The marine sedimentary nitrogen isotope record

Introduction Conclusions References


Introduction
Nitrogen is a critical nutrient element in marine ecosystems, and its supply is currently being modified anthropogenically in multiple ways (Gruber and Galloway, 2008).Future changes in the nitrogen cycle have the potential to significantly impact the oceanic nutrient regime, with significant implications for the marine ecosystem, but uncertainty remains regarding how sensitive the nitrogen cycle will prove to be.
Measurements of nitrogen isotopes in marine sediments provide a unique way of understanding the past and present marine nitrogen cycle and its relationship to climate change.The nitrogen isotope ratio ( 15 N/ 14 N) in a sample is expressed relative to the nitrogen isotope composition of a standard, conventionally, atmospheric nitrogen The sedimentary δ 15 N database is available for use by the scientific community to assist in further research and analysis by download from the PAGES website (http: //www.pages-igbp.org/workinggroups/nicopp).The database will be updated as new records become available, and the authors welcome additional contributions.

Database description
The sources of data are listed in Table 1.All data represent measurements of dry, homogenized bulk sediment, typically combusted using an Elemental Analyzer, coupled to a GC-IRMS with a Go-Flo apparatus and using He as a carrier gas, with local air as the standard.The combusted material is dominated by marine organic matter at most locations, although there is significant contamination by clay-bound inorganic (Kienast et al., 2005) and terrigenous nitrogen (Schubert and Calvert, 2001) in some locations.Given the known problems with acidification of samples (Brodie et al., 2011) we flagged samples that had been acidified where possible, though acidification is not always reported.Reported errors for the bulk combustion method are generally better than ±0.3 ‰ for replicates.

Seafloor sediments
Thus far, the collection of seafloor sediment δ 15 N contains more than 2300 sites, which include only real sediment measurements from any depth and region of the ocean seafloor (Fig. 1).Most of these sites (ca.regions samples represent a much longer time scale (order 10 3 yr) compared with highsedimentation sites (order 10 2 yr).
The seafloor δ 15 N values range from 2.5 to 16.6 ‰ (Fig. 1, inset).The average isotopic composition is 6.7 ‰, higher than the average isotopic composition of nitrate in the ocean (∼5 ‰, Sigman et al., 1997;1999).However, the dataset is positively skewed towards lower δ 15 N, such that a majority of sites have values between 4 and 6 ‰ (Fig. 1, inset).Given the highly irregular sampling pattern (Fig. 1), a significant spatial bias is likely, and may contribute to the elevated mean value.In addition, alteration of the δ 15 N during sinking and sedimentation is likely to produce higher values in the sediment (Altabet and Francois, 1994).
The global map of seafloor δ 15 N (Fig. 1) reveals regionally-consistent patterns, aligned with large-scale oceanic features.In some regions there are strong gradients of surface δ 15 N, such as in the eastern Arabian Sea, where gradients as large as 4.5 ‰ occur within ∼1 • of latitude/longitude.This variance could be due to oceanic fronts, sedimentary processes, or even to discrepancies in measurement techniques.In order to more clearly identify regions of pronounced small-scale variability, we analyzed the similarity of neighbouring seafloor samples within 100 km of each other (not shown).
The average difference between neighbouring surface samples is less than 1 ‰ in most of the ocean, with significant differences occurring at only a few sites, found in the eastern Arabian Sea, central equatorial Pacific, and the Benguela Current.

Sub-seafloor
The sub-seafloor database includes δ 15 N measurements and corresponding sediment depths, as well as (where available) a published age model, nitrogen content (% N), total organic carbon content (% C) and dry bulk density.We identified 173 sub-seafloor records for which bulk sedimentary δ 15 N data exist (Table 1).We obtained δ 15 N data for 147 core sites, while we could only obtain a complete set of ancillary measurements (% N, % C, dry bulk density) for 14 of those core sites, while an additional 74 core sites Figures

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Full have at least % N or % C. Age models were entered as published in the original references with no alteration (with a few exceptions where the age model was unavailable and needed to be regenerated from the original age control data).
The majority of sub-seafloor records are situated along the coast, where sedimentation rates are high and there has been greater confidence in the fidelity of sediment δ 15 N (Thunell et al., 2004;Altabet et al., 1999).In addition, records are concentrated at regions of interest, including those associated with coastal upwelling and/or suboxic conditions with water-column denitrification.Thus, the database has a high degree of spatial bias.In particular, as illustrated in Figs. 2 and 3a, a considerable number of records are from the Arabian Sea (17 %), the South China Sea (13 %), the Eastern Equatorial Pacific (19 %), the west coasts of North (12 %) and South America (14 %), and the southwest coast of Africa (11 %).As in the case of surface sediment sampling (Fig. 1) we have vast regions of the ocean, especially in the Southern Hemisphere, where the spatial coverage of δ 15 N records is very poor.For example, there were no published downcore records found in the Bay of Bengal, the Southwest Pacific or most of the North Atlantic (Fig. 2).The temporal coverage of the available δ 15 N records shows, unsurprisingly, a strong bias toward the more recent timescales of the Holocene and late Pleistocene periods.We find the maximum number of records between 5 and 20 kyr before present (BP) (Fig. 3b).As one might expect, most of the records that go beyond the last glacialinterglacial cycle (i.e., the last ∼120 kyr), have the disadvantage of being considerably coarser in resolution.In fact, the average sampling frequency for the most recent times is as high as 5 measurements per kyr, dropping to less than one measurement per kyr by about 65 kyr, and to one measurement every 4 kyr by 100 kyr BP.To compare these unequally spaced time series of δ 15 N records quantitatively, we created a common age axis by interpolating all δ 15 N records.Figures

