https://doi.org/10.1130/G46977.1 Manuscript received 14 April 2019 Revised manuscript received 8 November 2019 Manuscript accepted 12 November 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 17 December 2019 Atmospheric dust stimulated marine primary productivity during Earth’s penultimate icehouse Mehrdad Sardar Abadi1, Jeremy D. Owens2, Xiaolei Liu1, Theodore R. Them II2,3, Xingqian Cui4, Nicholas G. Heavens5,6 and Gerilyn S. Soreghan1 1 School of Geosciences, University of Oklahoma, Norman, Oklahoma 73019, USA 2 Department of Earth, Ocean, and Atmospheric Science, Florida State University, and National High Magnetic Field Laboratory, Tallahassee, Florida 32306, USA 3 Department of Geology and Environmental Geosciences, College of Charleston, Charleston, South Carolina 29424, USA 4 Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 5 Department of Atmospheric and Planetary Sciences, Hampton University, 21 E. Tyler Street, Hampton, Virginia 23668, USA 6 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA ABSTRACT BACKGROUND AND METHODS The importance of dust as a source of iron (Fe) for primary production in modern oceans Pennsylvanian–Permian carbonate strata is well studied but remains poorly explored for deep time. Vast dust deposits are well recog- of the study area were located at ∼30°S at the nized from the late Paleozoic and provisionally implicated in primary production through Fe time of deposition, adjacent to the northeastern fertilization. Here, we document dust impacts on marine primary productivity in Moscovian Arabian margin of Gondwana, in relative prox- (Pennsylvanian, ca. 307 Ma) and Asselian (Permian, ca. 295 Ma) carbonate strata from peri- imity (∼1000 km) to Gondwanan ice centers Gondwanan terranes of Iran. Autotrophic contents of samples, detected by both point-count (e.g., Arabian shield; Sardar Abadi et al., 2019). and lipid-biomarker analyses, track concentrations of highly reactive Fe, consistent with the The upper Moscovian of the Sanandaj-Sirjan hypothesis that dust stimulated primary productivity, also promoting carbonate precipita- zone and lower Asselian of the Alborz Basin tion. Additionally, highly reactive Fe tracks the fine-dust fraction. Dust-borne Fe fertilization record deposition of warm, shallow-marine car- increased organic and inorganic carbon cycling in low- and mid-latitude regions of Pangaea, bonate isolated from fluvial-deltaic sediment maintaining low pCO2. sources (Sardar Abadi et al., 2019). These strata record multiple glacioeustatic sequences, and INTRODUCTION Establishing a link between atmospheric dust high-resolution (20 cm) sampling was conduct- Atmospheric dust carries nutrients such as and productivity that extended to mid-latitudes ed through 28 m of the study intervals to capture iron and fixed nitrogen that fertilize marine pri- would clarify global biogeochemical responses several sequences. Petrographic and biomarker mary producers, impacting carbon cycling (e.g., and carbon cycling during the late Paleozoic ice data indicate a diverse biotic assemblage includ- Martin, 1990; Boyd et al., 2004; Krishnamurthy age (LPIA). These effects also have implications ing speciose cyanobacteria, phylloid algae, other et al., 2010). Today, eolian delivery of nutrients for Earth’s climate projections, given the urgent calcareous green and red algae, and heterozoans, to iron-limited marine systems controls phyto- need to achieve negative carbon emissions (e.g., as well as abundant abiotic components. plankton populations (Guo et al., 2012; Chien Hauck et al., 2016; IPCC, 2018). We point-counted (400 points per sam- et al., 2016) and can intensify carbonate precipi- For the first time in the ancient record, ple) thin sections from 21 Asselian and 25 tation through stimulation of microbial activity we investigated the relationship among con- Moscovian samples, selected to capture facies (Bressac et al., 2014; Guieu et al., 2014; Swart centrations of dust, bioavailable iron, auto- variation. Components were categorized into et al., 2014; Zhu and Dittrich, 2016). troph content, and bacterial biomarkers from abiotic, autotrophic, heterotrophic, and symbi- Despite the importance of eolian dust in sup- shallow-marine carbonate of the Central Per- otic attributes (following Schlager, 2003; Table plying marine nutrients, the biotic consequences sian terrane (present-day Iran) once located in DR1 in the GSA Data Repository1). In this ter- of dust in Earth’s deep-time record remain poor- southeastern mid-latitude Pangaea (Fig. 1A) to minology, “abiotic” refers to physically precipi- ly known. Dust deposits are well documented assess the strength and spatial extent of