Secular variation in the elemental composition of marine shales since 840 Ma: Tectonic and seawater influences
Introduction
The elemental composition of marine shales and muds can potentially provide information about sediment provenance, weathering processes, and seawater chemistry through time (Fig. 1). Differences in source terrains (e.g., mafic versus felsic parent rocks) can lead to differences in sediment composition (Johnsson and Stallard, 1989, Fralick and Kronberg, 1997, Dhuime et al., 2011). Chemical weathering intensity can strongly influence the composition of particulate sediments (Gaillardet et al., 1999, Oliva et al., 2003). Areas of limited weathering yield compositionally immature sediments that are altered only modestly relative to their parent rocks (MacNaughton et al., 2005, Adiotomre et al., 2017), whereas areas of intense weathering yield mature sediments characterized by strong cation depletion and concentration of resistate elements such as Al, Ti, and Zr (Vital and Stattegger, 2000, Selvaraj and Chen, 2006). Weathering intensity is strongly modulated by tectonic, climatic, and biotic boundary conditions. Tectonic uplift increases rates of physical weathering relative to chemical weathering and contributes to the textural and mineralogical immaturity of sediments through rapid erosion and deposition as well as cooler weathering conditions at higher elevations (West et al., 2005, Ruddiman, 2013). Climate influences weathering at all spatial and temporal scales, with global differences documented for greenhouse versus icehouse climate regimes (White and Blum, 1995, Riebe et al., 2004). Chemical weathering is strongly mediated by plant roots, leading to influences related to the evolution of terrestrial floras through time (Drever, 1994, Algeo and Scheckler, 1998, Sheldon, 2006). Uptake of ionic species from seawater by clay minerals, Fe-Mn-oxyhydroxides, (bio)phosphate, and organic matter can provide information regarding ancient seawater chemistry (Fleet, 1984, Rasmussen et al., 1998, Bekker et al., 2014).
Long-term secular variation in the elemental composition of marine shales and muds has received only limited attention to date. Mackenzie and Garrels (1971) compiled a shale dataset for the Archean to Recent, identifying pronounced increases in K2O/Na2O and Th/Sc ratios at the Archean/Proterozoic transition and decreases in the younger part of the geological record (∼0.5–0 Ga). They attributed the first pattern to a major cratonization event during the Archean/Proterozoic transition that resulted in upper continental crust (UCC) of a more felsic nature, and the second pattern to an increase in tectonic settings preserving immature siliciclastic assemblages. Condie (1993) estimated the average chemical composition of UCC and shale through the Archean and Proterozoic, showing a significant compositional change at the Archean-Proterozoic boundary. He linked high Cr and Ni contents in Archean shales to the large amounts of basalt and komatiite in UCC of that age, and increases in Na, Ca and Sr content in post-Archean shales to a decrease in chemical weathering intensity. Cox et al. (1995) studied the chemical compositions of mudrocks deposited on the Colorado Province from 1.8 to 0.2 Ga, showing decreases in mobile major elements (e.g., Ca and Na) and compatible transition elements (e.g., Sc, Cr and Ni), and an increase in incompatible high field-strength elements (HFSEs; i.e., Th, Hf, Zr, and U). They linked these changes to progressive weathering of sedimentary material through multiple crustal recycling events. Recently, Gaschnig et al., 2016, Li et al., 2016 have used glacial diamictite compositions to evaluate long-term changes in continental crustal composition. All of the studies cited above span longer time intervals (i.e., eons) but have substantially lower temporal resolution than the present study.