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Full 3 Relationships between LH and neighbouring seafloor sedimentary δ 15 N It was shown above that the δ 15 N of neighbouring seafloor sediment samples is consistent within a 100 km radius for most of the oceans.To better understand the translation of nitrogen isotopes in this seafloor material to what we observe in sub-seafloor records, we compared seafloor samples with sub-seafloor sediments of late Holocene age (LH, 0-5 kyr BP).In general, one would expect the average LH δ 15 N of a sediment record to be about the same as the δ 15 N of the surrounding surface sediments, assuming that the δ 15 N of sinking organic nitrogen has been constant over this time period (Thunell et al., 2004).
To test this hypothesis, we examine the correlation between the average LH subseafloor δ 15 N and the average δ 15 N of neighbouring seafloor samples within a radius of 100 km.First, we examine LH-surface correlation within the Eastern Equatorial Pacific, since there is relatively good coverage of both seafloor and sub-seafloor data, and large-amplitude δ 15 N gradients.We average the core-top δ 15 N that fall within a 100 km radius of a chosen core site (see large circles, Fig. 4) and plot them against the LH average of that site (Fig. 5a).The LH and seafloor averages correlate well (r = 0.81) and show the same trend of lower δ 15 N along the equator and coastline and increased δ 15 N toward higher latitudes (Fig. 4).Interestingly, the line of best fit is shifted, nearly parallel to the 1:1 line, suggesting a small, uniform δ 15 N enrichment from LH to seafloor values.We used a non-parametric bootstrap resampling of the data to test the consistency of the regression coefficients; this showed that the y-intercept has much higher uncertainty than the slope.
We then extended this analysis to the global dataset (Fig. 5b).The correlation for the global ocean is even stronger than in the Eastern Equatorial Pacific (r = 0.92, R 1:1 2 = 0.80-0.83),with little deviation from the 1:1 line.There are a number of outliers, but these are either from regions with large variability between neighbouring seafloor measurements (large vertical whiskers, Fig. 5b), or with only one neighbour (no vertical whiskers).This correlation is highly encouraging for the use of bulk sedimentary Figures

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Full nitrogen isotope measurements as a record of past changes in the sinking flux of organic nitrogen.Despite the small magnitude of the discrepancies between seafloor and shallow sub-seafloor sediments, it is worthwhile considering their occurrence.Surprisingly, the relationship between surface and LH values, δ 15 N seafloor − δ 15 N LH , shows some spatial coherence, with clear contrasts between oceanic regions (Fig. 6).The δ 15 N seafloor − δ 15 N LH is consistently negative on the western coast of Africa, consistent with observations made by Freudenthal et al. (2001) who argued that down-core increases of δ 15 N in sediments near Mauritania reflected diagenetic enrichment.However, the picture is reversed along the western coasts of South and North America, where no sites show significant negative δ 15 N seafloor − δ 15 N LH .Elsewhere in the Pacific and Indian Oceans, the changes between LH and surface δ 15 N are generally minor or mixed.
We can conceive of two possibilities to explain these contrasts.One, that postdepositional alteration of δ 15 N varies between basins, due to changes in sediment characteristics, organic matter composition, or seafloor biota.Given that similar seafloor environments show opposite patterns (for example, the Benguela and Peruvian upwellings), while different environments within the same region show similar patterns (for example, Angola and the Mediterranean), this explanation seems unlikely.The second, more likely explanation, is that the δ 15 N of sinking organic matter changed significantly over the late Holocene.If this is true, it implies that the fidelity with which seafloor δ 15 N is transferred into the sedimentary record is somewhat better than would be indicated by our seafloor-LH comparison (Fig. 5), since our assumption of temporal invariance is incorrect.The sense of change between the LH and seafloor sediments, with decreasing δ 15 N in the Atlantic and increasing δ 15 N in the Pacific, would be consistent with an increase of water column denitrification (dominantly in the Indo-Pacific) and/or an increase of N 2 fixation rates (the isotopic imprint of which is stronger in the Atlantic) over this time period.We leave the resolution of this observation to future work.

Conclusions
Sedimentary δ 15 N data from the ocean are now available in a single database in order to quantitatively compare observations with each other and with biogeochemical ocean models.The variability in seafloor δ 15 N is small over spatial scales of 100 km for most regions.However, a few places do exist where strong oceanographic and/or sedimentary gradients lead to elevated spatial variance in the surface δ 15 N.
The spatial coverage of seafloor δ 15 N data is poor in vast regions of the oceans, such as the southern Indian Ocean, the subtropical gyres of the Pacific, and the North Atlantic.The clustered spatial distribution is even more prevalent in the collection of sub-seafloor sediment records, with most sites located on the continental shelves and slopes of the Pacific, Arabian Sea and southeastern Atlantic.
We tested the fidelity of sedimentary δ 15 N as a tracer of organic matter by comparing the available surface data with the LH average from the upper parts of sub-seafloor records.We found strong correlations between seafloor and sub-seafloor δ 15 N, which suggest reliable translation of sedimented δ 15 N into the buried sediment record.
We also discern relatively weak, but regionally coherent patterns of change between LH and seafloor sediments, suggestive of an acceleration of the marine nitrogen cycle over the late Holocene period (i.e.greater rates of water column denitrification and/or N 2 fixation).
Further studies with the synthesized dataset will help to improve our understanding of the marine nitrogen cycle, and thus of ocean biogeochemical dynamics in general.The database will continue to grow as new sedimentary δ 15 N records become available.Introduction

Conclusions References
Tables Figures

Table 1 .
List of δ 15 N records in the NICOPP database (as of 03/2012).