Riverine export of aged terrestrial organic matter to the North Atlantic Ocean

Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00±6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural C and C data from four rivers that discharge to the western North Atlantic Ocean and ®nd that these rivers are sources of old (C-depleted) and young (C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These ®ndings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in C from nuclear testing, and hence newly produced. We also ®nd that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter signi®cantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans. We sampled four rivers that discharge to the western North Atlantic Ocean: the Amazon (Brazil), Hudson (New York, USA), York (Virginia, USA) and Parker (Massachusetts, USA). The Amazon was sampled during base ̄ow in 1991, and the three US rivers were sampled during moderate to high ̄ow regimes from 1996 to 1998. The four sampled rivers range over four orders of magnitude in freshwater discharge (160,000, 375, 70 and 11 m s , respectively) and watershed size (7,050,000, 21,000, 4,350 and 609 km, respectively). All water samples were taken from tidal freshwater sites except the Parker River sample, which was taken from above a dam feeding the Parker estuary. We report radiocarbon (C) signatures for two operationally de®ned size fractions of organic matter: dissolved organic carbon (DOC; the fraction passing through a 0.7-mm glass-®bre ®lter) and particulate organic carbon (POC; the fraction collected on a 0.7-mm glass®bre ®lter). Together these two size classes comprise the total organic carbon in these river waters. We also present for comparison data from four other western North Atlantic rivers collected previously. Mean DOC and POC concentrations for all rivers were 357 6 280 mM and 95 6 33 mM, respectively (Tables 1 and 2). The DC of DOC exported from these rivers was highly variable, ranging over 4151⁄2 (DC is de®ned in Table 1.) The Hudson River DOC had the most depleted DC value at -1581⁄2 (equivalent radiocarbon age of 1,384 years before present, yr BP), while the DOC from the Susquehanna and Rappahannock rivers was also low in DC-DOC, with corresponding ages of 680 and 766 yr BP (Table 1). In contrast, the DC of DOC in the York, Parker, Potomac and Amazon rivers all contained `bomb' C (that is, DC . -501⁄2), introduced by nuclear weapons testing in the 1950s and 1960s. The corresponding DC values for POC (Table 2) were all signi®cantly depleted compared to DOC (Table 1). The average DC-POC values for the Amazon, Hudson, York and Parker rivers were -145, -437, -68 and -1381⁄2 and had corresponding average radiocarbon ages of 1,258, 4,609, 690 and 1,210 yr BP, respectively; the greatest ages for POC from these four rivers were 1,258, 4,763, 1,690 and 1,696 yr BP, respectively (Table 2). This suggests that on average, the bulk POC discharged from these rivers was photosynthetically ®xed several millennia ago, and a signi®cant fraction of this POC aged on land and in river basins for hundreds to thousands of years. Very old riverine POC (.10,000 yr BP) has also been found in a subtropical mountainous river in Taiwan. The average radiocarbon ages of POC in the present study were 1,260, 3,225, 690 and 1,210 years older than DOC in the Amazon, Hudson, York and Parker rivers, respectively (Tables 1 and 2). The DC values of Amazon DOC and POC were ,2001⁄2 lower than values found further upriver in 1984 for dissolved humic substances and POC, and re ̄ects a decrease in atmospheric DC-CO2 of 1401⁄2 over the same period, spatial variation in POC sources, as well as our