In the present study, we analyzed or compiled bulk geochemical data in marine shales and muds with a near-global distribution ranging in age from the Cryogenian (∼840 Ma) to the Recent (Section 2). We selected for investigation a set of 10 elements, including both major (Mg, Na, and K) and trace elements (Ba, Sr, Y, Th, Rb, Nb, and Zr). The principal selection criteria were that each element must (1) be present in typical X-ray fluorescence (XRF) datasets, and (2) show little to no redox sensitivity. The elemental chemistry of shales is more commonly studied using XRF than other analytical procedures, so the first criterion ensured that we were able to compile sufficient data for each element to permit a meaningful assessment of its long-term pattern of secular variation. The second criterion excluded elements such as Co, Cr, Cu, Mo, Ni, U, V, and Zn that are commonly present in XRF datasets because their concentrations are typically controlled by aqueous redox conditions (Morford and Emerson, 1999) rather than tectonic, weathering, or seawater chemical influences, which are the main focus of the present study. Finally, we elected not to examine Ca and Si because of our inability to reliably separate the clay-hosted Ca and Si fractions from other fractions containing Ca (e.g., carbonates) and Si (e.g., quartz silt and biogenic silica) in bulk-rock datasets.
For each element, Al-normalized concentrations were plotted against geological age, and a locally weighted scatterplot smoothing (LOWESS) curve was calculated in order to identify patterns of secular variation (Section 3.1). Because subsets of the 10 elements analyzed in this study show similar patterns of secular variation, we determined relationships among the study elements using two statistical procedures (PCA and CA; Section 3.2). The results of these analyses provided a theoretical framework for interpretation of controls on their secular variation (Section 4). One elemental subset, comprising K, Y, Th, Rb, Nb, and Zr, can be linked to subaerial weathering and tectonism (Section 4.3) and a second elemental subset, comprising Mg, Na, and Sr, to seawater chemistry and submarine hydrothermal alteration (Section 4.4). More tentatively, secular variation in Ba can be linked to changes in Earth-surface oxygenation and seawater sulfate concentrations (Section 4.5). Other influences such as bioevolutionary events, although not strongly supported by our dataset, may have operated and are given passing consideration (Section 4.6). This study thus documents long-term changes in the elemental composition of marine shales and provides an integrated theory for understanding these changes.
Section snippets
Data compilation and analytical methods
The elemental geochemistry database used in this study consists of 268 datasets for individual formations (14,531 total samples) from fine-grained siliciclastic marine units (i.e., shales and muds) with a minimum of 10 analyses per dataset. About 75% of the data was compiled from published literature sources and ∼25% is from unpublished datasets generated by the second author over the past 25 years (see supplement S1 for analytical methods). The compiled datasets have a near-global
Secular elemental variation
For Nb/Al, the median formation-mean value in our database is 2.1 (range 1.3–2.5), which is somewhat higher than PAAS (1.8) and UCC (1.5; Table 1). The LOWESS curve varies from ∼1.5 to 2.7, showing significant peaks in the Cryogenian (∼2.5), Ordovician to Devonian (∼2.7), Permian-Triassic (∼2.3), and late Cretaceous-Paleogene (∼2.0) (Fig. 5a).
For Th/Al, the median formation-mean value in our database is 1.6 (range 1.1–2.1), which is close to PAAS (1.5) and UCC (1.3; Table 1). The LOWESS curve
Primary character of secular elemental records
The utility of our geochemical database as an archive of primary compositional information for marine shales through time depends on the studied formations not having been strongly altered by secondary fluids. Given our reliance on published sources as well as the large number of datasets analyzed in this study (n = 268), a rigorous test of secondary alteration in each study formation is not possible. However, secondary alteration is unlikely to be a major influence on our database because of
Conclusions
This study documents significant relationships of marine shale composition to first-order geological controls. Patterns of secular variation in the concentrations of 10 non-redox-sensitive major and trace elements in marine shales and muds of Cryogenian to Recent age are consistent with long-term control by tectonism, continental weathering, submarine hydrothermal alteration, and seawater chemistry. The dominant influence on a subset of “detrital elements” (K, Rb, Th, Nb, Y, and Zr) appears to
Acknowledgments
We thank Hui Jian for assistance with drafting figures. This research was funded by National Science and Technology Major Project (2016ZX05027001-005, 2017ZX05049004), National Natural Science Foundation of China (41690134, 41802175), and China Scholarship Council (CSC). The project was supported by the Fundamental Research Funds for National Universities, China University of Geosciences (Wuhan).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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