REPLICATION OF ANNUAL CYCLES IN MN IN HUDSON RIVER CORES: MN PEAKS DURING HIGH WATER FLOW

Using the results from an ITRAX, XRF scanner, we previously reported apparent annual cycles in Mn in a single, high sedimentation rate Hudson River core, LWB1-8, taken off Yonkers, NY (Carlson et al., 2016). We replicated these results in three more high sedimentation rate cores and found stratigraphic markers that verify our inferences about the annual nature of the Mn cycles. The three new cores are LWB4-5 taken off Peekskill, NY, and LWB3-44 and LWB3-25, both taken in Haverstraw Bay. The cores are from water depths of 7-9 meters and all have high magnetic susceptibilities (typically > 30 cgs units) in their upper 1 to 2 meters. The high susceptibilities are primarily produced by magnetite from modern industrial combustion. One core, LWB1-8, has reconnaissance Cs dates that verify the annual nature of the cycles. More Cs dates are expected before the meeting. We developed several new methods of verifying the annual nature of our layer counts. The first is looking at the grain size distribution and age of layers with unusually high Mn peaks. Peaks in Si, Ni and Ti and peaks in percentage of coarse material typically accompany the peaks in Mn. Some are visible as yellow sandy layers. The five highest peaks in Mn in LWB1-8 have layer counted ages that correspond (within 1 year in the top meter and within 2 years in the bottom meter) to 1996, 1948, 1913, 1857 and 1790. The latter three events are the three largest historical spring freshets on the Hudson. 1996 is a year of unusually high flow rate during the spring freshet. Based on our work and previous work on Mn cycling in rivers, we infer that the peaks in Mn are produced by extreme erosional events that erode sediment and release pore water Mn into the water column. The other methods of testing our chronology involve marine storms that increase Ca and Sr and a search for fragments of the Peekskill meteorite that fell in October 1992. More information on the latter will be available by the meeting.