Modeling Mud: Flocs as Global Meltwater Indicators in Ice-Proximal Glacimarine Sediments

Flocculation of fine sediment is pervasive in particle-rich nearshore hypopycnal plumes. Floc formation is seen in field studies of northern-hemisphere, meltwater-rich fjords. Few exist for Antarctica, but indicate lateral transport of fine sediment, likely as flocculated material. Subglacial meltwater discharge contains abundant fine sediment, which rapidly flocculates and settles to the seafloor along with single grains of silt and sand transported in the turbulent plume discharge. Curran et al. (2004; doi: 10.1111/j.1365-3091.2004.00647.x) infer the flux of flocculated and single-grain sediment to the seafloor with an inverse model applied to disaggregated grain size distributions (DIGS). We apply the same model and sediment mass accumulation rates to quantify the mass flux of flocculated mud in glacimarine environments. Grain size distributions are from terrestrial tills, sub-ice stream and sub-ice shelf cores, and grounding line proximal marine sediment cores. Meltwater varies from minor (Ross Sea and East Antarctic Ice Sheet) to voluminous (Svalbard, Greenland, Alaska). Modeling is within context of published glacimarine facies to control for termini position on texture. We hypothesize that more meltwater leads to increased floc formation, accumulation, and hydrodynamic sorting of single-grain silt sub-populations. We observe that sub-ice stream and terrestrial till deposits are extremely poorly sorted and polymodal, often with discrete modes in the clay-size fraction; no evidence of flocs exists. Suspended sediment DIGS from N. Hemisphere fjords are primarily floc with secondary single-grain fine-silt components. Similar distributions are seen in sediments from same settings, but with increased single-grain silt contributions having narrow size distributions, which we attribute to meltwater sorting during subglacial and plume transport. Antarctic sediments contain both floc and single-grain silt fractions, but floc component is reduced relative to meltwater-rich settings. Single-grain modes also are more poorly sorted. We attribute both to minimal hydrodynamic sorting from reduced transport in meltwater prior to deposition. Floc accumulation rates are orders of magnitude larger in meltwater-dominant settings reflecting likely elevated glacial erosion rates.