Carbon Reservoir In Low-latitude Oceans And Orbital Cycles Of Monsoon Climate

The discovery of glacial cycles in the ice-core CO₂ records has raised some fundamentalscientific questions: How does the oceanic carbon reservoir respond to orbital cycles? Is it drivenby the waxing and winning of ice sheets, or can it directly respond to orbital changes throughlow-latitude procession? To answer these questions, cyclicity in variations of the oceanic carbonreservoir should be first studied.In the present work, the carbon reservoir changes of the South China Sea (SCS) and theMediterranean Sea since the Pliocene have been investigated, on the basis of the oxygen andcarbon stable isotopes of planktonic foraminifer Globigerinoides ruber, carbonate and organiccarbon percentages from Site MD052901 in the SCS and Punta Piccola section in the southernItaly. The orbital cyclicities of African Monsoon and East Asian Monsoon, and the low latitudeprocess responses to the orbital forcing have also been discussed. All the analyses have been donein the State Key Laboratory of Marine Geology, Tongji University. The results are compared anddiscussed with many published and unpublished data from the low latitude oceans.A stacked oceanδ¹³C record of 200 kyr (SCS PFδ¹³C) is established on the basis of G ruberδ¹³C data from 8 sites in the SCS. The glacial/interglacialδ¹³C cycle mostly reflects the influenceof the continental biomass on the ocean carbon reservoir. Threeδ¹³C minima occurring atglacial/interglacial terminations may have been caused by Antarctic warming prior to Arctic icemelting. The enrichment trend ofδ¹³C from MIS6 and MIS4 to MIS2 represents the longeccentricity cyclicity, implying that the Earth system has been entering into a newδ¹³Cmax stage.SCS PFδ¹³C shows clear precession signals, suggesting that the carbon reservoir change can bedriven by low latitude process, such as monsoon circulation. At the same time, a stacked oceanCaCO3 record of 200 kyr (SCS CaCO₃ %_stacked and SCS MAR_stacked) is obtained according toCaCO₃% data from 6 sites in the SCS. There are obvious glacial-interglacial fluctuations at SCSCaCO₃ MAR which due to the influence of high latitude ice caps.The 450 kyr records ofδ¹⁸C,δ¹³C, CaCO₃ % and Corg % in core MD052901 from theupwelling area off Vietnam show obvious 41kyr obliquity cycles, implying the ice-sheet influenceon carbon reservoir. Meanwhile, the pronounced 23 kyr precession and 11 kyr hemi-precessionindicate that the stronger East Asian monsoon at precession minima likely drives the upwellingdevelopment. The 100 kyr short eccentricity cycle and 400 kyr long eccentricity cycle are alsopresent in the SCS carbon reservoir. Moreover, the mineral clay percentages of this site, whichindicate the weathering varieties caused by the monsoon, represent the foregoing orbital cyclicitiesAll of these cyclicities show that the SCS carbon reservoir and the East Asian Monsoon changewere not only influenced by the polar ice-sheets, but also directly responded to the orbital forcing.There are 400 kyr long eccentricity cycles with heavyδ¹³C values, labeled asδ¹³Cmax,in the isotope records of Site ODP1143 over the last 5 Ma. They occurred withδ¹³O maxima at theeccentricity minima, while theδ¹³C andδ¹⁸O records are coupled over the long eccentricity band in the Pliocene. With the boreal ice-sheet growth,δ¹³C andδ¹⁸O are decoupled in the Quaternary,suggesting the ice-sheet changes effect on the monsoon system. For example, stronger monsoonand enhanced precipitation occurred in all tropical oceans duringδ¹³Cmax-â…¡period, whenδ¹³C incore MD052901 and ODP1143 shifted negatively, subsequently leading to a major ice sheetexpansion.The 400 kyr long eccentricity cyclicity is present most clearly in the ocean reservoir recordsfrom the Rossello Composite section in Sicily. As an example, the Punta Piccolasection shows obvious 400 kyr cycle in theδ¹⁸O,δ¹³C and CaCO₃ % records. At the sametime, the 100 kyr eccentricity cyclicity is found in these records and in planktonic foraminiferalpercentages. Furthermore, clear precession cycles occur with lowδ¹³C, CaCO₃ % and high Corg% at precession minima in carbon reservoir records of this section, which denote the low latitudeprocess (monsoon system) responding to the orbital forcing. The mineral clay component and theBa/A1 ratio show the strong precession cycles. It's suggested that the stronger Africa monsoon atprecession minima causes the enhanced precipitation and the Nile River flooding, and muchcontinental fresh water inputs bring the elevated productivity, resulting in the Mediterraneancarbon reservoir changes.Comparing the records between the SCS and the Mediterranean, the carbon cyclicities at SiteODP 1143 are more complicated than the Punta Piccola section, likely due to many differentfactors and origin from the complex East Asian monsoon for its geographical position. Influencedby the West Pacific Warm Pool, Australian monsoon and the Siberia High, the East Asianmonsoon becomes more complex than the African monsoon.In the Pliocene, the ocean carbon reservoir records show most obvious 400 kyr and 100 kyreccentricity cycles, 20 kyr precession and 10 kyr half-precession cycles. But the orbital cyclicitiesof the ocean carbon reservoir in the Pleistocene are more complicated than in the Pliocene, thoughthe 100-kyr eccentricity and 20-kyr precession cycles are still clear. The 40-kyr obliquity cyclesare found in the carbon records of the SCS, the Mediterranean and the Atlantic oceans, whichindicate the strong ice sheet forcing for the Earth climate. As to the long eccentricity band, theδ¹³C andδ¹⁸O decoupling in the Pleistocene is associated with oceanic and climatic eventspreceding the eccentricity minimum concomitantly with drastic expansion of the boreal ice sheet.Instead of the 400 kyr eccentricity cycle, the 500 kyr eccentricity cycle appears in the Plioceneunder the influence of the boreal ice sheet.The 400 kyr long eccentricity cycle persists in the ocean carbon reservoir since the middleCenozoic and is the heartbeat of the climate system. It remains quite clear after the Antarcticice-sheet formation. With the formation and development of the boreal ice-sheet, the carbonrecords show the 500 kyr cycle. There are different responses to the orbital parameters at differentlatitudes. The high-latitude area is mainly affected by obliquity, and the low-latitude area isprimarily influenced by precession. Furthermore, the climate precession is modulated byeccentricity. Therefore both 100 kyr and 400 kyr eccentricity cycles are representing low-latitudeprocesses.In summary, the oceanic carbon reservoir is directly subject to orbital forcingthrough the monsoon system, resulting in precession and eccentricity cycles as itsprimary periodicities. The Earth climate system has both "ice-sheet response" and "monsoon response" to orbital changes. The low-latitude processes representedchiefly by monsoons have their own responding mechanisms, rather than merelypassively following the ice-sheet changes. On the other hand, the low-latitude recordsalso carry climate signals from the high latitude. In fact, the complicated records ofthe East Asian monsoon are resulted from interaction between tropical andhigh-latitude processes.