Katja M. Meyer

ENVIRONMENTAL CONTROLS ON THE GENESIS OF MARINE MICROBIALITES AND DISSOLUTION SURFACE ASSOCIATED WITH THE END-PERMIAN MASS EXTINCTION: NEW SECTIONS AND OBSERVATIONS FROM THE NANPANJIANG BASIN, SOUTH CHINA

Abstract A widespread marine microbialite and underlying truncation surface occur in Permian–Triassic sections of South China. We interpret the microbialite to have formed as a shallow, open-marine benthic framework stimulated by high seawater CaCO3 saturation. The widespread distribution across platform interiors and lack of asymmetry or thickening toward platform margins is incompatible with an alternative hypothesis, that microbialite deposition was stimulated by upwelling anoxic, alkaline waters. The truncation surface beneath the microbialite is irregular with overhangs and small caverns extending up to 30 cm beneath the surface indicating a dissolutional origin. Petrographic observations refute the interpretation that strata immediately beneath the surface contain pendant cements, meniscus cements, and vadose silt. Measurements of the anisopachous fibrous cements show that thickened areas have random, not downward orientations. Pores retain the pointed geometry consistent with isopachous cement. Carbon and oxygen isotope measurements, from immediately beneath the surface, do not show a negative shift as would be expected with subaerial exposure. Also incompatible with a subaerial origin is the occurrence of only one truncation surface within a subtidal succession ~ 50 m thick below the surface and the limited vertical penetration of dissolution. The surface closely resembles a hardground containing a micritized alteration zone with stromatolites encrusted on the surface. We interpret the surface to have formed by submarine dissolution driven by a pulse of ocean acidification associated with Siberian Traps eruptions and the end-Permian extinction. After a hiatus of ~ 30–100 kyr, seafloor dissolution would have brought seawater back to saturation coupled with increased delivery of calcium to the oceans as the result of elevated continental weathering and caused a rebound in carbonate saturation and precipitation of microbialites.

The influence of the biological pump on ocean chemistry: implications for long-term trends in marine redox chemistry, the global carbon cycle, and marine animal ecosystems.

Abstract The net export of organic matter from the surface ocean and its respiration at depth create vertical gradients in nutrient and oxygen availability that play a primary role in structuring marine ecosystems. Changes in the properties of this ‘biological pump’ have been hypothesized to account for important shifts in marine ecosystem structure, including the Cambrian explosion. However, the influence of variation in the behavior of the biological pump on ocean biogeochemistry remains poorly quantified, preventing any detailed exploration of how changes in the biological pump over geological time may have shaped long‐term shifts in ocean chemistry, biogeochemical cycling, and ecosystem structure. Here, we use a 3‐dimensional Earth system model of intermediate complexity to quantitatively explore the effects of the biological pump on marine chemistry. We find that when respiration of sinking organic matter is efficient, due to slower sinking or higher respiration rates, anoxia tends to be more prevalent and to occur in shallower waters. Consequently, the Phanerozoic trend toward less bottom‐water anoxia in continental shelf settings can potentially be explained by a change in the spatial dynamics of nutrient cycling rather than by any change in the ocean phosphate inventory. The model results further suggest that the Phanerozoic decline in the prevalence ocean anoxia is, in part, a consequence of the evolution of larger phytoplankton, many of which produce mineralized tests. We hypothesize that the Phanerozoic trend toward greater animal abundance and metabolic demand was driven more by increased oxygen concentrations in shelf environments than by greater food (nutrient) availability. In fact, a lower‐than‐modern ocean phosphate inventory in our closed system model is unable to account for the Paleozoic prevalence of bottom‐water anoxia. Overall, these model simulations suggest that the changing spatial distribution of photosynthesis and respiration in the oceans has exerted a first‐order control on Earth system evolution across Phanerozoic time.

REPLY: PERMIAN–TRIASSIC MICROBIALITE AND DISSOLUTION SURFACE ENVIRONMENTAL CONTROLS ON THE GENESIS OF MARINE MICROBIALITES AND DISSOLUTION SURFACE ASSOCIATED WITH THE END-PERMIAN MASS EXTINCTION: NEW SECTIONS AND OBSERVATIONS FROM THE NANPANJIANG BASIN, SOUTH CHINA

We appreciate Kershaw et al.s comment and continued interest in our Permian–Triassic research in the Nanpanjiang Basin. Kershaw et al. comment on aspects of our recent study that contradict results from the study of Collin et al. (2009). Specifically their comments center around two questions: (1) whether the truncation surface at the base of the Permian–Triassic microbialite shows evidence for subaerial exposure, and (2) whether data support the model of genesis of the microbialites by upwelling of anoxic, alkaline waters with elevated carbonate saturation from bacterial sulfate reduction (Kershaw et al. 1999, 2007, 2012). We note that our recent study is much broader in scope than the issues addressed in this comment and reply. Our study presented extensive stratigraphic, geochemical, biostratigraphic and petrographic analyses from multiple sections, introduced new sections that have not been presented in the literature, and evaluated multiple models for the genesis of the microbialite and underlying truncation surface. We welcome this opportunity to respond to Kershaw et al.s comment and to further illuminate studies of the Permian–Triassic boundary in the Nanpanjiang Basin. ### Two Generations of Fibrous Cement Fringe? Maybe, But There is Still no Evidence for Pendant Geometry Kershaw et al. comment that there are two generations of bladed fringing cement adhered to grains and that only the second generation has a pendant geometry. Collin et al. (2009) made the same interpretation. From the photographs in Collin et al. (2009, fig. 5Aa–5Ab) and from our thin section observations, we interpreted this to be a single generation of fibrous cement fringe adhered to grains, with a faint zonation of clear-cloudy-clear alternation within the fringe, in which the cloudy zone contained a greater concentration of fluid inclusions (fig. 1A, 1B). Note that the photos presented in the comment by Kershaw et al. (figs. 3, 4) are also consistent with this interpretation. The entire bladed cement fringe was used …