Abstract The effects of elevated CO 2 on tropical ecosystems were studied in the artificial rain forest mesocosm at Biosphere 2, a large‐scale and ecologically diverse experimental facility located in Oracle, Arizona. The ecosystem responses were assessed by comparing the whole‐system net gas exchange (NEE) upon changing CO 2 levels from 900 to 450 ppmV. The day‐NEE was significantly higher in the elevated CO 2 treatment. In both experiments, the NEE rates were similar to values observed in natural analogue systems. Variations in night‐NEE, reflecting both soil CO 2 efflux and plants respiration, covaried with temperature but showed no clear correlation with atmospheric CO 2 levels. After correcting for changes in CO 2 efflux we show that the rain forest net photosynthesis increased in response to increasing atmospheric CO 2 . The photosynthetic enhancement was expressed in higher quantum yields, maximum assimilation rates and radiation use efficiency. The results suggest that photosynthesis in large tropical trees is CO 2 sensitive, at least following short exposures of days to weeks. Taken at face value, the data suggest that as a result of anthropogenic emissions of CO 2 , tropical rain forests may shift out of steady state, and become a carbon sink at least for short periods. However, a better understanding of the unique conditions and phenomena in Biosphere 2 is necessary before these results are broadly useful.
During the Early Jurassic, cyst-forming dinoflagellates began a long-term radiation that would portend ecological importance of these taxa in the pelagic plankton community throughout the rest of the Mesozoic era. The factors that contributed to the evolutionary success of dinofla- gellates are poorly understood. Here we examine the relationship between oceanographic and cli- matic conditions during the Hettangian-Toarcian interval in relation to the radiation of dinofla- gellates and other organic-walled phytoplankton taxa in the Tethys Ocean. Our analysis is based on two data sets. The first includes d 13Ccarb, d13Corg, total organic carbon (TOC), and quantitative palynological observations derived from the Mochras Core (Wales, U.K.), which spans the complete Early Jurassic. The second is a coupled Mg/Ca and d 18O record derived from analyses of belemnite calcite obtained from three sections in northern Spain, covering the upper Sinemurian to Toarcian. From these two data sets we reconstructed the influence of sea level, trophism, temperature, and salinity on dinoflagellate cyst abundance and diversity in northwest Europe. Our results suggest that organic-walled phytoplankton (acritarchs, prasinophytes, and dinoflagellates) diversity in- creased through the Early Jurassic. The radiation coincides with a long-term eustatic rise and over- all increase in the areal extent of continental shelves, a factor critical to cyst germination. On shorter timescales, we observed short bursts of dinoflagellate diversification during the late Sinemurian and late Pliensbachian. The former diversification is consistent with the opening of the Hispanic Corridor during the late Sinemurian, which apparently allowed the pioneer dinoflagellate, Liasi- dium variabile, to invade the Tethys from the Paleo-Pacific. A true radiation pulse during the late Pliensbachian, with predominantly cold-water taxa, occurred during sea level fall, suggesting that climate change was critical to setting the evolutionary tempo. Our belemnite d 18O and Mg/Ca data indicate that late Pliensbachian water masses cooled (DT 268C) and became more saline (DS 12 psu). Cooling episodes during generally warm and humid Early Jurassic climate conditions would have produced stronger winter monsoon northeast trade winds, resulting in hydrographic instability, increased vertical mixing, and ventilation of bottom waters. During the late Pliensba- chian, dinoflagellates replaced green algae, including prasinophytes and acritarchs, as primary pro- ducers. By producing benthic resting cysts, dinoflagellates may have been better adapted to oxi- dized ocean regimes. This hypothesis is supported by palynological data from the early Toarcian ocean anoxic event, which was marked by highly stratified anoxic bottom water overlain by low- salinity, warm surface waters. These conditions were advantageous to green algae, while cyst-pro- ducing dinoflagellates temporarily disappeared. Our results suggest that the rise in dinoflagellate diversity later in the Jurassic appears to correspond to deep water ventilation as a result of the opening of the Atlantic seaway, conditions that appear to have simultaneously led to a loss of pra- sinophyte dominance in the global oceans.
[1] Instrumental data suggest that major shifts in tropical Pacific atmospheric dynamics and hydrology have occurred within the past century, potentially in response to anthropogenic warming. To better understand these trends, we use the hydrogen isotopic ratios of terrestrial higher plant leaf waxes (δDwax) in marine sediments from southwest Sulawesi, Indonesia, to compile a detailed reconstruction of central Indo-Pacific Warm Pool (IPWP) hydrologic variability spanning most of the last two millennia. Our paleodata are highly correlated with a monsoon reconstruction from Southeast Asia, indicating that intervals of strong East Asian summer monsoon (EASM) activity are associated with a weaker Indonesian monsoon (IM). Furthermore, the centennial-scale oscillations in our data follow known changes in Northern Hemisphere climate (e.g., the Little Ice Age and Medieval Warm Period) implying a dynamic link between Northern Hemisphere temperatures and IPWP hydrology. The inverse relationship between the EASM and IM suggests that migrations of the Intertropical Convergence Zone and associated changes in monsoon strength caused synoptic hydrologic shifts in the IPWP throughout most of the past two millennia.
[1] In the paper “Cenozoic Seawater Sr/Ca Evolution” by S. M. Sosdian, C. H. Lear, K. Tao, E. L. Grossman, A. O'Dea, and Y. Rosenthal (Geochemistry, Geophysics, Geosystems, 13(10), Q10014, doi:10.1029/2012GC004240, 2012) an error was made in Figure 9. The corrected figure showing the alkenone-based atmospheric carbon dioxide (pCO2) record of Pagani et al. [2005] plotted on the correct time scale, appears below. We also include pCO2 estimates from foraminiferal boron isotope proxies [Pearson and Palmer, 2000; Pearson et al., 2009; Foster et al., 2012] although the relationship between pCO2 and saturation state on these time scales is not straightforward [Hönisch et al., 2012]. Our conclusion that the increase in aragonite deposition as inferred from our estimated DSrSHELF record lags the increase in seawater Mg/Ca above 2 mol/mol by several million years remains unchanged.
Paleontological data were collected using microscopes and recorded in the JRSO description software. All data for a species group (e.g., diatoms or nannofossils) were collected in a Microsoft Excel worksheet by hole. A zip file of the entire expedition's observations is also available.
