Paleoproductivity

A B S T R A C T During the early stages of sediment diagenesis most of the organic matter reoxidizes, leaving behind a residual fraction of organic carbon which does not typically reflect its original quantities. Paleo-productivity reconstructions are therefore based on changes in the chemical composition of carbonate shells or, alternatively, use the abundances of inorganic elements in the bulk sediments, that have been shown to be proxies for organic matter contents. To examine the applicability of bulk inorganic elements composition for this task, we compare recorded changes in known anthropogenic nutrient fluxes to the oligotrophic and oxygenated Gulf of Aqaba in the north Red Sea, with the sedimentary records of barium, cadmium, copper and nickel over the last decades. Among these elements, nickel and copper strongly correlate with recorded nutrient fluxes and primary productivity in the region. In the present case, nickel is a more reliable proxy since part of the copper is possibly contributed from airborne pollution sources. The applicability of cadmium to serve as a tracer for nutrient additions could not be reliably tested because contribution of cadmium associated with phosphate ore loading in the adjacent ports may be significant. We do not observe any bulk sediment barium enrichments associated with increased nutrient fluxes. Overall, it appears from these correlations that nickel and probably also copper reliably record past changes in nutrient availability and organic matter fluxes while sedimentary barium and barite, which are commonly attributed to productivity, do not.

Reconstruction of palaeoproductivity from Southern Ocean is crucial for understanding the functioning
of the Southern Ocean biological pump in the past. High resolution records of multi-proxy parameters
(calcium carbonate, opal, total organic carbon biogenic barium and planktonic carbon isotope ratios
(δ13C)) were investigated in two well-dated sediment cores (SK200/22a and SK200/27) from the Indian
sector of Southern Ocean situated to the north and south of Antarctic Polar front (APF), respectively. The
palaeoproductivity records extending
95 ka BP (SK200/22a) and 75 ka BP (SK200/27) revealed inverse
relationships between the calcite and opal productivity, indicating the influence of shifting nutrient
regimes. At core SK200/22a, reduced calcite productivity during marine isotope stage (MIS) 2, 4, and part
of MIS 3 suggest an equatorward migration of the frontal regimes during glacial intervals. Compared to
this, the region south of the APF (core SK200/27) was characterized by the near absence of calcite
content during the last glacial period and increased opal productivity during MIS 1 and MIS 3,
supporting a southward migration of APF during warmer intervals. Ba(bio) records exhibit good
correlation with opal records in both the cores and also correlate with that of calcite record at SK200/
22a, indicating that Ba is influenced by the combined opal and calcite productivity. The enhanced opal
productivity during the glacial periods north of the APF is attributed to the northward shifting of oceanic
fronts and associated transfer of nutrients. Diatom productivity records of SK200/22a reveal significant
similarities with the dust records from the Antarctic and Southern Ocean, but showed no significant
relationships with the diatom record of SK200/27. It is proposed that the dust-derived Fe input had
apparently influenced the palaeoproductivity north of the modern APF, but had a minor influence on
opal productivity south of the APF. Comparison with the ice core climate records from Antarctica and
Greenland revealed that bioproductivity peaks in the study region are nearly synchronous with the
millennial Antarctic warming events. Remarkably, the calcite and opal productivity records at SK200/22a
responded differently to the Antarctic warming events, with opal productivity lagging behind the calcite
productivity peaks by
1–2 ka.

RESUMO A produtividade é um importante parâmetro paleoceanográfico porque permite inferências sobre as condições atmosféricas, de circulação oceânica superficial e sobre o clima no passado. Uma das formas de estudá-la ao longo do Quaternário é através das assembleias de microfósseis contidas nos sedimentos, tais como cocolitoforídeos, foraminíferos planctônicos e bentônicos, diatomáceas e dinoflagelados. No Atlântico Sul, os principais locais de elevada produtividade são as áreas de ressurgência equatorial e costeira (margem leste). Durante o Quaternário, as mudanças da produtividade são direcionadas pela ciclicidade glacial/interglacial e pela insolação, devido à associada intensidade dos ventos. ABSTRACT The productivity is an important paleoceanographic parameter because it allows inferences about atmospheric conditions, surface ocean circulation and climate in the past. One way to study it throughout the Quaternary is through microfossil assemblages in the sediments, such as coccolithophorids, benthic and planktonic foraminifera, diatoms and dinoflagellates. In the South Atlantic, the main locations of high productivity are the areas of equatorial and coastal upwelling (east margin). During the Quaternary, changes in productivity are driven by the glacial/interglacial cyclicity and the insolation, due to the associated intensity of the winds.

