Mark A. Altabet

Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the δ15N of core top sediments were strongly related to [NO3−] in surface waters. With distance from the equator in the equatorial Pacific, δ15N increased from 7‰ to 16‰ as [NO3−] decreased from 8μM to < 0.1 μM. Going from 60° to 30° S in the SE Indian Ocean, core top δ15N increased from 5‰ to 11‰ as surface [NO3−] decreased from 25μM to < 0.1 μM. These results are strong evidence that sedimentary δ15N in these regions is recording the increasing isotopic enrichment of near-surface NO3− with its depletion by phytoplankton. In the case of the equatorial Pacific, δ15N values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near-surface generated isotopic signal. These equatorial Pacific data sets have variations with near-surface [NO3−] consistent with Rayleigh fractionation kinetics for a fractionation factor (ϵu) of 2.5‰. This value is substantially lower than previously found for temperate or polar regions, perhaps as a result of differences in phytoplankton species assemblage or growth condition. In the southern ocean south of the polar front, comparison of δ15N values for opal-rich sediments south and sinking particles indicates an apparent +5‰ diagenetic enrichment relative to the surface-generated signal that requires further investigation. This exception aside, our observations show that the surface-water relationship of increasing δ15N with increasing NO3− depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and application of this important paleoceanographic tool.

Spatial and temporal distributions of prochlorophyte picoplankton in the North Atlantic Ocean

Over the past several years red-fluorescing picoplankton, believed to be prochlorophytes, have been shown to be extremely abundant in the North Atlantic and Pacific Oceans. The dim fluorescence of these tiny cells initially limited studies to the relatively highly pigmented cells near the bottom of the euphotic zone; however, improvements in sensitivity of flow cytometry now enable us to detect the prochlorophytes in surface waters as well. In the Sargasso Sea in May 1988 and May 1989 prochlorophytes were present throughout the upper water column, and we observed the highest concentrations in surface waters within the Gulf Stream. Further south, the prochlorophytes formed subsurface maxima; the median depth of prochlorophyte (but not Synechococcus) populations followed the deepening of the nitracline. Prochlorophytes were not present in waters north of the Gulf Stream in May, although we had observed them there on a previous September cruise. They were present year round near Bermuda, with lowest concentrations in the winter, when Synechococcus was most numerous; the prochorophytes appear to “bloom” later than the Synechococcus, after the onset of seasonal stratification. The latitudinal variations in prochlorophyte and Synechococcus distributions during spring resembled the seasonal pattern near Bermuda.

The δ15N of nitrate in the southern ocean: Consumption of nitrate in surface waters

We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton. The δ15N of nitrate increases by 1–2‰ from deep water into the Antarctic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarctic depth profiles yields values in the range of 5–6‰ in east Indian sector and 4–5‰ in the east Pacific sector. Surface transect data from the Pacific sector also yield values of 4–5‰. The major uncertainty in the profile-based estimation of the isotope effect involves the δ15N of nitrate from the temperature minimum layer below the summertime Antarctic surface layer, which deviates significantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertically from the Subantarctic thermocline because of the distinctive relationships between the δ15N and concentration of nitrate in these two potential sources. Our Subantarctic samples, collected during the summer and fall, indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Pacific sector of the Subantarctic suggest an isotope effect of 4.5‰, indistinguishable from the Antarctic estimates in this sector.

The δ15N of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior

We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0.2‰ observed in Lower Circumpolar Deep Water, North Atlantic Deep Water, and central Pacific deep water. The isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean, the Southern Ocean surface being an important exception. In the Subantarctic Zone (north of the Polar Frontal Zone) the nitrate δ15N of Upper Circumpolar Deep Water is ∼0.7‰ greater than that of Lower Circumpolar Deep Water. This isotopic enrichment appears to result from denitrification in the low-latitude water masses with which Upper Circumpolar Deep Water communicates. The isotopic enrichment of Upper Circumpolar Deep Water is diminished in the Antarctic, probably because of the remineralization of sinking organic N, which has a low δ15N in the Antarctic. Relative to the other water masses of the Southern Ocean, the Subantarctic thermocline has a very low nitrate δ15N for its nitrate concentration because of exchange with the low-latitude thermocline, where this isotopic signature appears to originate. This signature of the low-latitude thermocline has two probable causes: (1) mixing with low-nitrate surface water and (2) the oxidation of newly fixed N.

Phytodetritus at the abyssal seafloor across 10 of latitude in the central equatorial Pacific