the iron- tated carbonate (e.g., ooids), “autotrophic” com- from the late Paleozoic (Soreghan et al., 2008, fertilization effect in deep time. The findings ponents are primary producers (e.g., calcareous 2015), with the possibility of large dust-fertil- have potential implications for the influences of green algae), “heterotrophic” components are ization effects, but evidence is preliminary and atmospheric dust on carbon cycling in Earth’s consumers (e.g., bryozoans), and “symbiotic” limited to the paleo-equator (Sur et al., 2015). penultimate icehouse and beyond. components are two cooperating organisms, 1GSA Data Repository item 2020064, Figures DR1–DR5, Tables DR1 and DR2, and supplemental data set with iron speciation, ICP-MS results, and grain-size analysis, is available online at http://www.geosociety.org/datarepository/2020/, or on request from [email protected]. CITATION: Sardar Abadi, M., et al., 2020, Atmospheric dust stimulated marine primary productivity during Earth’s penultimate icehouse: Geology, v. 48, p. 247–251, https://doi.org/10.1130/G46977.1 Geological Society of America | GEOLOGY | Volume 48 | Number 3 | www.gsapubs.org 247 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/3/247/4945811/247.pdf?casa_token=H-C9BbNj9tkAAAAA:oY_sW3kxZ3aScIjb4pHuXDY49K6_x7sjUQk9g-SJjCHKe6urnItUZqIgp0ZHlZgGKkBw3ek by Florida State University user on 29 March 2020 A Figure 1. (A) Paleogeo- the 2-methyl hopane index (2-MHI) (Summons graphic reconstruction, et al., 1999). All samples (n = 50) were analyzed including climate and for bulk-rock geochemistry using inductively oceanic currents during Early Permian time (ca. coupled plasma–mass spectrometry. 295 Ma; modified after Samples were processed for dust extraction Sardar Abadi et al., 2019). by sequential steps to remove carbonate, or- Note that present-day Iran ganic, pyritic, iron-oxide, and authigenic silica (including Alborz, Sanan- daj-Sirjan, and central components (Sur et al., 2010); subsequently, the Iran) is traditionally posi- extracted dust was analyzed by laser particle- tioned near the Arabian size analysis (Malvern Mastersizer 3000). A section of the Gondwanan standard sequential extraction (Poulton and Can- margin. Asterisk (*) indi- field, 2005) was used on (unprocessed) sample cates paleogeographic B location of the Midland splits to assess the amount of highly reactive iron Basin sampled by Sur (FeHR) species (cf. Raiswell et al., 2018). FeHR et al. (2015). (B) Facies is generally determined by summing dithionite logs of study sections, iron (Fe ), acetate iron (Fe ), oxalate iron (Fe ) Alborz Basin, and Sanan- D ac OX daj-Sirjan zone, including and pyrite iron (Fepy). However, we did not in- schematic sedimentologic clude Fepy in our calculation of FeHR because logs with facies and cycle samples (n = 5) with petrographic evidence of boundaries (horizon- pyrite documented very low Fepy (0.01–0.03 tal dashed lines). Cycle wt%). The ratio of Fe to total iron (Fe ) is a boundaries were chosen HR T based on the presence proxy for initially bioavailable iron; while this of one or more of the fol- approach does not quantify bioavailable iron, lowing features: evidence this is the best available proxy for quantifying for micro-exposure (e.g., potential ancient iron bioavailability (Sur et al., circumgranular cracking, alveolar structures, micro- 2015). Additionally, iron extraction quantifies discontinuity), fenestrate the various fractions of Fe corresponding to dif- fabric, red siltstone, and/ ferent iron species that are highly reactive. Im- or abrupt deepening portantly, FeT does not vary appreciably through of facies. Successions the analyzed samples, thus the ratio of Fe to record incomplete facies HR progradation with subtle FeT reflects changes in FeHR in the sample. evidence for subaerial We used linear and polynomial regressions exposure that indicates to examine relationships between FeHR/FeT and the influence of high- proportions of autotrophic, heterotrophic, sym- frequency (inferred glacioeustatic) falls. biotic, and abiotic components. We performed Plots on the right indi- a redundancy analysis (RDA) to examine rela- cate percent autotroph tionships among FeHR, dust grain size, and biotic content, ratio of highly and abiotic components. RDA is an ordination reactive iron to total iron technique that allows hypothesis testing for mul- (FeHR/FeT), and percent fine dust <( 31 μm) out of tiple driver and predictor variables (Legendre total dust. The ratio FeHR/ and Legendre, 1998). RDA was conducted us- FeT is shown as a percent ing the vegan package (Oksanen et al., 2018) to graph it on the same x- in the R program version 3.5.3 (R Core Team,