Benthic foraminiferal response to the emergence of the Isthmus of Panama and coincident paleoceanographic changes

Late Cenozoic benthic foraminiferal faunas from the Caribbean Deep Sea Drilling Project (DSDP) Site 502 (3052 m) and East Pacific DSDP Site 503 (3572 m) were analyzed to interpret bottom-water masses and paleoceanographic changes occurring as the Isthmus of Panama emerged. Major changes during the past 7 Myr occur at 6.7–6.2, 3.4, 2.0, and 1.1 Ma in the Caribbean and 6.7–6.4, 4.0–3.2, 2.1, 1.4, and 0.7 Ma in the Pacific. Prior to 6.7 Ma, benthic foraminiferal faunas at both sites indicate the presence of Antarctic Bottom Water (AABW). After 6.7 Ma benthic foraminiferal faunas indicate a shift to warmer water masses: North Atlantic Deep Water (NADW) in the Caribbean and Pacific Deep Water (PDW) in the Pacific. Flow of NADW may have continued across the rising sill between the Caribbean and Pacific until 5.6 Ma when the Pacific benthic foraminiferal faunas suggest a decrease in bottom-water temperatures. After 5.6 Ma deep-water to intermediate-water flow across the sill appears to have stopped as the bottom-water masses on either side of the sill diverge.The second change recorded by benthic foraminiferal faunas occurs at 3.4 Ma in the Caribbean and 4.0-3.2 Ma in the Pacific. At this time the Caribbean is flooded with cold AABW, which is either gradually warmed or is replaced by Glacial Bottom Water (GBW) at 2.0 Ma and by NADW at 1.1 Ma. These changes are related to global climatic events and to the depth of the sill between the Caribbean and Atlantic rather than the rising Isthmus of Panama. Benthic foraminiferal faunas at East Pacific Site 503 indicate a gradual change from cold PDW to warmer PDW between 4.0 and 3.2 Ma. The PDW is replaced by the warmer, poorly oxygenated PIW at 2.1 Ma. Although the PDW affects the faunas during colder intervals between 1.4 and 0.7 Ma, the PIW remains the principal bottom-water mass in the Guatemala Basin of the East Pacific.     

A Late Pleistocene time-series of bottom-current speed in the Vema Channel

A coarsening of the mean particle size of the carbonate-free silt fraction from sea-floor samples below 4000 m in the Vema Channel has been used to separate high-velocity Antarctic Bottom Water (AABW) from the overlying, slower North Atlantic Deep Water (NADW). A time-series of fluctuations in bottom-current speed within the modern AABW/NADW transition zone was examined by determining the particle-size distribution of sediments from eight gravity cores with a high-resolution stratigraphy for the past 250 kyrs. The bottom-current paleospeed was inferred from a correlation of particle size in seafloor samples with mean current speed from nearby current-meters. The mean bottom-current speed at depths comparable to modern AABW was highest (7–10 cm/s) during interglacial to glacial transitions corresponding to the oxygen isotopic stage 6/7 and 4/5 boundaries and at present. The mean bottom-current speed at depths comparable to modern NADW was nearly uniform for most of the past 250 kyrs except during glacial oxygen isotopic stage 2 when the speed dropped to 2 cm/s, or one-half of the present speed. The application of the “calibrated” particle-size method to examine bottom-current paleospeed allows testing of paleoceanographic models which rely on assumptions or inferences of changes in bottom-water production rate during the late Pleistocene paleoclimatic fluctuations.     

A benthic foraminiferal record of middle to late Pliocene (3.15-2.85 Ma) deep water change in the North Atlantic

