In this study we present hydrography, biogeochemistry and sediment trap
observations between 2003 and 2012 at Porcupine Abyssal Plain (PAP)
sustained observatory in the Northeast Atlantic. The time series is valuable
as it allows for investigation of the link between surface productivity and
deep ocean carbon flux. The region is a perennial sink for CO
The Porcupine Abyssal Plain (PAP) sustained observatory is situated in the
Northeast Atlantic Ocean (49
The pathway by which a small fraction (< 1 %;
Martin et al., 1987) of the carbon fixed by photosynthesis in the sunlit
upper ocean is exported to great depths, thereby constituting a sink for
atmospheric CO
Primary production in the surface ocean can be measured by several
techniques (broadly separated into vitro incubations or changes in bulk
properties; Platt et al., 1989), however,
from the perspective of the oceanic carbon cycle the most important rate is
the net community production (NCP). NCP is the net primary production (NPP)
minus heterotrophic respiration, and represents the sum of the particulate
and dissolved organic carbon available for export or utilization by higher
trophic levels. NCP is traditionally measured by bottle O
The new production, Dugdale and Goering (1967), is the production supported by the input of new nitrogen into the euphotic zone through upwelling and horizontal mixing, but also by processes such as atmospheric deposition and nitrogen fixation (Sarmiento and Gruber, 2006; Gruber, 2008). On an annual basis, assuming the system is in a steady state, export production is considered equivalent to new production (Eppley and Peterson, 1979).
From a climate change perspective, the long-term (> 100 years)
removal of carbon from the atmosphere is important to quantify, which is
often defined as the flux of carbon below 1000 m (Lampitt et al.,
2008a), known as the sequestration flux. The sequestration flux is smaller
than the export flux out of the euphotic zone or mixed layer, and is
In this study we present a time series of surface ocean measurements and
particle trap data from the PAP observatory station from 2003 to 2012. Our
aim is twofold; firstly, we will quantify NCP and new production from the
average monthly drawdown of dissolved inorganic carbon (DIC) and
NO
Hydrographical and biogeochemical parameters were measured using data from
instruments at a nominal 30 m depth on a mooring at the PAP observatory (49
Overview of ancillary data.
The sediment trap mooring at the PAP observatory was deployed in the depth
range 3000 to 3200 m, which is around 1800 m above the seabed. The
methodology is described in Lampitt et al. (2010). Briefly, a Parflux
sediment trap was used with mouth area 0.5 m
To interpret and expand on the data from the PAP observatory, the following
parameters were used from external data sources (see Table 1): temperature
and salinity profiles from Argo floats, atmospheric CO
Temperature and salinity profiles were extracted from the global fields for
the PAP observatory (49
Time series of available data from PAP surface mooring and sediment
trap with
Monthly climatology (black dots) with
The atmospheric CO
The air–sea CO
The alkalinity (Alk) was calculated from Argo temperature and salinity (30 m),
following the relationship for the North Atlantic developed by Lee et
al. (2006). The dissolved inorganic carbon (DIC) was calculated from Alk and
measured
The seasonal drawdown of DIC and NO
Monthly changes in MLD integrated NO
A different source of uncertainty in NCP and new production estimates come
from measurement uncertainty, which propagates into calculated values.
However, for new production, the variability associated with measurement
uncertainty is negligible compared to the natural variability. The
measurement uncertainty for the NO
The monthly changes in DIC concentrations
Physical mixing processes, such as vertical entrainment, diffusion and
advection, will to some degree contribute to monthly DIC changes, however
they are difficult to quantify without information on vertical and horizontal
gradients. Following the approach by Körtzinger et al. (2008) we have
performed a simplified calculation of seasonal NCP and new production for
the summer period when the mixed layer is relatively stable and the
biological drawdown in DIC (and NO
The same rationale can be applied to the monthly changes in NO
Summing up the months with a net drawdown in NO
Net primary production (NPP) was estimated from satellite data using the Vertically Generalised Production Model (Behrenfeld and Falkowski, 1997), which requires inputs of chlorophyll concentration, sea surface temperature and photosynthetically available radiation data, here taken from NASA's MODIS Aqua satellite (reprocessing R2012.0). The NPP data were downloaded from the Ocean Productivity website (see Table 1).
In addition to estimating the surface origin of particles sinking to the
sediment trap using a simple 100 km box around the PAP observatory, we also
used modelled velocity fields to determine the likely source region. The
velocity field (
The cross-correlation between the sediment trap data and either NPP in a
100 km box around the PAP observatory or in source locations identified by
particle tracking, were calculated using the ccf function in R (R
Development Core Team, 2012). The cross-correlations were performed on
monthly anomalies (monthly climatology – observed monthly value), to avoid
possible inflation of
Time series data from 2003 to 2012 from the PAP surface mooring and sediment
trap are shown in Fig. 1. The temperature and salinity (both PAP sensors and
Argo 30 m) varied in the range 12–18 and 35.4–35.8
The monthly climatology (or average seasonal cycle) for temperature showed a
seasonal warming of around 5
The monthly MLD-integrated NO
Tracking of the particles arriving at the sediment trap at 3000 m at the PAP
observatory (see Sect. 2.4), revealed that the source locations of particles
could vary substantially between years, and also on an annual timescale
(Fig. 4). The satellite NPP in these source regions also varied markedly, and
the highest NPP of around 210 mmol C m
Satellite NPP (mmol C m
There was a high cross-correlation between the seasonal anomalies of NPP in
source locations identified by the particle tracking and the volume flux in
the sediment trap (
Cross-correlations between sediment trap data (top: volume flux, middle: dry weight, bottom: POC) and NPP in the source regions defined by particle tracking (left) or in a 100 km box around the PAP observatory (right). The dashed lines show the 95 % confidence intervals. The unit of the lags is months.
