Phosphogenesis during the Cenozoic transition from greenhouse to icehouse conditions: Upper Oligocene to lower Miocene siliceous, phosphate, and organic‐rich sediments near La Purísima, Baja California Sur, Mexico

Abstract

Upper Oligocene and lower Miocene, siliceous, organic‐, and phosphate‐rich sediments are widespread in Baja California Sur (Mexico). A representative section at La Purísima was analysed for its sedimentology, stratigraphy, geochemistry, and mineralogy. A corresponding age model was obtained by dating zircons from ash layers (27.84 ± 0.33 to 21.21 ± 0.59 Ma). The sediments were deposited in an upwelling‐dominated, hemipelagic setting, for which the presence of lamination, the scarcity of benthic organisms (except for in gravity‐flow deposits), and redox‐sensitive trace‐element enrichments indicate oxygen‐depleted conditions. Anoxic conditions were particularly strong around 27 and 24–22 Ma. Gravity‐flow deposits are frequent and predominantly composed of phosphatic‐coated grains. They were generated by seismic and volcanic activity, as is indicated by the close association with volcanic ash layers. The phosphatic‐coated particles were formed in a more proximal, better‐oxygenated shelf environment. They precipitated also in situ within the hemipelagic sediments, where they were often concentrated by subsequent winnowing. In situ phosphogenesis also partly cemented the gravity‐flow deposits. At La Purísima, phosphogenesis occurred throughout the time interval investigated and was particularly important around 28–25.5 and 23.5–21.5 Ma. These two time intervals correspond to the late Oligocene glacial maximum and the Oligocene–Miocene and early Miocene glacial intervals Mi1 and Mi1a. This provides evidence for the increasing importance of glacial denudation during the Oligocene, which led to an enhanced phosphorus flux into the ocean. Cooler climates also promoted the efficient transfer of phosphorus to thermocline waters by increased upwelling. Subsidiary phases of phosphogenesis during the intervening warm periods are explained by the weathering of glacial legacy sediments. These observations suggest that during the transition from greenhouse to icehouse conditions in the Oligocene and Miocene, new and radical changes in the global phosphorus cycle affected and partly inverted feedback mechanisms between climate, geochemical cycles and life, and profoundly influenced the biosphere and its evolution.

Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments

“…Our data show that phosphate accumulation in sediments on the Namibian Shelf is tightly related to redox conditions of the sediments as inferred from the distribution of redox sensitive Mo and Re and stable isotope signatures. Similar relationships with a transition from sulfidic organic-rich sediments to overlying suboxic apatite rich sediments reported in this study have been described in other Cenozoic phosphorites at upwelling margins near Peru (Arning et al, 2009b), Baja California (Föllmi et al, 2019) and Monterey (Föllmi et al, 2005;John et al, 2002) as well as in geologically older deposits (Glenn and Arthur, 1988), suggesting an analogous environmental setting and formation mechanism. The prevalent interpretation regarding the formation of the phosphatic component of these sediments is believed to progress by (a) formation of phosphatic pore fillings during early diagenesis near the SWI (sediment depth of <5 cm; Schulz and Schulz, 2005), (b) formation of phosphatic laminae by repeated episodes of non-deposition that (c) act as a sedimentary "seal", aiding the preservation of organic matter below, (d) leading to sulfidic conditions in the sediment (Arning et al, 2009b;Föllmi et al, 2005;John et al, 2002).…”

“…Nearly all of the largest Phanerozoic phosphorites have formed at upwelling continental margins during periods of fluctuating climate and enhanced weathering (Compton et al, 2002;Compton and Bergh, 2016;Föllmi, 1996;Föllmi et al, 2019;Pufahl and Groat, 2017;Schöllhorn et al, 2020;Shields et al, 2000). Similarly, modern phosphogenesis occurs on the continental shelfs of Peru and Chile (Arning et al, 2009b(Arning et al, , 2009aBurnett, 1977), Namibia (Baturin, 2000;Compton et al, 2002;Compton and Bergh, 2016), Baja California (Jahnke et al, 1983), the Arabian Sea (Schenau et al, 2000) and Western Australia (Heggie et al, 1990;O'Brien and Veeh, 1980).…”

Early Miocene Intensification of the North African Hydrological Cycle: Multi‐Proxy Evidence From the Shelf Carbonates of Malta

“…These phosphorite episodes (Schöllhorn et al., 2019) started during the Late Oligocene (28–29 Ma) along the present day United States Pacific coast (Schöllhorn et al., 2019). A belt of phosphorite deposits developed in a west‐east trend with major phosphatic episodes occurring during the late Oligocene along the shallow marine coastal areas of California, Mexico (Föllmi et al., 2019; Schöllhorn et al., 2019), Puerto Rico, the Dominican Republic (Ortega‐Ariza et al., 2015), and Florida (Schöllhorn et al., 2019). Across the Atlantic, phosphogenesis started later, and by ∼25 Ma periodic phosphorite deposits are observed in Malta and Sicily (Carbone et al., 1987; Föllmi et al., 2008; Gruszczynski et al., 2008; Pedley & Bennett, 1985), Central Italy (Auer et al., 2016), and Galicia bank along the Atlantic Spanish/Portuguese submarine coast (González et al., 2016).…”

Macroalgae from two coastal lagoons of the Gulf of California as indicators of heavy metal contamination by anthropogenic activities

“…Differences in metal concentrations in algae between sites can be found because of the source of metals and background metal concentrations, such as discharges of household and industrial effluents (Osuna-López et al ., 1989; Páez-Osuna et al ., 2000) that influence metal bioavailability (Marín-Guirao et al ., 2008; Vizzini et al ., 2013; Hatje et al ., 2016; Páez-Osuna et al ., 2017). For example, algae from SJC and AL generally showed the highest metal concentrations (Mn, Cd and Ni in SJC and Fe, Cu, Zn, Pb and Ni in AL); both sites are influenced by anthropogenic activities (one site is located in the vicinity of a mine and the second at the cargo port) and phosphorite mining operations at SJC might increase minerals draining off from the mine to coastal water during the extraction processes (Föllmi et al ., 2019).…”