Records of benthic foraminifera from North Atlantic DSDP Site 607 and Hole 610A indicate changes in deep water conditions through the middle to late Pliocene (3.15 to 2.85 Ma). Quantitative analyses of modern associations in the North Atlantic indicate that seven species, Fontbotia wuellerstorfi, Cibicidoides kullenbergi, Uvigerina peregrina, Nuttallides umboniferus, Melonis pompilioides, Globocassidulina subglobosa and Epistominella exigua are useful for paleoenvironmental interpretation. The western North Atlantic basin (Site 607) was occupied by North Atlantic Deep Water (NADW) until ~2.88 Ma. At that time, N. umboniferus increased, indicating an influx of Southern Ocean Water (SOW). The eastern North Atlantic basin (Hole 610A) was occupied by a relatively warm water mass, possibly Northeastern Atlantic Deep Water (NEADW), through ~2.94 Ma when SOW more strongly influenced the site. These interpretations are consistent with benthic δ¹⁸O and δ¹³C records from 607 and 610A (Raymo et al., 1992). The results presented in this paper suggest that the North Atlantic was strongly influenced by northern component deep water circulation until 2.90–2.95 Ma. After that there was a transition toward a glacially driven North Atlantic circulation more strongly influenced by SOW associated with the onset of Northern Hemisphere glaciation. The circulation change follows the last significant SST and atmospheric warming prior to ~2.6 Ma.     

Late Quaternary paleoproductivity and deep water circulation in the eastern South Atlantic Ocean: Evidence from benthic foraminifera

Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction.

During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic δ¹³C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge.     

Benthic ostracods and deep water-masses in the Atlantic Ocean

Ostracod faunas at six locations are compared, and related to distributions in an Atlantic Ocean-wide data base. Five, widely developed, vertical faunal sequences are recognised at particular levels within deep water-masses: Henryhowella Fauna (lower part of Antarctic Intermediate Water); Krithe Fauna (Upper North Atlantic Deep Water); Poseidonamicus-Bosquetina Fauna (upper part of Lower NADW); Dutoitella Fauna (lower part of Lower NADW); Legitimocythere Fauna (Antarctic Bottom Water). These faunas are correlated with previously established deep water benthic foraminiferal assemblages, and their possible palaeo-oceanographic use is discussed.     

Stepwise postglacial migration of benthic foraminifera into the abyssal northeastern Norwegian Sea

Accumulation rates of individual species (SpecAR) and relative abundances (percentages) of benthic foraminifera of an AMS ¹⁴C-dated high resolution sediment core from the Norwegian Seas (water depth: 2707 m) provide a record of the faunal fluctuations from the last glacial maximum across the Weichselian deglaciation to the Holocene. During glacial times, the total foraminifera accumulation remains at a very low level (< 100 specimens cm⁻² kyr⁻¹) and is dominated by two endofaunal species: Oridorsalis umbonatus (Reuss) and Siphotextularia rolshauseni (Phleger and Parker) and reworked specimens of the genus Elphidium. The following deglaciation period exhibits an increase of the AR of the total fauna at 14 kyr B.P. The species distribution is marked by the last appearance of S. rolshauseni and the first postglacial appearance of suspension feeding Cibicidoides wuellerstorfi (Schwager) 13 kyr B.P. The absolute maximum of benthic foraminiferal AR (2750 spec cm⁻² kyr⁻¹ occurred near 9 kyr B.P. at the end of the deglaciation. This maximum also marks the re-appearance of the agglutinating species Cribrostomoides subglobosus (Sars). The post-glacial interval is characterized by a twofold reduction of the total accumulation of benthic foraminifera. The species distribution shows two new species: Ammobaculites agglutinans (d'Orbigny, at 6 kyr B.P.) and Epistominella exigua (Brady, at 3.5 kyr B.P.). The total AR indicates benthic activity during glacial times was at a low level. It was significantly higher during the Holocene with an abrupt increase of benthic foraminiferal abundance from 10 to 9 kyr B.P. The Stepwise re-invasion into the postglacial deep-sea environment maybe related to specific habitat preference.     

Variations in bone collagen δC and δN values of fauna from Northwest Europe over the last 40?000 years

We report here a signal in the temporal variation of stable isotopes in protein from surviving animal bone in Northwest Europe over the past glacial cycle. There is a change in the average δ¹³C values of fauna in the Holocene, and there is also a significant reduction in δ¹⁵N values of herbivore bone collagen towards the end of the last glaciation, with a subsequent recovery soon after the start of the Holocene. This change is observed for several species and is restricted to those regions most affected by the glacial advance. Comparison with ice core data shows that there is a strong correlation between the average δ¹³C values of three herbivore species and ice core CO₂ concentration. The data presented here show how readily available faunal bone collagen δ¹³C and δ¹⁵N measurements provide a record of past climate and environmental change on a regional to continental scale.     

