LONG-TERM PRESERVATION (SEQUESTRATION) – OR NOT -- OF BIOGENIC CARBONATE: SHARED STAGES, BUT NOT RATES, OF MICROSTRUCTURAL STABILIZATION IN TROPICAL, TEMPERATE, & ARCTIC SEABEDS

New age-dating of aragonitic bivalve shells from the bioturbated surface mixed layer (SML) of Alaskan continental shelves reveals much shorter scales of time-averaging than in temperate and tropical shelves: Arctic death assemblages have median shell ages of <50 y and maxima of 850-1600 y (Meadows et al. in prep), as opposed to ~50y and 10-20ky elsewhere (many studies). Calculated loss rates are thus overall higher in the Arctic. However, this owes to a “failure of sequestration” –delay in long-term stabilization -- rather than to significantly faster initial rates of destruction: immediately post-mortem, Arctic shells exhibit the same decadal half-lives as shells elsewhere. Low sequestration has several possible explanations, including (a) more rapid and deeper burial of shells below the SML than on lower-latitude shelves (unlikely, given similar or shallower depths of bioturbation and ~identical ~0.2 cm/y sedimentation rates) and/or (b) a slower rate of diagenetic stabilization, i.e. delayed attainment of a less reactive shell surface, e.g. through crystallite coarsening, precipitation of crusts, and/or change in mineralogy. To advance mechanistic understanding of shell preservation, we launched SEM and other analysis of nanoscale microstructural alteration of age-dated shells. We find that, remarkably, shells – both crossed-lamellar and homogeneous microstructures -- undergo the same sequence of stages in all three settings: (1) (microbial) removal of organic matrix between mineral crystallites; (2) ‘ruffling’ of x-lamellar crystallite ends and suturing of shared long-edges, reducing reactive surface area; (microbial-mediated) dissolution/back-precipitation is implied. (3) Development of ‘amoeboid plates’, which are actually a crust of (microbially precipitated) syntaxial mineral prisms, possibly calcite. Once plates develop, at least in patches, (4) euhedral (abiotic) calcite rhombs form, signaling supersaturated porewaters and thus residence below the SML. Tantalizingly, syntaxial crusts occur on shells only a few decades old in temperate and tropical shelves, but the youngest Arctic examples are a few 100s y, consistent with crusts playing a critical role in sequestration. No other diagenetic features are common to all three study systems.