Stable isotope constraints on the nitrogen cycle of the Humboldt Current System

(with Jaime Soto, and Karen Casciotti from WHOI)

Models of isotope fractionation associated with nutrient uptake by microalgae suggest extensive fractionation when nutrients are abundant.Thus, nitrogen isotope distribution in marine sediments gives the possibility of tracking changes in the marine nitrogen cycle through time.

The use of nitrogen isotopes as paleo-oceanographic tracer rests on the premise that sedimentary organic matter retains the nitrogen isotopic signal imprinted on cellular nitrogen at the time of synthesis.However, during the transit of particulate organic nitrogen (PON) to the sediment-water interface and in sediments, diagenesis may alter the d15N signal.In addition, the isotopic signature of the source nitrate can also be altered in the susbsurface if dissimilatory reactions take place.Those effects would mask the original signal imprinted in cellular nitrogen during photosynthesis as PON sinks out the photic zone and is buried in sediments.One way to reconstruct the isotopic signal of phytoplankton in sediments is to make compound specific stable N-isotope measurements on chlorophyll (Sachs and Repeta 2000, Org. Geochem. 31, 317-329) and nitrate (Casciotti et al. 2002, Anal. Chem. 74, 4095-4912).Chlorophyll is a light harvesting pigment present in all marine photoautotrophs, and as such, N isotope composition of chlorin (chlorophyll and degradation products) would be a tracer for algal biomass unaffected by detritus and diagenesis.Nitrate d15N will allow us to track isotope fractionation associated to changes in isotope composition of nitrate supplied to the photic zone.

The stable nitrogen isotopic composition of nitrate, particulate nitrogen, and surface sediment were determined in the Oxygen Minimum Zone (OMZ) of northern Chile (23oS) where the OMZ is permanent and more intense than in a central-south area (37oS), where the OMZ is seasonal.In both sampling zones, sedimentary δ15N decreases by about 2‰ with distance from the coast.This trend is opposite to what others have found in highly productive areas (Holmes et al. 1996 Mar. Geol. 134, 1-12), where sedimentary δ15N increases with distance from the coast by about 1.5-2‰ presumably associated with depletion of surface water column nitrate.Nitrate-N isotope composition ranged from 12.3 to 30.9‰ in northern Chile with maximum enrichment (>25‰) between 80-100 m depth, within the OMZ.Maximum depletion in δ15N-NO3- (ca. 12‰) was observed at 30 m depth (redoxcline) that coincided with the lowest nitrate concentration.We examine the relative influence of transformation of PON during its transit to surface sediments and alteration of the nitrate source signal in surface waters to explain the absence of the expected fertilization signature on sedimentary δ15N.

Funding:FONDECYT Grant 1040503 Paleoceanographic changes associated with intensity of El Niño events in the eastern south Pacific recorded in sedimentary organic matter in the continental shelf off Chile