Reconstruction of a subalpine grass-dominated ecosystem,Lake Rutundu,Mount Kenya: a novel multi-proxy approach

Palaeoecological reconstructions based on a single proxy are limited, but by combining pollen, biogeochemistry and grass cuticle analysis, ecosystem structure and function can be better understood. Lake Rutundu is a small, subalpine lake on the northeast flank of Mt Kenya. During the last glacial, pollen evidence suggests a shrub grassland dominated by Afroalpine taxa and Poaceae, representing a dry, cold, open environment. The δ¹³C values of terrestrial biomarkers imply a high proportion of C₄ plants. Grass cuticle analysis allows resolution of the different C₄ subtypes and shows that the vegetation was dominated by tall C₄ panicoid grasses, prone to frequent fires. During the Holocene, Poaceae pollen declined while subalpine shrubs increased. The δ¹³C values of terrestrial biomarkers imply a C₃-dominated vegetation. Together with an expansion of rainforest at lower altitudes, this suggests wetter conditions more favourable to C₃ plants. Increased percentages of C₃ pooid grass cuticles confirm a reduction in moisture stress.     

Glacial–interglacial deep-water changes in the NW Pacific inferred from single foraminiferal δO and δC

Oxygen and carbon isotope values of single benthic foraminiferal tests in a core from the Shatsky Rise, NW Pacific Ocean, show greater intra-horizon variance during the Holocene than during the Last Glacial Maximum (LGM). This greater variance is caused by the introduction of glacial specimens some 20 cm upward from their original deposition layer due to bioturbation. In contrast, foraminiferal populations belonging to glacial layers do not include Holocene specimens. The difference in direction of bioturbation greatly modifies climate information in horizons formed during and after deglacial events. After omitting glacial specimens from Holocene sediments, the glacial–interglacial difference in δ¹⁸O suggests that Pacific deep-water temperature changed by 2.4–3.8°C at the most. The δ¹³C values suggest that nutrient concentration was higher during the LGM than the Holocene. The glacial deep North Pacific Ocean apparently was influenced by cold deep waters of southern origin.     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

Palaeoceanographic record of the last deglaciation in the Strait of Sicily

The palaeoceanographic evolution of the Levantine waters during the last deglacial time is investigated using the sedimentary record of a deep sea core, CS 70-5, from the Linosa basin (35° 44.4′N/13° 11.0′E, 1486 m water depth). Radiocarbon dating and oxygen isotope stratigraphy based on¹⁸O changes inGlobigerina bulloides allow us to recognize and to date the different steps of the deglaciation. These steps are synchronous with those reported in the North Atlantic, but correspond to a δ¹⁸O decrease of higher amplitude than in the Alboran sea or in the North Atlantic Ocean. Major faunal events permit the establishment of a local biozonation which differs from those reported either for the Alboran sea or the Eastern Mediterranean basin. Major breaks in the faunal assemblages occurred during Termination I_A, within the first step of the deglaciation around 14 kyr B.P., and near 10.6 kyr B.P. within the Younger Dryas. The onset of the last deglaciation induced important changes in the characteristics of the Levantine and Atlantic water masses which occupied the Strait of Sicily. The δ¹³C records ofGlobigerina bulloides andCibicidoides pachyderma indicate that the modifications observed in the assemblages of deep faunas are controlled by the oxygenation of the water column. δ¹³C records ofGlobigerina bulloides are similar throughout the West Mediterranean as well as in sediments located below the present Mediterranean outflow. The distributional pattern as well as its δ¹³C record suggest that this species could be a good recorder of the upper Levantine waters and, more precisely, of the mixing layer of the overlying Atlantic waters with the Levantine ones.Major influxes of Atlantic waters during Terminations I_A and I_B could have slowed down the vertical mixing of the different water layers present in the Strait of Sicily and caused a decrease in the oxygenation of the water column. During Termination I_B the effect of Atlantic influxes was reinforced by the occurrence of a low salinity layer in the eastern basin which led to the stagnation of the deep waters. The two episodes of decreased oxygenation in the Levantine waters also favored the precipitation of inorganic magnesium calcite.     

