Figure 4

Schematic overview illustrating the shift of δ⁵⁶Fe_aq towards negative values. A combination of two consecutive processes could have caused extremely negative δ⁵⁶Fe_aq values at Site C0023: (a–b) repetitive DIR and (c) adsorption of Fe(II)_aq onto Fe (oxyhydr)oxide surfaces. The blue color scheme represents the isotopic composition of Fe(II)_aq, where pale blue colors indicate low and dark blue colors higher δ⁵⁶Fe_aq values. The combination of repetitive DIR (Step I) and the subsequent adsorption of Fe(II)_aq onto mineral surfaces progressively removed Fe(II)_aq from the pore water (Step II), which led to a shift of δ⁵⁶Fe_aq towards extremely negative values. However, the adsorptive removal of Fe(II)_aq (Step II) can only lead to extremely low δ⁵⁶Fe_aq values if DIR has ceased (details given in the text). Here, while the zone in which dissimilatory Fe reduction occurs is referred to as ‘suboxic’ zone, where oxygen, nitrate and HS⁻ are absent, the zone in which Fe(II)_aq is oxidized is described as ‘oxic’ zone.