Fresh phytoplankton detritus (or phytodetritus) has been reported from numerous deep seafloor sites in the temperate North Atlantic and Pacific Oceans following seasonal phytoplankton blooms. Here we report the first strong evidence for abyssal accumulations of phytodetritus in the tropics, in the central equatorial Pacific. In November–December 1992 we obtained photographs and/or sediment-core samples from 61 abyssal stations (water depths of 4280–5012 m) between 12°S and 9°N along ∼ 140°W. Greenish flocculent material was recovered from the top of multiple-core samples from 5°S to 5°N; this material was most abundant from 2°S to 2°N, in some areas forming continuous layers at least 5 mm thick, and individual aggregates > 1 cm in diameter. The greenish material was clearly visible in bottom photographs as a green veneer that covered >95% of the seafloor near the equator, and as individual cm-scale aggregates covering <1% of the seafloor. Occasionally, thick accumulations of cm-scale aggregates occurred in biogenic pits. Cleared trails and feeding traces suggest that surface-deposit-feeding holothurians and echiurans grazed the greenish material. Microscopic examination of greenish material recovered from core tops and a burrow lumen revealed relatively intact diatoms (including Rhizosolenia sp.) and other microalgae with chloroplasts containing chlorophyll. The greenish material was 1–12.5% organic carbon by weight, i.e. 5–39 times richer than associated seafloor sediments. It also contained high excess activities of 234Th, suggesting arrival from the water column in the previous 100 days. Samples of the greenish flocculent material from 0° and 5°N incubated at simulated environmental pressure and temperature with 14C-labeled glutamate exhibited ⩾ 5-fold higher rates of microbial activity than underlying sediments or brown floc from 9°N. Surface-sediment samples (which included the greenish flocculent material) from 5°S to 5°N also contained significant concentrations of chlorophyll a and other chloropigments; the chloropigment concentrations were roughly comparable to deep-sea phytodetritus collected in the North Atlantic. We conclude that fresh, organic-rich phytodetritus was present on the seafloor from 5°S to 5°N along 140°W in November–December 1992, with highest concentrations within 2–3° of the equator. This material is likely to be a concentrated, high-quality food resource for deep-sea microbes and metazoans. We estimate an upper limit for the standing stock of this phytodetritus to be ∼2.6 mmol Corg/m2; this corresponds to ∼3% of the annual flux of organic carbon to the seafloor at these latitudes in 1992. Because the degradation rate of this material appears to be very high, its presence at the seafloor for several months per year could yield significant phytodetrital contributions to the annual seafloor organic-carbon budget. We also suggest that the phytodetrital aggregates are formed at intense convergence zones resulting from seasonal passage of tropical instability waves within 5° of the equator; if so, phytodetrital accumulations are likely to recur seasonally over broad areas of the abyssal equatorial Pacific.

Changes in the δ13C of surface water particulate organic matter across the subtropical convergence in the SW Indian Ocean

We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59°S and 30°S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC-δ13C and [CO2aq], we concurrently measured surface water pCO2, temperature, salinity, nitrate concentration, POM concentration, chlorophyll a and the δ13C of total dissolved inorganic carbon. While we found the previously reported general negative correlation between POC-δ13C and [CO2aq], we also observed a prominent maximum in POC-δ13C in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a, and with a minimum in pCO2. The increase in POC-δ13C between 59°S and the Subtropical Convergence is consistent with the trend expected if [CO2aq] were the main factor controlling the isotopic composition of POM. In contrast, data from the region north of the Subtropical Convergence clearly illustrate that POC-δ13C can also vary independently of [CO2aq] as a 5 per mil decrease in POC-δ13C was found in a region characterized by nearly constant [CO2aq]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO2aq] from the carbon isotopic composition of sedimentary organic matter.

Glacial/interglacial changes in sediment rain rate in the SW Indian Sector of subantarctic Waters as recorded by 230Th, 231Pa, U, and δ15N

High-resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high-sedimentation rate core (MD84-527: 43°50′S; 51°19;′E; 3269 m) by normalization to 230Th. This method estimates paleofluxes to the seafloor on a point-by-point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment δ15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on 14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate-induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa-231/Th-230 ratios, suggesting increased scavenging from the water column. Sediment δ15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply rates to surface waters.

Glacial to interglacial changes in surface nitrate utilization in the Indian Sector of the Southern Ocean as recorded by sediment δ15N

We present a new approach for paleoceanographic reconstruction of surface nutrient utilization in the southern ocean. It has been observed in the contemporary ocean that, due to preferential uptake of 14NO3− during photosynthesis, the δ15N of planktonic organic matter increases with increasing nitrate depletion in surface waters. Our results demonstrate that the δ15N signal produced in surface waters is reflected in the underlying surface sediments; core top δ15N is inversely correlated with surface nitrate concentration along a transect across the Subtropical Convergence and the Polar Front in the southeast Indian Ocean. These results are consistent with a four-box model showing that the nitrogen isotopic composition of sinking organic matter depends on percent nitrate utilization in the euphotic zone. By comparing the δ15N of surface sediments with that measured in the glacial sections of several cores, we infer changes in the intensity and latitudinal distribution of nitrate uptake in this region during the last glacial maximum. These preliminary results argue against increased biological uptake of nutrients in southern polar waters as a major mechanism for glacial lowering of atmospheric CO2. They also suggest that Subantarctic waters in the SE Indian sector became more nutrient depleted as they migrated northward. Increased nitrate depletion might have also occurred slightly south of the glacial Polar Front. We use a six-box model to explore the possible impact of this observation on atmospheric CO2.

Nitrogen isotopic ratios in fecal pellets produced by marine zooplankton

At each site and in every season studied, zooplankton in the upper ocean produced fecal pellets that were 1.3% lower in {delta}{sup 15}N than their body tissue but 2.2% higher than their apparent food source. {sup 14}N-containing molecules are evidently preferentially assimilated in zooplankton intestinal tracts, though other isotopic effects must account for the enrichment in {sup 15}N of these organisms relative to their food. These results also show zooplankton to be important modifiers of nitrogen isotopic ratios for marine particulate matter. {delta}{sup 15}N values for sinking particles and fecal pellets are similar, supporting the perspective that macrozooplankton are important factors in producing and processing particles that sink into the oceans interior and sediments. In contrast, the relationship in {delta}{sup 15}N between fecal pellets and suspended particles in the euphotic zone is more variable. It appears that zooplankton select food particles of varying {delta}{sup 15}N from the suspended particle pool. These results suggest that both zooplankton feeding behavior and their digestive chemistry are important in determining the composition of sinking particulate matter in the ocean with respect to the suspended particle source in the euphotic zone.