Benthic foraminiferal extinctions linked to late Pliocene–Pleistocene deep-sea circulation changes in the northern Indian Ocean (ODP Sites 722 and 758)

During the late Pliocene–middle Pleistocene, 63 species of elongate, bathyal–upper abyssal benthic foraminifera (Extinction Group = Stilostomellidae, Pleurostomellidae, some Nodosariidae) declined in abundance and finally disappeared in the northern Indian Ocean (ODP Sites 722, 758), as part of the global extinction of at least 88 related species at this time. The detailed record of withdrawal of these species differs by depth and geography in the Indian Ocean. In northwest Indian Ocean Site 722 (2045 m), the Extinction Group of 54 species comprised 2–15% of the benthic foraminiferal fauna in the earliest Pleistocene, but declined dramatically during the onset of the mid-Pleistocene Transition (MPT) at 1.2–1.1 Ma, with all but three species disappearing by the end of the MPT (∼0.6 Ma). In northeast Indian Ocean Site 758 (2925 m), the Extinction Group of 44 species comprised 1–5% of the benthic foraminiferal fauna at ∼3.3–2.6 Ma, but declined in abundance and diversity in three steps, at ∼2.5, 1.7, and 1.2 Ma, with all but one species disappearing by the end of the MPT. At both sites there are strong positive correlations between the accumulation rate of the Extinction Group and proxies indicating low-oxygen conditions with a high organic carbon input. In both sites, there was a pulsed decline in Extinction Group abundance and species richness, especially in glacial periods, with some partial recoveries in interglacials. We infer that the glacial declines at the deeper Site 758 were a result of increased production of colder, well-ventilated Antarctic Bottom Water (AABW), particularly in the late Pliocene and during the MPT. The Extinction Group at shallower water depths (Site 722) were not impacted by the deeper water mass changes until the onset of the MPT, when cold, well-ventilated Glacial North Atlantic Intermediate Water (GNAIW) production increased and may have spread into the Indian Ocean. Increased chemical ventilation at various water depths since late Pliocene, particularly in glacial periods, possibly in association with decreased or more fluctuating organic carbon flux, might be responsible for the pulsed global decline and extinction of this rather specialised group of benthic foraminifera.     

The evolution of ecological specialization in southern African ungulates: competition‐ or physical environmental turnover?

Using long‐term diet reconstructions spanning the past one million years, we contrast hypotheses that biotic interactions versus physical environmental changes are primary drivers of evolutionary turnover in mammals. We use stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios in tooth enamel carbonate to trace herbivore niche shifts through the Late Quaternary Land Mammal Ages (LMAs) of grassland savannas in the South African interior (Cornelian‐1.0 to 0.6 Ma; Florisian‐500 to 10 ka; and Holocene/modern). Data reveal niche separation amongst closely related coeval taxa, and dispersals through time into empty niche spaces following extinctions. This suggests a primary role of competitive exclusion and niche displacement for speciation and extinctions in these early grassland environments. However, niche changes through time show a similar trend in many taxa, entailing increased δ¹³C (elevated C₄ grass consumption) from the Cornelian to the Florisian, and from the Florisian to the Holocene/modern, and elevated δ¹⁸O in Holocene/modern taxa that reflect global aridification around the terminal Pleistocene. Commonality in isotopic trends implies universal environmental forcing of ecological, and ultimately macroevolutionary, turnover. Yet some taxa shift from a mixed C₃/C₄ diet in the Florisian to a near‐pure C₃ diet today. Indeed, we find that while δ¹³C data are normally distributed for Cornelian fossils, non‐normal distributions characterize more recent time intervals. Such distributions are in line with the bimodal distribution of δ¹³C and diet in contemporary African ungulates. Thus, while environmental forcing did not, by necessity, lead to increases in C₄ intake, the results show changes from mixed to more specialized diets. We propose that this niche specialization was a function of long‐term exposure to C₄ grasslands, consistent with predictions that relatively high metabolic demands of C₄ grazing in subtropical environments forced the differentiation of herbivores into one of two highly specialized feeding niches, i.e. C₃ browsing or C₄ grazing.     

