Growth rates for freshwater ferromanganese concretions indicate regional climate change in eastern Canada at the Northgrippian-Meghalayan boundary

Abstract

The existence of freshwater ferromanganese concretions has been known for decades, but we are not aware of a generally accepted explanation for their formation, and there has been little research into their potential use as records of Holocene climate and paleohydrology. A conceptual model is presented to describe the environmental and geochemical processes which result in the formation and growth of freshwater ferromanganese concretions. In order to evaluate their potential as historical geochemical records, a concretion from Magaguadavic Lake, New Brunswick, Canada is the focus of a detailed geochronological and geochemical investigation. The radiocarbon data provide a coherent growth curve and a maximum age for the concretion of 8448 ± 43 years, consistent with the establishment of Magaguadavic Lake as a stable post-glacial lacustrine system. The data suggest accretion rates of 1.5 and 3.4 mm per 1000 years during the Northgrippian and Meghalayan stages of the Holocene, respectively. The abrupt change in growth rate observed at the stage boundary may be an indicator of Holocene climate change. These features are consistent with inferences from previous research that warmer climate in the Northgrippian led to eutrophication in some lakes in eastern North America. The results confirm that freshwater Fe-Mn concretions may yield important information about past climatic and environmental conditions.

Keywords: Holocene; climate; ferromanganese concretions; geochemistry; radiocarbon.

Figure 1.

Locations of Magaguadavic Lake and Harvey Lake showing the bedrock geology (Fyffe et al., 2005) and the sampling sites indicated by open circles.

Figure 2.

Upper surface (top photo) and lower surface (bottom photo) of Fe–Mn concretion ML2014 from Magaguadavic Lake, New Brunswick, Canada. A lithic fragment forms the central nucleus. The surrounding brown-black material is the concretion, comprised mostly of Fe and Mn oxyhydroxides.

Figure 3.

(a) Back-scattered-electron image (BSEI) for a cross-section of a concretion from Harvey Lake, NB, showing the three principle structural features that are common to many concretions. The growth direction is from left to right – the pebble nucleus was located at the arcuate boundary along the left side. The location for the inset image is indicated by the white box in (b). (b) BSEI from the area defined by the white box in (a). From top left to bottom right, the basal Fe-rich zone is evident at the top left, the radial growth zone is in the middle and the upper low-density zone occurs at the right side. (c) Composite element map for Fe (yellow) and Mn (magenta) acquired with SEM-EDS.

Figure 4.

Plot of radiocarbon dates against distance from the outer edge of the concretion toward the nucleus. The error bars represent the thickness of individual samples. Error bars for the Y-axis are smaller than the symbols.

Figure 5.

X-ray diffraction patterns from a concretion collected from Harvey Lake, New Brunswick. The upper pattern represents material from the inner, or oldest, segment of the concretion, and the lower pattern represents the outer segment.

Figure 6.

Plot of Ce and Eu anomalies for concretion ML2014 against ¹⁴C age. Error bars represent analytical error but the error bars for the horizontal axis are smaller than the symbols.

Figure 7.

Correlation matrix for ML2014 samples showing 17 measured elements together with ¹⁴C age and calculated REE anomalies (using “corrplot” from Wei et al., 2017).

Figure 8.

Conceptual model for the formation of freshwater Fe-Mn concretions illustrating: (1) basal Fe-rich zone, (2) radial growth zone, (3) upper low-density zone.

Figure 9.

Conceptual diagram representing conditions that would be conducive to (a) Mn accretion, or (b) Fe accretion, at the leading edges of lakebed concretions. The profiles of relative concentration versus depth for O₂, Fe(II) and Mn(II) are not to scale. The concentration profiles for Mn(II) and Fe(II) in the water column above the sediment-water interface are assumed to result from diffusive transport upward from the sediment porewater. Dispersive effects of currents in the water column are not considered.