The Nitrogen Isotopic Record From the Peru-Chile Suboxic Zone; Distinguishing Internal and External Signals Across the Last Deglaciation M. A. Altabet 1, S.C. Bova 2, T. Herbert 2, Y. Rosenthal 3, J. Kalansky 3 Acknowledgments: Funding from the NSF P2C2 program, Jen Larkum and Rehka Singh for technical support 1) Introduction The Peru-Chile suboxic zone is one of 3 major open ocean regions where vanishingly small subsurface O 2 concentrations enable microbially-mediated fixed N loss. This loss associated with denitrification and anammox processes globally is a predominate control on marine N cycling and oceanic N inventory. Past variations in the extent of low O 2 and N-loss are recorded in the δ 15 N of underlying sediments of these regions and prior work has shown climate sensitivity on centennial to orbital time scales. Amongst a number of findings has been a sharp and early rise in δ 15 N at the beginning of the last deglaciation. Outstanding questions include the nature of the forcing of this rapid deglacial increase in N loss and the relative contributions of imported vs. system generated signals. Of the latter, changes in surface NO 3 - utilization have the potential to also contribute to the sediment δ 15 N record. # B43C SMAST, University of Massachusetts Dartmouth 2 Geological Sciences, Brown University 3 Institute of Marine and Coastal Sciences, Rutgers University 2) Core Location and Regional Context To address these issues, δ 15 N records were constructed from two long pistons cores recently collected on the northern Peru margin in the vicinity of 4°S (Fig. 1). CDH-23 and CDH-26 were raised from 350 and 1000 m depth. These sites were chosen in expectation of a) high continuous sedimentation rates from the LGM to present and b) overlying water column characteristics are representative of the primary source water for the Peru- Chile OMZ. The primary subsurface water mass constituting the Peru- Chile suboxic zone is sourced in the equatorial undercurrent (‘13°C water’) and enters this system through the Peru-Chile Undercurrent (Fig. 2). From north to south along its flow path, O 2 decreases along the margin and reaches levels sufficient to enable subsurface N-loss in the vicinity of 7 to 10° S as indicated by the appearance of NO 2 - and N deficits (negative N’) and increasing δ 15 NO 3 -. CDH-23 and CDH-26 at 4°S are thus located just upstream of the low O 2, N-loss region with cores sites we have previously studied within a N-S gradient of increasing OMZ and N-loss intensity. Figure 1. Locations of cores discussed in this poster. CDH 23 & 26 PUC 3) Dating and Age Models Unlike cores to the south along the Peru margin (Fig. 1), forams were sufficiently abundant for radiocarbon dating in both CDH-23 & 26. This is likely a consequence that, just outside of the OMZ, bottom water conditions along the margin remain conducive to carbonate preservation. Numerous 14 C dates were obtained on both cores to achieve well resolved age models (Fig. 3). Accumulation rates were high, ranging from 0.5 to 1.5 m/kyr. Also in contrast to previously studied margin cores, sedimentation was continuous. Whereas shallower CDH-23 reached to the middle of the last deglaciation, CDH-26 spanned the LGM to late Holocene. Benthic foram  18 O confirms these age assignments. The offset between cores reflects the warmer overlying water for shallower CDH-23. Figure 3. (A) Age models for CDH-23 & 26 based on radiocarbon dating of planktonic foraminifera. Calendar ages were derived after subtraction of a constant 14 C reservoir age effect (~700 yr) and calibration using InterCal 4.0 High and continuous sedimentation is evident for both cores. (B) Benthic foram  18 O vs. age for each core in comparison to the Epica Antarctic ice core record. Both the quality of the age model and the time periods covered by each core are evident. A. B. 4) CDH-23 & 26  15 N Modest variations in sediment δ 15 N were observed over the last 25 kyr. Where CDH-23 & 26 overlap (last 14 kyr), the records are practically identical supporting their fidelity in reflecting near- surface conditions overlying these two nearby cores. The most prominent feature is a 1‰ increase between 18 and 14 kyr followed by a near-steady decrease of 1.5‰ to the late Holocene. Core top values of 5‰ appear to represent the modern δ 15 N average, but EUC source waters actually have a δ 15 NO 3 - of ~6‰. The difference is likely due to HNLC conditions at the site of CDH-23 & 26. Figure 4. Sediment  15 N records for CDH-23 & 26. 5) Comparison to OMZ  15 N Records Within the Peru-Chile OMZ, margin sites between 9 and 30° S are marked by a large and sharp rise in δ 15 N. The increase is from 4 to 6‰ and takes place, at most over 2 kyr. The CDH 23 & 26 cores, in contrast, have a deglacial δ 15 N increase that is a fraction of this magnitude and takes twice as long. The high temporal resolution of these cores indicates that record quality cannot explain the difference. We do note that both cores see subsequent decreases in δ 15 N into Holocene though there are also clear differences. Our major conclusion is that the CDH 23 & 26 reflect changes in the δ 15 NO 3 - of the system and/or local HNLC conditions. Thus most of the δ 15 N signal found within the OMZ is generated by changes in OMZ intensity and corresponding N- loss. The early rapid rise in OMZ δ 15 N thus represents a corresponding rapid increase in N- loss. Figure 5. Sediment  15 N records for cores with the OMZ along the Peru margin (Fig. 1). GeoB 7139 is at 30°S and not shown on Fig 1. These data were previously published by De Pol-Holtz (2006, Paleoceanography) Figure 2. Biogeochemical maps of the Peru OMZ from January and February of Properties are shown along a constant density surface (σ θ = 26.3 kg m -3 ) within the upper portion of the OMZ corresponding to a depth range of 100 to 170 m (Altabet et al., 2012, Biogeosciences). (A) O 2 concentration (µmol kg -1 ), station locations (B) NO 2 - concentration (µmol kg -1 ). (C) Nitrogen anomaly – N’ (µmol kg - 1 ) calculated as [NO 3 - ] + [NO 2 - ] – 16 x [PO 4 -3 ]. (D) The δ 15 N of NO 3 -. When southward intensification of suboxic conditions reaches [O 2 ]<3 µmol kg -1, the onset of N-loss processes is evident.