Long‐term effects of climate change on carbon flows through benthic secondary production in small lakes

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Late glacial to recent deep-sea benthic foraminifera from the northeastern Atlantic (Cadiz Gulf) and western Mediterranean (Alboran Sea): Paleooceanographic results

Deep-sea benthic foraminifera of the region of water exchange between the Mediterranean and the Atlantic Ocean, at the Gibraltar Strait, show significant water-mass relationships for the past 18,000 years.Cores used in this benthic foraminifera analysis were selected, according to the context of the distribution of various deep bottom water masses, on each side of the Gibraltar Strait:

• 1.(1) on the Atlantic side of the Strait, two cores (KS 8228, KS 8229) are located in the present day NADW, and one (KC 8221) in the upper part of the present Mediterranean outflow water,
• 2.(2) on the Mediterranean side of Gibraltar, two cores (KC 8241, SU 8107) are located in the deep water mass, and one (KS 8230) in the intermediate water mass.

In the two deep basins (1500–2800 m depth in the Gulf of Cadiz, 1200–1300 m depth in the Alboran Sea), the paleooceanographic changes appear to be in an opposite way for the past 18,000 years. The Alboran basin shows a paleooceanographic evolution from a well-oxygenated, nutrient-rich environment at about 18,000 yr B.P. to a nutrient-poor, oxygen-depleted environment from 13,000 to the present time; moreover, for the time-span synchronous with the well-known development of sapropels in the eastern Mediterranean basins between 10,000 and 7000 yr B.P., the faunal assemblage shows most unusual characteristics implying drastic environmental conditions. Conversely, in the Gulf of Cadiz, the environment pass as from a biotope occupied by an oxygen-depleted, nutrient-poor water mass at about 18,000 years B.P. to a biotope occupied by NADW since the Younger Dryas; this agrees with previous data obtained in the northeastern Atlantic Ocean (Caralp, 1987).In the epibathyal zones (550–800 m depth), on both sides of the Gibraltar Strait, paleohydrographic changes do not seem so important. According to the present and very late Holocene assemblages, which are similar on both sides of the Strait with a strong east-west flow, two other stratigraphic episodes have shown the same conditions of water exchanges: the end of the isotopic stage 2 and the Younger Dryas. Conversely, during the last glacial maximum, the Bølling-Allerød and the lower Holocene, westward water fluxes were probably lower. At no time, the hypothesis of a reversal or a stop in the east-west exchanges between Mediterranean and Atlantic Ocean may be justified.     

Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

The relationship between carbon and nitrogen stable isotopes of zooplankton and select environmental variables in low-latitude reservoirs

Cyclopoids were collected from 18 reservoirs in southern China during August (a wet month) and December (a dry month) of 2010 for the analysis of carbon and nitrogen stable isotopes (δ¹³C_Zoo and δ¹⁵N_Zoo). The objectives of this study were to examine whether δ¹³C_Zoo and δ¹⁵N_Zoo can be better indicators of primary productivity and trophic state than the stable isotope composition of suspended particulate organic matter (POM), and to evaluate the relationship between δ¹³C_Zoo and δ¹⁵N_Zoo and select environmental variables. The δ¹³C_Zoo in these reservoirs was enriched in August and depleted in December, and varied significantly along the continuum of trophic levels. By contrast, δ¹⁵N_Zoo was depleted in August and enriched in December, and did not increase significantly with an increase in trophic state. Both δ¹³C_Zoo and δ¹⁵N_Zoo were more strongly correlated with environmental factors than δ¹³C_POM and δ¹⁵N_POM were. In addition, more environmental factors were significantly correlated with δ¹³C_Zoo and the δ¹⁵N_Zoo than with δ¹³C_POM and δ¹⁵N_POM. When data from two seasons were pooled, δ¹³C_Zoo was strongly correlated with dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and the DIN:SRP ratio, while δ¹⁵N_Zoo was weakly correlated with nutrient concentrations. This study indicates that, compared to the stable isotope composition of POM, δ¹³C_Zoo is a better indicator of primary productivity and trophic state, while δ¹⁵N_Zoo may be used as a proxy for nitrogen sources in aquatic ecosystems.     

