long-term isolation of the lower brine led to (super 14) C decay and an increase in the reservoir age. Yet, enhanced runoff input with atmospheric (super 14) C brings the reservoir age down. The highest reservoir age of 6 ka was recorded after the sharp fall of the Dead Sea at approximately 8.1 ka cal BP. The lower reservoir age of zero was recorded between 36 and 32 ka cal BP, when the Lake Lisan mixed layer was frequently replenished by runoff.

Radiocarbon in seawater intruding into the Israeli Mediterranean coastal aquifer.

Saline groundwaters from the Israeli coastal aquifer were analyzed for their radiocarbon and tritium content to assess the rate of seawater penetration. The low (super 14) C values (28-88 pMC versus 100-117 pMC in seawater) imply an apparent non-recent seawater source, or water-rock interactions along the penetration route. The latter process is supported by measurable tritium values at some locations, which imply a relatively rapid rate of seawater intrusion. In other locations, low tritium values (

Extreme 13C depletions in seawater-derived brines and their implications for the past geochemical carbon cycle

lt;2 T.U.) indicate that recent seawater (

Groundfish overfishing, diatom decline, and the marine silica cycle: Lessons from Saanich Inlet, Canada, and the Baltic Sea cod crash

lt;50 yr) did not penetrate inland. The low delta (super 13) C values in saline groundwater (average of -5.3 per mil versus 0 per mil in seawater) indicate that the dissolved carbon pool is comprised of a significant fraction of organic carbon. A linear negative correlation between delta (super 13) C and (super 14) C implies that this organic source is old (low (super 14) C values).