Benthic foraminifer accumulation rates (BFAR) and benthic to planktic foraminifer ratios (B/P) have been suggested to represent an index of paleoproductivity in the world oceans. It is shown here, however, that BFAR and B/P do not record the surface-water productivity signal in the intense upwelling and oxygen minimum zone (OMZ) along the Oman Margin. Greater BFAR and B/P values are noticed during the last glacial period, when the monsoon and upwelling strength were weaker and the surface-water productivity was reduced. This is in contrast to the suggestion that BFAR and B/P values are an indication of enhanced productivity. On the other hand, BFAR and B/P values were lower during the Holocene as a consequence of intensified monsoon and upwelling and resulting enhanced productivity in the Arabian Sea. This appears to suggest that BFAR and B/P do not reflect the productivity variations in the OMZ of the Arabian Sea. We speculate that dissolved oxygen concentrations might instead be controlling the benthic foraminifer abundance.

New magnetobiostratigraphic data for the Late Oligocene through early Miocene at Deep Sea Drilling Project (DSDP) Hole 516F provide a significantly revised age model, which permits reevaluation of developments that led to the Mi-1 glacial event at the Oligocene-Miocene boundary. Our new high-resolution paleomagnetic study, which is supported by quantitative calcareous nannofossil and planktonic foraminiferal analyses, significantly refines previous age models for Oligocene-Miocene sediments from DSDP Hole 516F, with ages that are systematically younger than those previously determined. In some parts of the Oligocene, the discrepancy with previous studies exceeds 450 kyr. Based on this new age model, we infer a progressive increase in sedimentation rate and paleoproductivity between ca 23.9 Ma and ca 22.9 Ma, with the highest rate coinciding with the Mi-1 glacial event at the Oligocene-Miocene boundary. This productivity increase would have resulted in higher rates of carbon burial and in turn a drawdown of atmospheric CO2. Immediately afterward, an abrupt decrease in sedimentation rate and paleoproductivity suggests that the Mi-1 deglaciation was associated with decreased carbon input into the ocean. Elevated sedimentation rates are also documented at ~24.5 Ma, coincident with the Oi2D glacioeustatic event. The presence of volcanic material within the sediments during these glacial events is interpreted to have resulted from redeposition of sediment scoured from nearby sites on the Rio Grande Rise due to transient variations in bottom-water flow patterns.

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Here we analyse a sedimentary record from the Agulhas Plateau, located close to the modern position of the subtropical front and use alkenones and coccolith assemblages to reconstruct oceanographic conditions over the past 300,000 years. We identify a strong glacial-interglacial variability in sea surface temperature and productivity associated with subtropical front migration over the Agulhas Plateau, as well as shorter-term high frequency variability aligned with variations in high latitude insolation. Alkenone and coccolith abundances, in combination with diatom and organic carbon records indicate high glacial export productivity. We conclude that the biological pump was more efficient and strengthened during glacial periods, which could partly account for the reported reduction in atmospheric carbon dioxide concentrations.

The history of the Somali upwelling during the last 18.5 ka has been reconstructed using biogenic silica fluxes estimated from a sediment core retrieved from the western Arabian Sea. The reconstructed record demonstrates periodic weakening and strengthening of the Somali upwelling during the past 18.5 ka. Variations in biogenic silica fluxes suggest weak upwelling during the last glacial period (18.5–15 ka BP). Strengthened upwelling during the Bølling–Allerød period (15–13 ka BP) points to post-glacial onset of the southwest monsoon. The Younger Dryas (13–11 ka BP) is again marked by reduced upwelling strength. Intensification of the Somali upwelling at the beginning of the Holocene and a decline at 8 ka BP are observed. Increases in upwelling strength recorded since 8 ka BP suggest strengthening of the southwest monsoon during the latter part of the Holocene. Linking these upwelling variations with southwest monsoon precipitation, a major shift in the relationship between the strength of the Somali upwelling and southwest monsoon rainfall from positive to negative has occurred during the pre-Holocene to the Holocene. The observed shift is attributed to the variation in the southwest monsoon strength due to the latitudinal shift of the Intertropical Convergence Zone (ITCZ) associated with changes in moisture sources.