Stories out of Mud
I think the big shifts in my sediment core data are recording a climate signal.
Between 1100 - 600 cm depth (roughly 3,000 years ago) there is a large increase in magnetic susceptibility, total inorganic carbon, % fine-grained material, and delta 13C (13 Carbon isotope). When water temperatures increase, CaCO3 becomes less soluble (in other words, it likes to be in colder water, and starts to "rain" out when temperatures increase). I think that air temperatures increased around 3k years ago, causing CaCO3 to precipitate out of the lake water -- an event called a "whiting." Sediments from whitings tend to be very fine-grained (consistent with my grainsize data) and can contain iron (in the form of siderite, an iron-carbonate).
Previous work supports this idea (which doesn't necessarily mean it's right, but is encouraging). Mudroch (1982) found that fine-grained, carbonate-rich sediments in a core from Lake Erie's central basin contained mollusc species that lived in much warmer waters (15-23 degrees C) than those found in coarser, low-carbonate sections (4-12 degrees C).
Graham and Rea (1980) also saw a sudden increase in fine-grained carbonate-rich sediments at roughly the same time, indicating that this is a regional rather than a basin-specific signal/event (points to climate).
I still have a lot of questions. Why does our % water decrease and then increase again with depth? Where does the Fe come from -- is there really siderite in our sediment? Why are there sections of the core with mostly whole, articulated mollusc shells, and other sections with shell hash? Does it fit the story? Why are my radiocarbon dates out of order stratigraphically? Turbidity currents/gravity flows?