The seasonal cycles of carbon (
The MLD-integrated seasonal NCP (from February to July) was
4.57
Converting the new production in terms of nitrogen to carbon units using the
Redfield ratio of 6.6 (Redfield, 1958), gives a value of 2.5 mol C m
Studies have found N
The degree to which the C overconsumed in the surface waters reaches the
deep ocean, and thus is sequestered on long timescales is important,
because it represents a potential negative feedback on atmospheric CO
The export flux of POC around the PAP observatory has been quantified in
several studies using different techniques (see overview in Fig. 4 in
Riley et al. (2012). The average POC flux in the upper 170 m obtained
from PELAGRA drifting sediment trap deployments for short periods of time
(3–5 days) between 2003 and 2005 was 72 mg C m
Using particle tracking to identify the source location of material arriving
in the sediment trap at 3000 m at the PAP observatory showed that the
particles could originate up to 140 km away (in 2007; Fig. 4). There was
large variation in the source location of particles between years, depending
on the prevailing current conditions in the given year. There was also large
variation within individual years, but the satellite NPP generally increased
during spring and decreased during autumn along the trajectory of the
particles reflecting the seasonal cycle. Mesoscale variability will also
contribute to the variability in source location of particles, both between
and within years. The highest NPP was found in 2009 (around 210 mmol C m
The transfer efficiency is a useful metric to describe the long-term removal
of carbon (> 100 years) from the atmosphere (cf. De
La Rocha and Passow, 2012). Using the same average POC flux of the surface
layer (0–170 m) as in the calculation of the export flux above and the
average flux between March and July of POC at 3000 m from the sediment trap
at the PAP observatory (5.1 mg C m
The PAP observatory currently sits near the boundary between the sub-polar and sub-tropical gyres of the North Atlantic. Seasonably variable areas, like the sub-polar region, are thought to export a higher fraction of labile material than sub-tropical regions (Lutz et al., 2007). As climate change is predicted to result in the oligotrophic gyres expanding over the next century (Sarmiento et al., 2004), the PAP observatory will likely transition into more sub-tropical conditions. This could result in more refractory material being exported at the PAP observatory, potentially reversing the pattern we report here with a high export ratio and low transfer efficiency. In addition, a more strongly positive NAO index is predicted due to climate change (Gillett, 2003), which is expected to increase diatom abundance at the PAP observatory, and result in reduced organic carbon flux to the deep ocean (Henson et al., 2012a). Although the precise response of the biological carbon pump to climate change is as yet unclear, transition-zone regions between gyres (such as the PAP observatory), could be among the systems that are most strongly affected by climate change (Henson et al., 2013).
The PAP observatory is characterized by strong interannual variability in
hydrography, biogeochemistry, and especially sediment fluxes. The seasonal
cycles of carbon and nitrogen show a winter maximum and summer minimum,
characteristic of highly productive sub-polar regions. The MLD-integrated
seasonal NCP (from February to July) was 4.57
The export ratio was 15 %, while the transfer efficiency was 4 %, which is typical of high-latitude ecosystems where, although a large proportion of the primary production is exported out of the euphotic zone, this material is relatively labile and therefore remineralized before it reaches the deep ocean. It is hypothesized that the export regime at the PAP observatory could change with climate change, as the region will probably transition into more sub-tropical conditions over the next century (Sarmiento et al., 2004; Lutz et al., 2007).
Using particle tracking to identify the source regions of material reaching the sediment trap at the PAP observatory, revealed higher correlations between NPP in the identified source regions and export flux than other methods. However, more observations are needed to establish if a particle-tracking approach indeed gives added value in sediment trap analyses.
We would like to acknowledge the various ship crew, engineers and scientists involved in preparation, deployment and recovery of the PAP sustained observatory moorings, especially Jon Campbell and Mark Hartman. We wish to thank Maureen Pagnani, Athanos Gkritzalis-Papadopoulos, Zong-Pei Jiang and Andres Cianca for compilation, quality control and calibration of PAP data. Mooring data and support for this research was provided by the European research projects ANIMATE (Atlantic Network of Interdisciplinary Moorings and Time Series for Europe), MERSEA (Marine Environment and Security for the European Sea), EUR OCEANS (European Network of Excellence for Ocean Ecosystems Analysis) and EuroSITES grant agreement EU 202955. The work was also supported through the Natural Environment Research Council (NERC), UK, project Oceans 2025 and National Capability. H. F. was supported by EU FP7, through projects MEECE (212085), EURO-BASIN (264933) and GreenSeas (265294). Edited by: C. Robinson