11:00 AM - 1:00 PM
[BCG05-P09] In-situ geochemical analysis of stromatolites in the Tumbiana Formation (2.7Ga), Western Australia: Reconstruction of phosphorus content of the late Archean shallow water
★Invited Papers
Keywords:Archean, Stromatolite, Tumbiana formation, Composition of seawater, Phosphorus, Evolution of surface environment
Phosphorus is one of the bioessential elements and ultimately limits marine primary productivity (Tyrell, 1999). Despite its importance, the phosphorus content in the ancient sea remains ambiguous; Both P-rich and depleted scenarios are proposed (Bjerrum and Canfield, 2002; Konhauser et al., 2007).
Phosphorus levels of the Archean seawater are commonly estimated from P/Fe ratios in iron formations and whole-rock P contents of shales (Planavsky et al., 2010; Reinhard et al., 2017). Significant uncertainties lie in the extent of phosphate scavenging by iron-oxyhydroxide depending on the presence of other elements such as Si, Ca, and Mg for the former estimate (Konhauser et al., 2007; Jones et al., 2015) and the physical, environmental, and biological relationships of P contents between seawater and shale for the latter (Reinhard et al., 2017), respectively. Therefore, another proxy is necessary to estimate the ancient seawater P levels; the P content of carbonate should be the candidate.
Recent studies revealed that the P is incorporated into the coral as phosphate defects in the aragonite crystal lattice (Mason et al., 2011; Chen et al., 2019). It is well known that trace elements inorganically incorporated into the hydrogenous carbonate can record the ambient water chemistry. Although carbonates are subjected to secondary alterations and diagenesis, the samples with primal sedimentary structures likely preserve the information of the depositional environment.
Limestones in the Tumbiana Formation (ca. 2.7Ga), Fortescue Group, Pilbara Craton, Western Australia contain one of the world’s best-preserved stromatolites. However, there is still an ongoing debate about the depositional environment of the Meentheena Member of the Tumbiana Formation: lacustrine, ephemeral lake (Buick, 1992; Bolhar and Van Kranendonk, 2007; Awramik and Buchheim, 2009) or shallow-marine settings (Thorne and Trendall, 2001; Sakurai et al., 2005).
We prepared thick sections from the stromatolites in the Tumbiana Formation, collected at Redmond section, Pilbara. Most samples are composed of low-magnesium calcite, silicate, and iron-rich minerals such as pyrite and hematite. As detrital minerals, we observed chlorite, alkali-feldspar group, sphene, apatite, and volcanic glasses. In whole rock analysis, contamination of these minerals and sparite can effectively obliterate the primal chemical signatures.
Here, we conducted in-situ major and trace elements (Al, Si, P, Ti, Mn, Sr, Zr, Ba, and REEs) analyses of the laminated micrite domains of the stromatolites using laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) at Gakushuin University. The obtained data were double-checked for the contamination by time profiles of signal intensities of LA-ICPMS and screening by Al contents ([Al] < 1wt%).
Most samples have flat PAAS-normalized REE patterns, consistent with lacustrine characteristics (Bolhar and Van Kranendonk, 2007). However, the stromatolite at the bottom of the Meentheena Member has high Y/Ho ratios ranging from 34 to 42 and possesses a small, negative Ce anomaly. Therefore, we concluded that at least the bottom of the Meentheena Member was deposited in oxic water accompanied by seawater input, suggesting oxygenic photosynthesis in the Neoarchean.
Phosphorus contents of carbonates range 17 to 73 ppm with ca. 30 ppm on average through the Meentheena Member, which is equivalent to those of the Cambrian shallow marine ooids (Shimura et al., 2014). This is possibly indicating that the Archean sea had relatively high levels of P, roughly equivalent to phanerozoic.
Phosphorus levels of the Archean seawater are commonly estimated from P/Fe ratios in iron formations and whole-rock P contents of shales (Planavsky et al., 2010; Reinhard et al., 2017). Significant uncertainties lie in the extent of phosphate scavenging by iron-oxyhydroxide depending on the presence of other elements such as Si, Ca, and Mg for the former estimate (Konhauser et al., 2007; Jones et al., 2015) and the physical, environmental, and biological relationships of P contents between seawater and shale for the latter (Reinhard et al., 2017), respectively. Therefore, another proxy is necessary to estimate the ancient seawater P levels; the P content of carbonate should be the candidate.
Recent studies revealed that the P is incorporated into the coral as phosphate defects in the aragonite crystal lattice (Mason et al., 2011; Chen et al., 2019). It is well known that trace elements inorganically incorporated into the hydrogenous carbonate can record the ambient water chemistry. Although carbonates are subjected to secondary alterations and diagenesis, the samples with primal sedimentary structures likely preserve the information of the depositional environment.
Limestones in the Tumbiana Formation (ca. 2.7Ga), Fortescue Group, Pilbara Craton, Western Australia contain one of the world’s best-preserved stromatolites. However, there is still an ongoing debate about the depositional environment of the Meentheena Member of the Tumbiana Formation: lacustrine, ephemeral lake (Buick, 1992; Bolhar and Van Kranendonk, 2007; Awramik and Buchheim, 2009) or shallow-marine settings (Thorne and Trendall, 2001; Sakurai et al., 2005).
We prepared thick sections from the stromatolites in the Tumbiana Formation, collected at Redmond section, Pilbara. Most samples are composed of low-magnesium calcite, silicate, and iron-rich minerals such as pyrite and hematite. As detrital minerals, we observed chlorite, alkali-feldspar group, sphene, apatite, and volcanic glasses. In whole rock analysis, contamination of these minerals and sparite can effectively obliterate the primal chemical signatures.
Here, we conducted in-situ major and trace elements (Al, Si, P, Ti, Mn, Sr, Zr, Ba, and REEs) analyses of the laminated micrite domains of the stromatolites using laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) at Gakushuin University. The obtained data were double-checked for the contamination by time profiles of signal intensities of LA-ICPMS and screening by Al contents ([Al] < 1wt%).
Most samples have flat PAAS-normalized REE patterns, consistent with lacustrine characteristics (Bolhar and Van Kranendonk, 2007). However, the stromatolite at the bottom of the Meentheena Member has high Y/Ho ratios ranging from 34 to 42 and possesses a small, negative Ce anomaly. Therefore, we concluded that at least the bottom of the Meentheena Member was deposited in oxic water accompanied by seawater input, suggesting oxygenic photosynthesis in the Neoarchean.
Phosphorus contents of carbonates range 17 to 73 ppm with ca. 30 ppm on average through the Meentheena Member, which is equivalent to those of the Cambrian shallow marine ooids (Shimura et al., 2014). This is possibly indicating that the Archean sea had relatively high levels of P, roughly equivalent to phanerozoic.