Ocean basin and depth variability of oxygen isotopes in cenozoic benthic foraminifera

One of the longest, most detailed quantitative records of oceanographic change in the Cenozoic is that provided by oxygen isotope measurements made on the tests of foraminifera. As indicated by measurements on benthic foraminifera, the deep waters of the world ocean have undergone an overall cooling of about 10°C in the Cenozoic. This change has been neither monotonic nor gradual. Rather, it is evidenced by a few, relatively rapid increases in the ¹³O content of the benthic shells. These “steps” in the isotopic record have been associated with major evolutionary changes in the mean state of the deep ocean. The variance around this mean state has also changed through the Cenozoic. From relatively high variance in the Middle Eocene, the oceans showed low variance in the Late Eocene and Oligocene. In the Miocene the variance of the isotopic measurement again increased, reaching a maximum of short duration in the Middle Miocene. This maximum as well as that which occurred during the Late Pliocene and Quaternary, may be attributable to fluctuations in the isotopic composition of the oceans caused by growth and decay of large ice sheets.In the Late Miocene the benthic oxygen isotopes in Atlantic sites less than 3000 m deep have a higher variance than sites at similar depths in the Pacific and Indian Oceans. It is thought that this high variance results from long-term changes in the importance of the cool and salty North Atlantic Deep Water relative to that of the warmer and less saline Antarctic Intermediate Water at Atlantic sites between 1000 m and 3000 m water depth. Such significant differences in benthic isotopic variance between the ocean basins have been demonstrated only in post-Middle Miocene intervals.     

The variation of upper ocean structure and paleoproductivity in the Kuroshio source region during the last 200 kyr

A core from the source region of the Kuroshio warm current (east of the Luzon Island) was analyzed using several proxies in order to study the variability of the Western Pacific Warm Pool (WPWP) during the last two glacial–interglacial cycles. Primary productivity (PP) variations were deduced from variations in the coccolith flora. Primary productivity was higher during glacial periods (the end of Marine Isotope Stage [MIS] 3, some periods in MIS 2 and 6), and decreased during interglacial periods (MIS 7, MIS 5e and probably MIS 5c–5d), with the lowest PP in MIS 5e. Variations in the δ¹³C difference in benthic and bulk carbonate, thus in the vertical gradient of δ¹³C in dissolved inorganic carbon (Δδ¹³C_{C. wuellerstorfi–N. dutertrei} and Δδ¹³C_{C. wuellerstorfi–coccolith}) coincided with the PP changes, showing that export productivity was low during interglacial periods (MIS 7, MIS 5e and Holocene) and high during glacial periods (MIS 6, probably MIS 5c–5d, late MIS 4 and late MIS 3). Comparison of foraminiferal carbonate dissolution indicators and PP changes reveals that nannofossil assemblage in core Ph05-5 is not sensitive to carbonate dissolution intensity. The depth of the thermocline (DOT) was estimated from planktonic forminiferal assemblages, and was relatively greater during interglacial periods (MIS 7, MIS 5e, probably MIS 5c and Holocene) than during glacials (middle MIS 6, probably MIS 5b and 5d, some periods in MIS 4, MIS 3 and MIS 2). Good coherence between the paleoproductivity records and the DOT suggests that the DOT changes could be the primary control factor in changes of paleoproductivity, and the glacial high productivity in the Kuroshio source region could be associated with a global increase of nutrient concentration in the intermediate waters that upwelled into the photic zone. The low CO₂ values derived for intervals of high productivity and a relatively shallow DOT suggest that the changes in biological productivity and DOT in the equatorial Pacific could have modified atmospheric CO₂ concentrations. High Sea Surface Temperatures (SSTs) during the warm MIS 5e in combination with intensified monsoonal rain fall could have resulted in a more intense stratification of the upper waters, resulting in low nutrient supply to the surface waters and a resulting decrease in productivity.     

Seabirds colonized Ny-Ålesund, Svalbard, Arctic ~9,400 years ago

A 118-cm-long and well-preserved sediment profile in a paleo-notch, which was formed by ocean wave action before rising to the terrace, was collected from the first terrace of Ny-Ålesund, Svalbard, Arctic. The bottom of this profile was dated as 9,400 years B.P. based on two radiocarbon dates of fossil mollusc shell fragments. The organic material in the sediment was identified by δ¹³C_org–C/N plot and δ¹⁵N_org characteristics to be predominantly composed of seabird guano, which was transported from the ocean via preying and excreting by seabirds. These results indicate that seabirds have inhabited Ny-Ålesund since 9,400 years B.P. after Kongsfjorden was completely deglaciated. This is the first report on Holocene seabird occupation on Ny-Ålesund and it provides the foundation for understanding the ecological history of seabirds in Svalbard in Holocene.